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CONTENTS
CHAPTER 1
:
1-23
NATURE OF GEOMORPHOLOGY
D e fin itio n and sc o p e o f g e o m o rp h o lo g y ; e v o lu tio n o f
geom orphological thoughts; Indian contributions to g eo m o rp h o lo g y ;
system c o n c e p t ; g eo m o rp h ic m odels ; m ethod s and ap p ro a c h e s to
the study o f landform s.
f-
CHAPTER 2
FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
A
24-56
C o n c e p t s r e l a t e d to u n i f o r m i t a r i a n i s m , g e o l o g i c a l s t r u c t u r e ,
g e o m o rp h o lo g ic a l p rocesses, stages o f time, g e o m o rp h ic sc a le (tim e
s c a l e - c y c l i c tim e , g r a d e d tim e a n d s te a d y tim e , s p a ti a l s c a l e ) ,
g e o m o rp h o lo g ic a l equation, com plex ity o f la n d fo rm s etc.
CHAPTER 3
:
TH EO RIES O F LANDFORM DEVELOPMENT
57-88
L a c k o f c o m m o n ly a c c ep tab le theo ry ; s ig n ific a n c e a n d g o a ls o f
g e o m o r p h ic th e o ries ; historical p ersp ectiv e ; b ases a n d ty p e s o f
g e o m o rp h ic theo ries (teleological theory, im m a n e n t th eory, h isto rical
th e o ry , ta x o n o m ic theory, functional theory, realist theory, c o n v e n t io n ­
alist t h e o r y ) ; m a jo r g e o m o rp h ic theories o f G. K. G ilb ert, W .M . D a v is ,
W . P en ck , L. C. K ing, J. T. H ack, M . M o ris a w a an d S. A. S c h u m m ;
g e o m o rp h ic th eories in Indian context.
CHAPTER 4
CLIMATIC GEOMORPHOLOGY AND MORPHOGENETIC
89-104
REGIONS
D ia g n o s tic la n d fo rm s ; g e o m o rp h o lo g ica l p ro c e sse s and c lim a tic c o n ­
trol ; d ire c t co ntrol o f cl i m a t e ; indirect clim atic c o n t r o l ; c lim a tic c h a n g e s
a n d la n d fo rm s ; m o rp h o g e n e tic regions.
CHAPTER 5
:
CONSTITUTION OF THE EARTH'S INTERIOR
105-113
S o u rc e s o f k n o w le d g e ; artificial sources, e v id e n c e s fro m th e th e o rie s o f
th e o rig in o f the earth, an d natural so u rces ; e v id e n c e s o f s e is m o lo g y ;
c h e m ic a l c o m p o s itio n and la y erin g sy stem o f the earth ; th ic k n e s s a n d
d ep th o f different layers o f the earth ; recent view s - crust, m a n tle and core.
CHAPTER 6
:
CONTINENTS AND OCEANS
114-131
In tro d u c tio n ; te tra h e d ra l h y p o th e sis ; co n tin e n ta l d rift th e o ry o f T a y l o r ;
c o n tin e n ta l d rift th e o ry o f W e g e n e r ; p late te cto n ic th e o ry .
CHAPTER 7
:
TH EO RY O F ISO STA SY
132-139
Introduction ; discovery o f the concept ; concept o f Airy ; concept o f
Pratt; concept o f Hayford and B ow ie ; concept o f Joly ; concept o f
Holmes ; global isostatic adjustment.
CHAPTER 8
:
ROCKS
140-157
Introduction; classification o f rock s; igneous rock s; sedimentary rocks ;
metmorphic rocks.
:
EARTH'S MOVEMENT
158-169
Introduction ; endogenetic forces (sudden forces and movements,
diastrophic forces and movements - epeirogenetic movements, orogenetic
m ovem en ts); folds ; faults ; rift valleys ; exogenetic forces.
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STRUCTURAL GEOMORPHOLOGY
CHAPTER 10
170-184
Geomorphic expressions of uniclinal structure ; topographic expressions
of fault structure (fault geomorphology) ; topographic expressions o f
folded structure (fold geomorphology), inversion o f relief, fluvial cycle
of erosion on folded structure ; topographic expressions o f domed
structure, fluvial cycle o f erosion on domed structure.
:
CHAPTER 11
185*199
PLATE TECTONICS
Meaning and concept ; plate margins ; palaeomagnetism-source of
g e o m a g n e ti c fie ld , r e m a n e n t m a g n e tis m , r e c o n s t r u c t i o n o f
palaeomagnetism, reversal of polarity ; sea-floor spreading ; plate m o ­
tion ; causes of plate motion ; plate tectonics and continental d r i f t ; plate
tectonics and mountain building ; plate tectonics and vulcanicity ; plate
tectonics and earthquakes.
CHAPTER 12
:
200-215
VULCANICITY AND LANDFORMS
Concept of vulcanicity ; components o f volcanoes ; classification o f
volcanoes ; volcanic types ; world distribution of volcanoes ; m echanism
and causes o f vulcanism ; hazardous effects of volcanic eruptions ;
topography produced by vulcanicity ; geysers ; fumaroles.
CHAPTER 13
:
216-246
MOUNTAIN BUILDING
Introduction ; classification of mountains ; block m ountains ; folded
mountains ; geosynclines ; theories of mountain building - geosynclinal
theory o f Kober ; thermal contraction theory of Jeffreys ; sliding co nti­
nent theory of Daly ; thermal convection current thery of H olm es ;
radiactivity theory o f Joly ; plate tectonic theory.
CHAPTE 14
:
WEATHERING AND MASSMOVEMENT
247-266
M eaning and concept ; controlling factors o f weathering ; types o f
weathering processes ; physical weathering ; chem ical w eath erin g ;
biotic weathering ; biochemical weathering ; geom orphic im portance o f
weathering ; m assm ovem ent and masswasting - m eaning and c o n c e p t ;
classification o f m assm ovem ents ; factors o f m assm ov em ents ; slides;
falls ; flows ; creep.
CHAPTER 15
:
HILLSLOPE
267-296
Classification o f s lo p e s ; slope e le m e n ts ; approaches to the study o f slope
development-slope evolution approach and process-form approach (m ono­
process concept and poly-process concept) ; slope decline theory o f
Davis ; slope replacem ent theory o f P enck ; A. W o o d 's m odel o f slope
e v o lu tio n ; hillslope cycle theory o f L.C. K i n g ; co n ce p t o f R. A .S av ig ear ;
F isher - L ehm ann model o f slope evolution ; pro cess-resp o n se m o d e l o f
A. Y oung ; slope failure ; hillslope p rocesses and erosion.
CHAPTER 16
:
CYCLE OF EROSION, REJUVENATION AND POLYCYCLIC RELIEFS
297-307
Origin and evolution of the concept ; geographical cycle of Davis ;
Penck's model of cycle of erosion; normal cycle of erosion; interruptions
in cycle of erosion ; rejuvenation ; topographic expressions of rejuvena­
tion and poly (multi) cyclic reliefs.
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DENUDATION CHRONOLOGY, EROSION SURFACES AND
PENEPLAINS
Meaning and concept ; erosion surfaces— meaning, identification o f
erosion surfaces, dating of erosion surfaces ; erosion surfaces o f
Cnotanagpur highlands ; denudation chronology o f peninsular India ;
(ii)
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CHAPTER 17
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denudation chronology and erosion surfaces o f Belan basin ; denudation
chronology and erosion surfaces o f Ranchi p lateau; p en ep lains; panplains.
CHAPTER 18
:
DRAINAGE SYSTEMS AND PATTERNS
334—352
M eaning and concept ; sequent drainage system s (consequent, sub se­
quent, obsequent and resequent streams) ; insequent d rainag e system
(antecedent and superim posed drainage systems) ; drainage patterns
(trellised, dendritic, rectangular, radial, centripetal, annular, barbed,
pinnate, herringbone and parallel p a t te r n s ) ; river capture.
CHAPTER 19
:
MORPHOMETRY OF DRAINAGE BASINS
353-384
M e a n in g and c o n c e p t ; historical perspective ; shortcom ings ; d rain ag e
basin : a geom o rp hic unit ; drainage basin : historical perspectiv e ;
d rain a g e basin hydrological cycle ; basin m orphom etry ; linear aspects :
stream ordering, bifurcation ratio, law o f stream num bers, length ratio,
law o f stream length, sinuosity indices, stream ju n c tio n angles ; areal
aspects : geo m etry o f basin shape, law o f basin perim eter, basin length
a nd basin area, area ratio, law o f basin area, law o f allom etric g row th,
stream frequency, drainage density, drainage texture ; relief aspects :
h y p so m etric analysis, clinographic analysis, altim etric analysis, av era g e
slope, relative reliefs, dissection index, law o f channel slope, profile
analysis.
RIVER V A LLEYS, GRADED RIVER AND PROFILE
OF EQUILIBRIUM
385-395
F o rm s o f valley d ev elo p m en t ; valley deepening ; valley w id e n in g ;
valley le n g th e n in g ; classification o f valleys ; graded curv e o f a riv er an d
p ro file o f eq uilibrium : longitudinal profile and graded curve, c o n c e p t o f
g rade, co n trollin g factors o f graded river, grading o f riv er ch an n e l a n d
p ro file o f eq u ilib riu m ; disturbed and regraded cu rv e : effects o f r e ju v e ­
n ation , effects o f deposition.
CHAPTER 21
:
CHANNEL MORPHOLOGY
396-412
C h a n n e l g e o m e try o r form ; hydraulic g eo m etry (at - a station re la tio n ­
ships, d o w n stre a m variations in channel form s, bed and b a n k m a te ria ls
a n d h y d ra u lic g eo m etry , sed im en t load and h y d raulic g e o m e t r y ) ; c h a n ­
nel b ed to p o g rap h y ; ch annel types (b ed ro ck c h a n n e ls and allu v ial
c h a n n e ls ) ; ch ann el patterns (straight ch annel, m e a n d e r in g c h a n n e l,
b raid e d ch an n e l, a n a s to m o s in g channel and a n a b ra n c h in g ch a n n e l).
CHAPTER 22
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FLUVIAL GEOM ORPHOLOGY
413-434
Erosional work of rivers; types of fluvial erosion ; base-level o f erosion ;
erosional landforms (river valieys-gorges and canyons, waterfalls, pot
holes, structural benches, river terraces, river meanders, ox-bow lakes,
and peneplains); transportational work of stream s; depositional works
o f streams ; depositional landforms (alluvial fans and cones, natural
levees, delta).
:
KARST GEOMORPHOLOGY
435-446
Groundwater: meaning and concept; geomorphic work o f groundwater ;
erosional work ; depositional work T lim estone (karst) topography ;
distribution o f karst areas ; erosional landforms (lapies, solution holes,
polje, sinking creek, blind valley, karst valley, caves or cavern s); karst
cycle o f erosion.
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CHAPTER 23
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CHAPTER 24
447-46*
COASTAL GEOMORPHOLOGY
A gents o f coastal erosion ; sea coast and sea shore ; processes and
m echanism o f marine erosion ; erosional landform s (cliffs, w ave-cut
platform, natural chimneys, stack, blow h o l e ) ; transportational w o rk ,
depositional landforms (beaches, bars, barriers and associated f e a t u r e s ) ,
classification o f coasts, and shorelines ; developm ent o f shorelines and
marine cycle o f erosion along a shoreline o f su bm ergence and e m e r­
gence.
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CHAPTER 25
ARID AND SEMIARID GEOMORPHOLOGY
463-477
Aeolian environm ents ; erosional works o f wind ; erosional la ndform s ;
transportational works o f w i n d ; depositional w ork o f w i n d ; depositional
landform s ( b e d f o rm s ) ; fluvial desert landform s (badland, playas, p e d i­
ments, b a j a d a s ) ; arid cycle o f erosion ; savanna cycle o f erosion.
CHAPTER 26
:
478-491
GLACIAL GEOMORPHOLOGY
Ice and related pheno m en a ; types o f glaciers ; m o v e m en t o f glaciers ;
ero s io n a l w o rk o f g laciers ; erosional and residual la n d fo r m s ;
tran sportational and depositional w orks o f glaciers ; dep o sitio n al
landform s ; glacio-fluvial deposits and landform s ; glacial geo m o rp h ic
cycle ; ice ages and pleistocene glaciation.
CHAPTER 27
492-505
PERIGLACIAL GEOMORPHOLOGY
M ean in g and concept ; periglacial clim ate ; periglacial areas ; p e r m a ­
frost ; active l a y e r ; m echanism o f periglacial processes (congelifraction,
frost heaving, congelifluction, nivation, fluvial process, and aeolian
pro cess) ; genetic classification o f periglacial land fo rm s ; periglacial
cycle o f erosion.
:
CHAPTER 28
REGIONAL GEOMORPHOLOGY
506-553
K u m a u n H im a lay a region ; G an g a plain ; S. E. C h o ta n a g p u r re g io n ;
R an ch i p l a t e a u ; P alam au u p la n d s ; B elan b a s i n ; B h a n d e r p l a t e a u ; G irn a r
hill region ; w est coastal plains.
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APPLIED GEOMORPHOLOGY
554-563
M e a n in g and co n ce p t ; applied g e o m o rp h o lo g y in I n d ia n c o n te x t ;
g e o m o rp h o lo g y and regional pla n n in g ; g e o m o rp h o lo g y and h a z a rd
m a n a g e m e n t ; g e o m o rp h o lo g y and u rb an iz atio n ; g e o m o rp h o lo g y an d
e n g in e e rin g w orks ; g eo m o rp h o lo g y an d h y d ro lo g y ; g e o m o rp h o lo g y
an d m in eral exploration.
/
CHAPTER 30
:
564-589
Meaning and concept; historical perspective; man's impacts on environ­
mental processes; man and hydrological p rocesses; man and weathering
and massmovement processes; man and coastal p rocesses; man and river
p rocess; man and periglacial processes ; man and subsurface processes ;
man and pedological processes ; man-induced soil erosion ; man and
sedimentation.
:
CLIMATE CHANGE AND QUATERNARY GEOMORPHOLOGY
590-629
Indicators o f climatic changes; causes and theories o f climatic changes;
quaternary climatic changes and landforms.
REFERENCES
631-639
INDEX
641-652
(iv )
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CHAPTER 31
ANTHROPOGENIC GEOMORPHOLOGY
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NATURE OF GEOMORPHOLOGY
D e f i n i t i o n a n d s c o p e o f g e o m o r p h o l o g y ; e v o lu tio n o f
g e o m o rp h o lo g ic a l thoughts; Indian contributions to geomorphology ;
s y s t e m c o n c e p t ; g eom orphic models ; methods and approaches to
th e s tu d y o f landform s.
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CHAPTER 1
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1
NATURE OF GEOMORPHOLOGY
forms (morphe) of the earth’s surface. T o be m o re
precise, forms mean topographic features o r g e o ­
metric features (relief features) o f the earth s sur­
face. P.G. Worcester (1940) prefered to d efin e
geomorphology as the intepretative description o f
the relief features of the earth's surface while W .D .
Thornbury (1954) pleaded for the inclusion o f su b ­
marine forms in addition to surface reliefs in the
realm of geomorphology.
The rapidly evolving discipline of geomorphology has undergone seachange in methodol­
ogy and approaches to the study of landforms and
related processes since 1945 when R.E. Horton
introduced quantitative methods for the analysis of
morphometric characteristics of fluvially originated
drainage basins. A clear-cut cleavage surfaced in the
discipline in the form of evolutionary approach
involving progressive changes in landforms through
long time periods and process-response approach
involving equilibrium model and steady state of
landform development after 1950. Thus, the need of
the hour is to integrate the cyclic concept involving
long-term historical evolution of landlorms and noncyclic concept involving dynamic equilibrium, func­
tional and process-reponse models on the one hand
and m icro-geom orphology involving smaller spa­
tial and temporal scales and mega-geomorphology
involving larger spatial and longer temporal scales
Geomorphology may be defined as the scien ­
tific study of surface features o f the earth's surface
involving interpretative description o f landform s,
their origin and development and nature and m e c h a ­
nism of geomorphological processes w hich evolve
the landforms with a view that ‘all landform s can be
related to a particular geologic process, or set o f
processes, and that the landforms thus developed
may evolve with time through a sequence o f form s
dependent in part, on the relative tim e a particular
process has been operating’ (Easterrook, 1969).
A.L. Bloom (1979) also defined g eom orphology as
the systematic description and analysis o f land­
scapes and the processes that change them.
on the other hand.
1.1 DEFINITION O F GEOM ORPHOLOGY
Geomorphology is significant branch of physi­
cal geography (geomorphology, oceanography, cli­
m atology and b io g e o g r a p h y ). The term
geomorphology stems from three Greek words i.e.
‘ge’ (rtieaning earth), ‘m orphe’ (form) and logos
(a discourse). Geomorphology, therefore, is defined
as the science of description (discourse) of various
1.2 SCOPE OF GEOMORPHOLOGY
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The subject matter o f geom orphology m ay be
organized on the bases o f (i) dim ension and scale o f
relief features (landforms), (ij) processes that shape
the landforms, and (iii) the app ro ac h es to the
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2
g e o m o r ph o l o g y
(2) RELIEF FEATURES OF THE SECOND ORDER
geomorphic studies. In fact, geomorphology, being
a study of landforms, has a well defined framework
of its subject matter. The systematic study of landforms
requires some fundamental knowledge of geology
as the genesis and development of all types of
lan dform s p rim arily d ep en d on the m aterials
(geomaterials or structure) of the earth's crust and
partly on the forces coming from within the earth
(endogenetic forces).Based on this connotation
geomorphology is, some times, equated with geol­
ogy (W.D. Thornbury, 1954) and sometimes is con­
sidered a branch of geology (A.K. Lobeck, 1939). In
fact, geomorphology has originated from geology
and in most of the American Universities it is still
housed in geology departments. Thus, some aspects
o f geology, even today, are included in the descrip­
tion and analysis of landforms e.g. structural and
dynamic geology. Theoretical geology helps in un­
derstanding the nature of landforms and, therefore,
the origin of different types of reliefs like mountains,
plateaus, continents and ocean basins on which the
microlandforms are evolved must be properly un­
derstood. Endogenetic forces particularly diastrophic
and sudden (vulcanicity and seismic events) should
be taken note of as they introduce irregularities on
the earth's surface which generate variety in landforms.
The structural forms developed over a con '
nent or part thereof as mountains, plateaus, lakes
faults, rift valleys etc. constitute the category 0f
relief features o f the second order. These forms owe
their genesis mainly to endogenetic forces particu­
larly diastrophic forces. The nature, mode and rate of
operation of these endogenetic forces must be stud­
ied properly so that general characteristics, nature
and mode of origin of the second order relief fea­
tures, upon which the third order reliefs are pro­
duced, are well understood. These are called as
constructional landforms.
(3) RELIEF FEATURES OF THIRD ORDER
Micro-level landforms developed on second
order relief features by exogenetic denudational
processes originating from the atmosphere are in­
cluded in this category. These landforms may be
erosional (e.g. glacial valley, river valley, karst
valley, cirques, canyons, gorges, terraces, yardangs,
sea cliffs etc.), depositional (e.g. drumlins, eskers,
flood plains, natural levees, delta, sea beaches, sand
dunes, stalactites, stalagmites etc.), residual (e.g.
monadnocks, inselbergs or bornhardts etc.) and some
times minortectonic features (by endogenetic forces).
In fact, the relief features of the third order are given
more importance in geomorphic studies as they
constitute the core of the subject m atter of
geomorphology. Besides, the nature, mode and rate
of operation of denudational processes, which pro­
duce the relief features of the third order, are also
studied at varying spatial and temporal scales. Be­
sides natural g e o m o rp h o lo g ic a l p ro c e s s e s ,
anthropogenic processes are also attached due im­
portance in geomorphic investigation because the
role of man as ‘economic and technological m an’
through his economic activities has augmented the
rate of natural processes beyond imagination (chap­
ter 30).
Thus, on the basis of dimension and scale, the
relief features of the earth's surface, the core subject
matter of geomorphic study, may be grouped in three
broad categories of descending order.
(1) RELIEF FEATURES OF THE FIRST ORDER
‘On the smallest scale and covering the larg­
est area is world geom orphology’ (C.A.M. King,
1966) which includes consideration of continents
and ocean basins. The consideration and interpreta­
tion of worldwide erosion surfaces requires the de­
scription and analysis of the characteristics and
evolution o f continents and ocean basins. Thus,
continents and ocean basins become the relief fea­
tures of the first order. The consideration of conti­
nental drift, in one way or the other, caused either by
the forces coming from within the earth (thermal
convective currents) involving plate tectonics or
from outer sources (tidal forces, gravitational forces
etc.), becomes desirable for the analysis of major
morphological features of the earth's surface. Plate
tectonics help in understanding the origin of conti­
nents and ocean basins.
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The subject matter of geomorphology may
also be organized on the basis of geomorphic proc­
esses (both endogenous and exogenous) that shape
the landforms and approaches to the study of
landforms. Davisian dictum that ‘landscape is a
function of structure, process and tim e’ and K.J.
Gregory's geomorphic equation (F=f (PM)dt, where
F = landforms, f = function of, P = processes, M =
geomaterials, dt = mathematical way of denoting
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n ature o f g e o m o r p h o l o g y
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X
change over time) clearly reveal that any geomorphic
study r e q u i r e s
care fu l
in v e stig a tio n
of
geomorphological pro cesses (m ainly denudational
processes), geom aterials (lithology, disposition of
rock beds and co m p o sitio n o f rocks, collectively
known as structure) and tim e factor, though the
advocates o f dynam ic equilibrium theory have pleaded
for exclusion o f tim e factor on the basic premise that
the landform s are tim e-independent. G eomorphic
studies incorporate tw o m ajor approaches viz. his­
torical studies in v o lv in g historical evolution of
landforms and functional studies involving timeindependent series o f landform evolution reflecting
association b etw een landform characteristics and
existing en viron m ental conditions. Both the ap­
proaches have their relevan ce in geomorphological
investigations.
(1) ANCIENT PERIOD
T h ough ‘geom orphology has d ev elo p ed from
the w ork o f late eighteenth and nineteen th century
geologists and hyd rolo gists’ (C .A .M . King, 1966)
but som e ideas regarding landform s w ere indirectly
postulated even in the ancient period w h en ph iloso ­
phers and historians o f G reece, R om e, E g y p t etc.,
the principal seats o f ancient culture an d civilization,
took the initiative in this precarious field. H ero d o tu s
(485 B.C.— 425 B.C), a noted G re e k historian,
made significant contribution in the field o f r iv e r s
alluvial behaviour during his ex tensiv e jo u rn e y o f
Egypt. After having a close observation o f depositional
work of the Nile he postulated that ‘E g y p t w a s th e
g ift o f N ile ’. He fu rth er re la te d th e s h a p e o f
depositional feature at the m outh o f the r iv e r to
Greek letter A and nam ed this feature as d e lta . H e
also postulated that ‘there is gradual g ro w th o f d elta
towards the sea. On the basis o f the p re s e n c e o f
marine fossils in the alluvium o f the N ile far inland
he opined that ‘the level o f sea is not p e r m a n e n t b u t
there is occasional rise and fall w hen sea a d v a n c e s
landw ard ( tr a n s g re s s io n a l p h a s e ) a n d r e tr e a ts
(regressional phase)’ Thus, we can infer the co n c e p t
of transgressional and reg ressio n a l p h a se s o f the
sea from the statements o f H erodotus.
1.3 EVOLUTION OF GEOMORPHOLOGICAL
THOUGHTS
T he present status o f geom orphology is the
result o f gradual but successive development of
geomorphic thoughts postulated in different periods
by i n n u m e r a b l e p h i l o s o p h e r s , e x p e r t s and
geoscientists in the subject and out side the subject.
Thus, the developm ental phases of geomorphology
indicate its dynam ic nature. After taking its birth in
the philosophical ideas o f the ancient Romans and
Greeks the su b ject has b lo sso m ed through the
geom orphological m ethodological nutrients of the
18th and 19th century and reached its golden status
in the 1st and 2nd decades o f the 20th century with
the postulation and w ider acceptance of cyclic con­
cept o f landscape d ev elo p m en t and denudation chro­
nology world over. After 1950, the science o f geomor­
phology w itnessed a m ajo r change in the m ethodo­
logical aspect in the form o f rejection o f Davisian
model o f cyclic dev elo p m en t o f landforms, intro­
duction o f quantitative m ethods in geomorphological
studies, postulation o f dynam ic equilibrium theory
of landscape d ev elo p m en t based on the concept o f
time-independent series o f landform evolution, more
emphasis on process geom orphology (process re­
sponse m o d e l) , e m e r g e n c e o f e n v iro n m e n ta l
geomorphology, shift from mega-geomorphology
to micro-geomorphology, from longer temporal scale
to shorter tem poral scale, and more attention to­
A ristotle (384 B.C.— 322 B.C.), a rep u te d
Greek philosopher, presented som e very interesting
ideas regarding w ater spring, origin o f stream s an d
behaviour of seas and oceans. A ccording to h im
spring-fed streams are seasonal and ephem eral (n o n ­
permanent). Limestones cannot m aintain p erm a n e n t
surface drainage as m ost o f the stream s d isap p ea r
and form subterranean drainage.' A cco rd in g to him
water springs get supply o f w ater through (i) ra in w a ­
ter, which reaches underground through percolation
and seepage, (ii) condensation o f underg ro und satu­
rated air, and (iii) w ater vapour. He w as also aware
o f changing nature o f sea-level and deposition o f
eroded materials by the rivers in the form o f allu­
vium. Strabbo (54 B.C— 25 A.D.). a noted h isto ­
rian, made significant contributions in the field o f
depositional work o f the rivers. A ccording to him thc^
size and shape o f delta depend on the nature o f
terrain through which the river makes its course. Am
extensive region having com paratively weaker rocks
gives birth to larger delta as weak rocks through
erosion yield more sedim ents to maintain large delta
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wards applied aspect o f the subject.
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4
GEOMORPHOLOGY
in the 18th century A.D. through his well knit con­
cept o f uniformitarianism but the postulation of
coherent scientific thoughts in the field of geomorphologv already began in the 15th, 16th and 17th centu­
ries when the preexisting concept o f everlasting
(permanent) landforms was rejected and theirchanging
nature through weathering and erosion was very
much realized. L eonardo da V inci (1452— 1519
A.D.) was o f the opinion that the rivers formed their
valleys themselves through vertical erosion and de­
posited the eroded materials elsewhere. Buffon
(1707— 1788 A.D.) rejected the catastrophists’ pos­
tulation of very little age o f the earth (thousands of
years). He further opined that the rivers were the
most powerful agent of erosion and they were capa­
ble of eroding the uplifted high land mass to sealevel. T argioni Tozetti (1712— 1784 A.D.), an Ital­
ian thinker postulated that the irregular courses (sym­
metry and asymmetry of the valleys) o f the rivers
depended on the nature of rocks through which they
flow. The regions of massive and resistant rocks
maintain deep and narrow courses (valleys) whereas
broad and meandering courses are developed in the
regions of soft and less resistant rocks. Thus, this
concept gives the glimpse of differential erosion.
According to G u e th a r d (1715— 1786 A.D.) not all
the eroded sediments are deposited by the rivers in
the seas rather some parts are also deposited in the
courses of the rivers as flood plains. He also at­
tached importance to the erosive power o f the m a­
rine processes. Dim arest (1725— 1815 A.D.) was of
the opinion that ‘the valleys through which rivers
flow have been formed by themselves through the
process of valley deepening’. He was probably the
first to postulate the concept of development o f
landforms through successive stages.
while the region of resistant rocks maintains smaller
delta because resistant rocks are less eroded and
hence produce less sediments. Thus, we may infer an
indirect glimpse of the concept o f differential ero­
sion from the statements o f Strabbo. Seneca main­
tained that ‘the rivers deepen their valleys through
abrasion.’
It may be mentioned that some incoherent
ideas were forwarded by ancient philosophers and
historians but they could not collectively come to
any definite conclusion.
(2) DARK AGE
V iith the fall of Roman empire a lull prevailed
in the development of geographical as well as
geomorphological thoughts for a very long period of
1400 years (from 1st century A.D. to 14th century
A.D.). Besides, some glimpses of geomorphological
ideas put forth by few thinkers e.g. Aviecena (980—
1037 A.D.), an .Arabian thinker, broke the academic
monotony. According to him mountains should be
divided into two categories i.e. (i) mountains origi­
nated due to upliftment and (ii) mountains origi­
nated due to erosion by running water.
(3) AGE OF CATASTROPHISM
The long continued academic silence of 1400
years was suddenly broken by the emergence of
catastrophists who believed in the quick and sudden
origin and evolution of all animate and inanimate
objects in very short period of time and thus new
pages of peculiar and fantastic concepts were added
to the treasure of geomorphological and geographi­
cal literature. The age of the earth was calculated to
be a few thousand years. Only those events could be
given cognizance which occurred in the life-time of
the people. It may be pointed out that sudden
endogenetic forces like volcanic eruption and earth­
quakes may be held responsible for convincing the
thinkers to postulate such fantastic and unreaslistic
ideas not only related to the landforms but to all of
the animate and inanimate objects. The concept of
sudden change and evolution also swept the biolo­
gists who believed in sudden evolution and destruc­
tion of all the living organisms.
The 18th century appeared with a new wave
of uniform itarianism on the academic stage of
geomorphology, with Jam es H utton as its postulator. His concept of uniformitarianism is based on the
basic tenet that the same geological processes which
operate today operated in the past and therefore the
history of geological events repeats in cyclic pattern.
His concept of ‘present is key to the p ast9 aimed at
the reconstruction of past earth-history on the basis
of the present. According to him the nature is sys­
tematic, coherent and reasonable and thus destruc­
tion ultimately leading to construction indicates
(4) AGE OF UNJFORMfTARtANISM
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The concept of catastrophism was finally
rejected and gradual cyclic nature o f earth's history
was postulated by Jam es Hutton (1726— 1797 A.D.)
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NATURE o f
5
g eo m o r ph o lo g y
orderliness o f nature. He w as the first geologist to
observe cyclic natu re o f the ea rth ’s history. His
glacial erosion, marine erosion, fluvial processes
and erosion, arid and karst landscapes.
work was published in the form o f a research paper
‘theory of the earth : or an investigation of laws
observable in the com position, dissolution and res­
toration of land upon the g lo b e’ in the Transactions
of the Royal Society o f Edinburgh in 1788. Later on,
his major work was published in the form o f a book
entitled, ‘Theory o f the Earth with Proofs and Illus­
trations' in two volum es in 1795. His concept, ‘that
topography is carved o u t and not built-up’ is a
significant contribution in geomorphology. John
Play-fair (1748— 1819), a professor of mathemat­
ics and a close friend o f Hutton, after making some
suitable modifications in the Huttonian concept and
adding some valuable contributions of his own elu­
cidated the Hutton's views on uniformitarianism
through his book entitled ‘Illustrations of Huttonian
Theory of the Earth' in 1802. Playfair also visual­
ized the erosive and transporting powers of fluvial
and glacial processes. On the origin of valleys Playfair
was also far in advance o f the views current at his
time’ (C.A.M. King, 1966). C harles Lyell (1797—
1873 A.D.), one o f the most active followers of
James Huttcn, laid the foundation of modern histori­
cal geology and he defined geology ‘as that science
which investigates the successive changes that have
taken place in the organic and inorganic kingdoms of
nature.’ Most o f his works appeared in his two
books ; ‘Principles o f G eo lo gy’(in two volumes)
and T h e Geological Evidences of the Antiquity of
Man’ in 1863. C.G. G reenw ood came to light through
his paper entitled ‘rain and rivers : or Hutton and
Playfair against Lyell and all com ers’ in 1857 and
was accepted as the father of modern subaerialism.
‘He put forward the idea o f the base-level o f erosion
before Powell in A m eric a’ (C.A.M. King, 1966).
Sir Charls Lyell ( 1797— 1873 A .D .) not only
endorsed the concept o f uniform itarianism put forth
by James Hutton but also popularised the concept
through his books, ‘Principles o f G eology (two
volumes). His significant contributions in biology
became the base of ‘p r ig in o f S p e c ie s’ o f Charles
Darwin. His book entitled, ‘T he G eological E v i­
dences o f the Antiquity o f M a n ’ (published in 1863)
accommodated most o f the concepts o f H utton.
Credit goes to E u ro p ea n school o f g e o ­
morphology for identification and recognition o f ice
ages. The geoscientists collected sufficient and c o n ­
vincing evidences in support o f total glaciation o f
northern Europe during Pleistocene period. L ou is
Agassiz (1807— 1873 A.D.) is given credit for an
early start in this precarious field. T h ou gh J ea n d e
C h a r p e n tie r postulated his concept o f continental
glacier and ice ages in 1841 but A gassiz is given
credit for the recognition and identification o f the
presence of ice age during Pleistocene period as he
presented his ideas in 1840. They opined that m o st
parts of northern Europe were covered w ith thick
sheets of continental glaciers during Pleistocene
period. It may be mentioned that the process o f study
of glaciation was started m uch earlier by J oh n
Playfair in 1815; V enetz o f Sw itzerland in 1821 and
1829, Norweigian scholar E sm ark in 1824, G erm an
scientist Bernhardi in 1832, Jean de C harpentier o f
Switzerland in 1834 than Louis Agassiz. T h e S co t­
tish geologist Jam es G eikie studied different as­
pects of ice age and published his ideas through his
book entitled. ‘The Great Ice A g e' in 1894. A ccord­
ing to him an ice age involving longer geological
period of time is comprised of distinct several glacial
periods which are separated by w arm interglacial
periods. A Penck and B ru ck n er after their observa­
tions of Pleistocene glaciation o ver the A lps identi­
fied four glacial periods during Pleistocene ice age
e.g. Gunz, Mindel, Riss and W u rm w hich were
separated by three warm interglacial periods.
(5) MODERN AGE (NINETEENTH CENTURY)
Geomorphology became an independent disci­
pline and a major branch o f geology at the beginning
of the 19th century w hen the developm ent of
geomorphic thoughts took place at regional level
and two distinct schools o f geomorphic thoughts can
well be identified e.g. (i) European School and (ii)
American School.
(A) E u rop ea n S ch o o l— S ignificant c o n ­
tributions were made in the fields o f recognition and
identification o f Pleistocene Ice Age and glaciation,
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L ■
In the field of m arine erosion , corrasion by
sea waves was given more attention and importance.
Sir A ndrew R am say (1814— 1891) presented de­
tailed description o f marine platforms made by ma­
rine erosion in W ales and S.W. England. It may be
mentioned that previously Ramsay attached more
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GEOMORPHOLOGY
ft
them into antecedent, superim posed, consequent
vU||cys etc. His most significant contribution is the
postulation o f lim it o f m a x im u m vertical erosion
(valley deepening or dow ncutting) by streams to
which he proposed the term o f base level, which is
determined by sea-level. Later on, C.A. mallot
(1928) inferred three types o f base level from his
writings viz. ultimate, local and temporary base
levels. He also opined that if the fluvial processes
(streams) were allow ed to e rode the landmass unin­
terruptedly for fairly a long period o f geological time
the high landmass m ight be ero d ed d ow n to a level
plain which may be slightly abo ve the sea level. This
erosional level plain was later term ed by Davis as
peneplain. He also observ ed the nature o f narrow­
ing and shifting o f w ater divides th ro u g h the process
significance to murine abrasion hut in Inter part ol
his life he gave more importance to Hu vial erosion.
Baron Ferdinand N on Richthofen (1833— 1905)
made significant contributions in the field ol marine
erosion during his visit to China. He ‘produced his
work on the genetic treatment o f landforms, in which
he supported a marine origin for plains found be­
neath marine transgressions, these being produced
when sea-love I is rising slowly' ( C A M . king. 1966).
C. G , G re e n w o o d , a British geologist, made
significant contribution in the field of subacrial
erosion. He is considered to be the first geoscientist
to postulate the concept o f base level o f erosion even
before M ajor Powell in the U.S.A. J u k e s (1862)
divided rivers into two categories e.g. (i) transverse
streams which flow across the geological structures
and (ii) longitudinal streams which follow the direc­
tion o f strikes o f rock beds or (low parallel to the
geological structures. According to him longitudi­
nal streams are subsequent to transverse streams i.e.
transverse streams originate prior to longitudinal
streams. Jukes also described various aspects of
river capture.
o f lateral erosion.
G . K. G ilb e r t (1 843 — 1918 A .D .) is consid­
ered as the first real g eo m o rp h o lo g is t o f A m erica
because ol his significant c o n trib u tio n s in system ­
atic and quantitative g e o m o rp h o lo g y . In fact, he was
much ahead o f his lime and p o s tu la ted such concepts
which still hold today. ‘He stressed the im portance
o f creative im agination, o f testin g a n u m b e r o f
h y p o th e se s , an d o f a n a l o g i e s in th e fie ld o f
geom orphology’ (C .A .M . King, 1966). G ilbert never
preferred to be called as the o re tic ian rather he took
him self as an investigator. A fte r a th o ro u g h study o f
different localities o f A m e r ic a (e.g. G reat Basin,
Bonnevile Lake, artesian w ells o f G reat Plains, Henry
M ountains, Siera M o u n ta in s etc.) he propo und ed a
num ber ot laws i.e. law o f u n ifo rm slope, law of
structure, law of divides, law o f in c re a sin g acclivity,
law ot tendency to e q u ality o f actio n s, dynamic
equilibrium, law o f the in te rd e p e n d e n c e etc. He was
the first geoscientist to p r o p o u n d the concept of
graded profile ot a riv er and to e s ta b lis h relationship
am ong load, v olum e, velo city a n d ch a n n e l gradient
on the basis o f q u a n tita tiv e a n a ly s e s o f these vari­
ables. His co n trib u tio n s h a v e b e e n elaborated in
m uch detail in the 3rd Chapter o f th is book.
(R) A m e ric a n S chool— American school is
credited for making m axim um contributions in the
field o f geom orphology. In fact, the last two decades
o f l^th century and first two decades o f 20th century
(i.e. from 1S75 to 1920) are considered as ‘golden
a g e’ not only o f American geomorphology but also
o f world geom orphology because it was this period
when for the first time general theory o f landscape
developm ent was propounded by VV.M. Davis and
the landform analysis attained its final shape. The
concept o f sequential changes o f landforms through
successive developmental phases in terms o f time
hased on the basic tenet o f time-dependent concept
o f Divisian model o f geographical cycle o f erosion
became the core o f landform analysis and guide-line
for geom orphologists and geologists not only in
North America but world over. Pow ell, G ilbert,
D utton and Davis made significant contributions in
the field o f subaerial denudation.
C. F. Dutton (1843— 1912 A.D.) was the
first gcoscientist to use the term isostasy to denote
equilibrium condition of upstanding and downstanding
landmasses of the earth’s surface. During his study
and investigations of Colorado Plateau and Grand
Canyon of the Colorado river he opined that the
present canyon was the result o f long continued
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M ajor J.W . Pow ell (1834— 1902 A.D.), a
major in American army after a thorough study o f
Colorado plateau and Uinta mountains (1876) sug­
gested geological structure as a basis for the classi­
fication o f landforms. He attempted a genetic classi­
fication o f river valleys and consequently classified
J
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7
NATURE OF GEOMORMOl.OOY
period of fluvial erosion to winch he assigned ihe
term of the period of great denudation. He ulso
presented evidences in support of Powell's concept
of base level of erosion.
W. Penck in Germany. His classical model o f geo­
graphical cycle propounded in 1899 and defined by
him ‘as a period of time during which an uplifted
landmass undergoes its transformation by the proc­
esses of land sculpture ending into a low featureless
plain (peneplain)’ dominated the geomorphological
investigations all over the world throughout 1st half
of the 20th century inspite o f its stiff opposition by
W. Penck and others in Germany. His model of
geographical cycle was variously termed, popular­
ised and applied by his followers world over e.g.
no rm al cycle, erosion cycle, g e o m o rp h ic cycle,
hum id cycle etc. It may be mentioned that his
‘geographical cycle’ does not represent his general
theory of landscape development as his general
theory states ‘that th e re is se q u e n tia l c h a n g e in
la n d fo rm s th ro u g h successive stag es an d the
changes a re directed to w a rd s a d efin ite end i.e.
attain m en t of featureless p la in (p e n e p la in ) ’. The
main goal of his theory was to present systematic
description and a gcnetic classification of landforms.
Davis also identified 3 basic factors which control
the evolution of landforms viz. ‘landscape is a
function of s tru c tu re , p rocess and tim e’, which
are termed as ‘trio o f D avis’. His concept o f geo­
graphical cycle was later on applied with all other
(other than fluvial) processes by Davis and his fol­
lowers e.g. arid cycle of erosion (Davis, 1903, 1905
and 1930), glacial cycle of erosion (Davis, 1900 and
1906), marine cycle of erosion (Davis, 1912, D.W.
Johnson, 1919), karst cycle of erosion (Beede, 1911,
Cvijic, 1918), periglacial cycle of erosion (L.C.
Peltier, 1950). His model was modified and pre­
sented in revised forms by a few geomorphologists
after 1950. Davis concept of historical evolution of
landscape became the pivot for the classical concept
of d en u d atio n chronology and erosion (planation)
surfaces in U.K. D avis’ major contributions (re­
search articles, papers and addresses) were pub­
lished in a book form entitled ‘G eograp hical E s ­
says’ in 1909. He is considered as a great definer,
analyser, interpreter, systematiser and synthesiser.
Only two quotes from S.W. W ooldridge and S.
Judson that ‘Davis towers above his predecessors
and successors, like a monadnock above one o f his
own peneplains’ (S.W. Wooldridge), and ‘his grasp
of time, space and change, his com mand o f detail,
and his ability to order his information and frame his
W.M. Davis (1850— 1934) was a professor
of physical geography at Harward University. He is
considered to he the patron o f the science of
geom orphology because o f his significant contribu­
tions in different fields of geomorphology and for
giving new direction to landform study. He covered
almost every nook and corner of geomorphology.
He is given credit to systematize and integrate hith­
erto seaitercd ideas of American geomorphologists
to present them in coherent and well defined frame­
work. His contributions were so significant and lie
w as so d o m in a n t am o n g the A m erican
geom orphologists that the American school of
geomorphology was recognized as Davisian school
o f geom orphology. Davis is credited for the postu­
lation of first general theory of landscape develop­
ment which, is in fact, a synthesis of his three major
concepts viz.. com plete cycle of river life (1889),
geographical cycle (1899) and slope evolution. He
emphasized progressive developm ent of erosional
stream valleys through the concept of complete
cycle of river life while sequential changes of land­
scapes through time involving historical evolution
of landforms (time-dependent series of landforms)
or cyclic developm ent of landform s were high­
lighted through the concept o f geographical cycle.
‘The reference system of Davisian model/theory of
landscape development is that the landforms change
in an orderly manner as processes operate through
time such that under uniform external environmen­
tal conditions an orderly sequence of landforms
develops’ (Robert C. Palmquist).
Since Gilbert and Davis also stepped in the
20th century and hence their further contributions to
the geomorphological thought are considered in the
succeeding heading. Further, the contributions of
Davis will be elaborated in detail in the 3rd chapter
of this book.
(6) MODERN AGE (20TH CENTURY : FIRST HALF)
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The beginning o f the 20th century was her­
ald e d by m e th o d o l o g i c a l
r e v o lu t io n in
geomorphological studies brought in by W.M. Davis
and his followers at home (UiS.A.) and abroad and
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GEOMORPHOLOGY
arguments remind us again that wc are in the pres­
ence o f a giant’ (Sheldon Judson. 1975) are suffi­
cient enough to demonstrate the greatness of Davis
in the e n r ic h m e n t and a d v a n c e m e n t of
geomorphological knowledge. C.G. Higgins' (1975)
remark that ‘Davis’ rhetorical style is justly admired
and several generations of readers became slightly
bemused by long though mild intoxication on the
limpid prose of Davis’ remarkable essays’ speaks of
the academic calibre o f W.M. Davis.
The American school of geomorphology was
further entriched by significant contributions of a
host of geomorphologists e.g. D.W. Johnson (ma­
rine process and coastal geomorphology), C. A. Malott
(fluvial processes and erosion), H.A. Mayerhoff and
E.W. Olmsted (evolution of Applachian drainage),
R.P. Sharp. C.P.S. Sharp. A.K. Lobeck, W.D.
Thornbury etc.
During the 1st half of the 20th century Euro­
pean school of geomorpholgy made significant con­
tributions in the advancement of geomorphological
thoughts. British geomorphologists made their inde­
pendent identity and there emerged an entirely dif­
ferent school o f geom orphology which laid empha­
sis on the chronological study of landscape develop­
ment in historical perspective better known as d en u ­
d ation ch ro n o lo g y based on the co n ce p t of
p alim p sest S.W. Wooldridge (his famous book
being the Physical Basis of Geography : An Outline
of Geomorphology, published in 1937), J.A. Steers
(The Unstable Earth, published in 1832) etc. made
significant contributions in different branches of
geomorphology.
A new branch o f geom orphology in the form
of climatic geom orphology was developed in France
and Germany on the basic tenet that ‘each climatic
type produces its own characteristic assemblage
o f landform s’. Sauer (1925), Wentworth 1928),
Saper (1935), Friese (1935) etc. paved the way for
the p o s tu la tio n o f the c o n c e p t o f clim atic
geomorphology and m orphogenetic or morpho
climatic regions by Budel (1944, 1948) and L.C.
Peltier (1950) in Germany. This concept of climatic
geomorphology was further advanced and estab­
lished by Tricart and C ailleux in France in the 2nd
half ot the 20th century.
The statistical techniques were first intro­
duced by Krumbein in geology in 1930s and the
work ol American engineer R.E. Horton (1932 and
1945) brought quantitative revolution in the field of
geomorphology when he presented quantitative analy­
sis ot morphometric characteristics o f fluvially origi*
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The Davisian model of geographical , cycle
met with strong criticism and his concept of rapid
and erosionless upliftment became the crux of
criticisms by the opponents of cyclic concept of the
evolution o f landforms particularly by the German
geoscientists. The German critics of Davisian model
of cycle o f erosion fall in two categories viz. the first
category of opponents pleaded for outright rejection
of cyclic concept while the second category of critics
suggested modifications and presented entirely new
model. According to Penck landform development
is not time-dependent as envisaged by Davis rather
it is time-independent. W. Penck, through his ‘M or­
phological Analysis’ and ‘Morphological System ’
tried to reconstruct and interpret past events of
crustal movements on the basis o f exogenetic proc­
esses and morphological characteristics. The refer­
ence system of Penck's model o f landscape develop,
mcnt is that the characteristics of landforms of a
given region are related to the tectonic activity of
that region. The landlorms, thus, reflect the ratio
between the intensity of endogenetic processes (i.e.
rate of upliftment) and the magnitude of displace­
ment o f materials by exogenetic processes (the rate
o f erosion and removal o f materials). According to
Penck landforms development should be interpreted
by means of ratios between diastrophic processes
(endogenetic or rate of upliftment) and erosional
processes (cxogen^tic, or rate of vertical incision).
‘Penck is supposed to have deliberately avoided the
use of stage concept in his model of landscape
development either to undermine the cyclic concept
of W.M. Davis or to present a new m o del’ (Savindra
Singh, 1995). In the place of D avis’ stage he used the
term entw ickelung meaning thereby development.
In the place of youth, mature and old stages he used
the terms aufsteigende entw ickelung (waxing or
accelerated rate of development), gleichformige
entwickelung (uniform rate of development) and
absteigende entw ickelung (waning or decelerating
rate of development). Detailed account of Penck's
contributions will be presented in the third chapter of
this book.
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9
NATURE OF GEOMORPHOLOGY
nated drainage basins. The criticism of Davisian
model o f landscape development and descriptive
geomorphology gained currency after 1940 and si­
ren was raised for the rejection and replacement of
time-dependent evolutionary concept of landscape
development. It may be mentioned that at a time
(1950) when majority of the geomorphologists world
over became fed up with evolutionary model ot
Davis and pleaded for alternative theory of land­
scape development which may envisage time-inde­
pendent series of landforms Pelltier presented the
concept o f periglacial cycle o f erosion in 1950 in
Germany which offered support to Davisian model
of cycle o f erosion.
(7) RECENT TRENDS (SECOND HALF OF 20TH CEN­
TURY)
Post-1950 geomorphology has undergone seachange in the methods and approaches to the study
o f landforms, conceptual framework, paradigm and
thrust areas o f study. The recent trends in the field of
geomorphological studies since 1950 include in­
creasing criticism o f Davisian model of cyclic de­
velopment o f landforms, concerted efforts for the
replacement o f cyclic model by non-cyclic (dy­
namic equilibrium) model, descriptive geomorpho­
logy (qualitative treatment o f landforms) by quanti­
tative geomorphology, inductive method of landform
analysis by deductive method, introduction of m od­
els and system approach, emergence of process
geom orphology, climatic geomorphology, applied
geomorphology,environmental geomorphology, shift
from larger spatial and longer temporal scale to
sm aller spatial and shorter temporal scale etc.
The landscapes were taken as open systems which
are in steady state of balance through continuous
input of energy and matter and output o f matter.
Though Hackian model o f landscape devel­
opment envisaged landscapes as the result o f bal­
ance between the resisting force of geomaterials and
erosive force of the geomorphological processes
acting on them but he laid more em phasis on geo­
logical control as he opined that ‘differences and
characteristics of forms are explicable in term s o f
spatial relations in which geologic patterns are the
primary consideration’ (Hack, 1960). It may be
pointed out that even Hack could not escape from
evolutionary concept as he h im self adm itted ‘that
evolution is also a fact of nature and that the inher­
itance of form is always a possibility’ (H ack, 1960).
R.C. Palmquist has opined that ‘Hack (1965) para­
phrases Davis’ ideal geographical cycle in term s o f
equilibrium concept and develops a sim ilar ev olu­
tionary scheme. An initial disequilibrium stage (youth)
of rapid stream incision is followed by an eq uilib­
rium stage (mature) wherein the rounded interfluves
are lowered as potential energy decreases though
they do not change in fo rm ’ (R.C. Palm quist). It may
be mentioned that continued criticism o f cyclic m odel
of landform development and ultimately its rejec­
tion caused a conceptual vacuum which could not be
filled up even by dynamic equilibrium theory. R e ­
cently, a few alternative geom orphic theories have
been advanced e.g. ‘geom orphic th resh old m o d e l’,
‘tectonic-geom orphic m od el’ (M. M orisaw a), ‘e p i­
sodic erosion model* (S.A. S chum m ) etc.
The most outstanding contribution to the ad­
vancement of geom orphological know ledg e in this
period is the adoption of quantitative approach
based on deductive scientific m ethod to the study o f
landforms and processes at short spatial and tem p o ­
ral scales. The time factor w hich w as taken as a
process in the landscape d evelopm ent in the cyclic
model has now been accepted as a variable. The
maga and m eso-scales used for landform studies
have now been reduced to m icro-scale w herein the
m echanism o f processes can be properly understood
through field instrum entation and m easurem en t of
the mode and rate o f operation o f geom orphic pro c­
esses. Thus, ‘form g eo m o rp h o lo g y 9 has been re­
placed by ‘p rocess geom orphology*. This quantita­
tive approach resulted in the form ulation o f 4func-
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The decade 1950— 60 was devoted more for
the quantitative study o f landforms and processes
and the consideration o f geom orphic theories occu­
pied a back seat. This is the reason that a set o f basic
concepts o f ‘the landscape cycle, the epigene cycle’,
‘the pediplanation c y c le ’ and ‘hillslope cycle' pos­
tulated by L.C. King and his ‘C anons o f L andscape’
(published in 1953) could not win support. The rejec­
tion o f D avisian concept o f ‘cyclic m o d e l’ based on
‘time dependent landform e v o lu tio n ’ culminated in
the postulation o f ‘dynam ic equilibrium theory’ of
landscape dev elopm en t by J.T .H ack, R.J. Chorley
and others based on the concept o f ‘tim e-independ­
ent evolution o f la n d sca p e’, and ‘system co n ce p t’.
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10
The decade* 1950-60 and 1960-70 saw a real
take off in the geomorphological research** wherein
more attention wa* paid towards the study o f differ­
ent physiographic regions o f peninsular India Sig­
nificant contributions came from R.P. Singh, A.K.
Sen Gupta, E. Ahmad, S.C. Bo**. W.D. West and
V.D. Chaubey, R. Vaidyanadhan. B. Venketesh.
G V. Rao etc. The 21st International Geographical
Congress held in 1968 in New Delhi aroused deep
interests in Indian geographers forgeom orpholopcal
researches in various parts of the country. Signifi­
cant co ntrib ution s in the field o f system atic
geomorphology came from B.C. Acharya *floods of
Mahanadi;, G.K. Datta (origin and evolution of
la n d fo rm s in L o w e r S o n e V a iJ e y ), M .K .
Bandopadhyay (glacial landforms;. D. Suza (evolu­
tion of drainage pattern of Goay, R.N. Mathur
(geohydrology of Meerut district/. H.S. S h an na {ra­
vine erosion/, A.K. Sengupta (denudation on ^.cntrai
Ranchi plateau;, L>. Niyogi. S.K. Sarkar and S.
Mallick (geomorphic mapping;, D. Niyogi (river
terraces;, A.K. Pal (Balasan river basin;. S. Subba
Rao (landforms of Deccan traps, physical features of
Girnar hills;, A.B. Mukerjee (inland streams in
Haryana;, S. Sen (outer bank slope steepness in
meandering rivers;, E. A hm ad (gull, erosion in
India;. H.R. Betal (identification of slope categories
in Damodar valley;. S.C. Bose<recession in Himalayan
glaciers;, R.S. Dubey (erosion surface on R e*a
plateau;, M.V. K ay erk ar and S.K . Badhaw an
(geomorphic classification o f terrain; etc.
tional theory o f landscape developm ent’ which
lays more emphasis on the logical analysis of rela­
tionship between ‘forms’ and related ‘processes’
based on quantitative data derived through filed
instrumentation.
The post-1950 geomorphology was also en­
riched by the introduction of system theory for the
explanation o f landforms and processes and postula­
tion ol different geomorphic models e.g. natural
analogue system, physical system and general sys­
tem. Process-response model has became the focal
theme of process-geomorphology.
Another significant contribution is the emer­
gence of e n v iro n m e n ta l geom orphology which is,
in fact, asignificantaspectofapplied geomorphology,
which envisages application of geomorphic knowl­
edge for the removal of environmental problems
arising out of interactions - o f ‘economic’ and 'tech­
nological m an’ with geomorphological processes
and natural system. For example, monitoring of
fluvial processes in man-impacted gully basin (cul­
tivation) enables the investigator to ascertain the
mode and rate of rill and gully erosion, siltation and
loss of soi 1and to suggest remedial measures (Savindra
Singh's study, 1996).
1.4 INDIAN CONTRIBUTIONS TO GEOM ORPHO­
LO GY
.T h e geomorphological researchesstartedauile
late in India due to late start of postgraduate teaching
o f geography (i.e. Aligharh Muslim University, 19 3 1.
Calcutta University, 19 4 1, Allahabad University
and Banaras Hindu University. 1946). In the begin­
ning sporadic geomorphic information in the form of
reports, articles, essays etc. were provided by ad­
ministrators. investigators and travelers (like Swami
Pranawanand of holy Kailash) and geologists. Like
overseas development of geomorphology, inde­
pendent status to geomorphology as a separate dis­
cipline was accorded by geologists in India too. The
subject was given initial start by eminent geologists
such as Heren. Wadia, Dunn, West, S.C. Chatterjee,
Auden, Arogyaswami, Radhakrishnan and geogra­
phers like C h ib b e r (basically geologist), S.P.
Chatterjee, S.C. Bose, R.P. Singh, E. Ahmad, K.
Bagchi, R.L. Singh etc. The works o f these scientists
and their followers were primarily based on Davisian
model o f ‘cycle of erosion’ and denudation chronol­
ogy approach’. The basic data were derived from
topographical maps and gazetteers.
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Post - 21st international geography congress
period witnessed an upheaval in geomorphological
researches by Indian geoscientists in the fields of
fluvial, arid, glacial, coastal, structural and quantita­
tive geomorphology w herein morphometric tech­
niques were widely used. Still most o f the wori^
were based on information derived from topographi­
cal maps and casual field observations. In fact,
morphometric analysis of terrain characteristics ba;>ed
on topographical maps was initiated bv R.L. Singh
in 1967 when he presented an exhaustive paper on
Morphometric Analysis o f T errain ’ in the form o f
presidential address at the joint session o f geologygeography section o f Indian Science Congress held
in 1967. H isefforts culminated in the presentation o f
a few Ph. D. dissertations on ’landforms and settle­
ments in the department o f geography, Banaras
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11
n a ture o f g eo m o r ph o lo g y
profiles on some residual hills in the Jamalpur-Kiul
Hills’ by Anil Kumar (981) in the prestigious jo u r­
nal, Zeitschrift fur Geom orphologie became a sin­
gular contribution in field-based geom orphology
bsed on A. Young's method of slope profiling. The
other sign ificant c o n trib u tio n s w ere m a d e by
S.C.Mukhopodhyay (1982, Tista Basin), R.K. Rai
(1980, Sonar-Bearm a Basin), Prudhvi Raju and R.
Vaidyanadhan (1981, Sarda Basin), B.S. M arh (1986,
Ravi Basin) etc. Three im portant contributions in the
form of international publications (in International
Geomorphology edited by V. Gardiner, 1987) cam e
from H .S. S h a r m a ( c l i m a t e a n d d r a i n a g e
morphometric properties), R.K. Rai (evidences o f
rejuvenation of the Deccan foreland) and S avindra
Singh and R.S. Pandey (m orphological analysis and
development of slope profiles over B han der Scarps).
V S. Kale published a field-based significant paper
on ‘western ghats’ in Zeitschrift fur geom orphologie.
A research paper entitled ‘rill and gully erosion in
the subhumid tropical riverine environment o f Teothar
tahsil, M .P.’ by Savindra Singh and S.P. A gnihotri
published in Geografiska A nnaler (1987) is a sig ­
nificant contribution in the field and laboratorybased geomorphological study o f m a n -im p ac ted
gullied area. The fluvial geom orphology w as e n ­
riched by substantial work undertaken by A.B.
Mukerjee, S.K. Pal. V.S. Kale, S.R. Jog etc. T he
International Conference on G eo m orph olo gy and
Environment held in 1987 at A llahabad U niversity
rejuvenated geormorphological researches in India
and encouraged field m easurem ent of s p atio -tem p o ­
ral variations in landform characteristics. R iver-bed
m o rp h o lo g y , alluvial m o r p h o lo g y a n d c o a s ta l
geomorphology became the centre o f intensive study
by Poona School o f G eom orphology led by K.R.
Dikshit, V S. Kale, S.R. Jog, S.N K arlekar and their
associates. The other positive result o f the said
conference was the establishm ent o f the Indian Insti­
tute of Geom orpliologists with its headquarters at
geography departm ent, A llahabad University. The
annual conferences organized at different places o f
the country under the agies o f the aforesaid organi­
zation since 1988 have encouraged several young
researchers from different parts o f the country to
peruse field-based geom orphic studies.
Hindu University, Varanasi, (e.g. S.C. Kharkwal,
1969. V.K. Asthana, 1968. K.N. Singh, 1967,Meera
Agarwal, 1970. O.P. Singh. 1977 etc.). Besides,
significant contributions were made in different as­
pects o f In d ia n g e o m o rp h o l o g y by S.C.
M ukhopadhyav (1968. geo m o rp h o lo g y of
Subamarekha basin), E. Ahmad (Ranchi to Ra jaroppa,
1969), S.C. Chakravarti (1970, geomorphological
evolution of W. Bengal), Swami Pranawanand (1970,
Sources o f four great rivers o f India), J.P. Singh
(1970. geomorphological evolution of Meghalaya),
K.R. Dikshit (1970, erosion surfaces and ploycyclic
reliefs of Deccan trap). R.P. Singh (1969. denuda­
tion chronology of C hotanagpur plateau, 1970,
periglacial cycle of erosion), Savindra Singh (1977,
altimeteric analysis as a significant morphometric
technique), K.R. Diksshit, S.N. Rajguru, N.S. Gupta
and J.P. Jog (1972. geomorphology of southern
K o n k a n a r e a ), S .C . M u h o p a d h y a y (1 973,
geomorphology ofSubam arekhabasin). Anil Kumar
(1974. morphological classification of landforms of
S.W. Ranchi plateau, 1979. geomorphology of
Simdega and its adjoining area), Savindra Singh and
Renu Srivastava (1976, denudation chronology and
erosion surfaces of the Belan Basin), Savindra Singh
(1977. tors o f Ranchi plateau). R K . Rai etc.
The recognition of drainage basins as ideal
geomorphic units for geomorphological investiga­
tions resulted in the systematic morphometric analy­
sis o f drainage basins consequent upon the presenta­
tion o f doctoral thesis on 'drainage basin character­
istics o f the Belan river’ by Renu Srivastava in 1976
in the departm ent o f geography, Allahabad Univer­
sity. This was followed by presentation of a number
o f doctoral theses in Allahabad University e.g. small
drainage basins o f Ranchi plateau (Savindra Singh,
1978), m orphom etric study of small drainage basins
o f P a l a m a u u p la n d (S .S . O jh a ,
1981),
geom orphological study o f small drainage basins o f
S.E. Chotanagpur region (D.P. Upadhyav. 1981)
etc.
The decadc 1980-90 was characterized by the
study o f causal relationship between landform s and
processes and formulation o f models and techniques.
Estimation o f drainage density on the basis o f drain­
age texture by Savindra Singh (1976 and 1981) is a
significant contribution in theoretical geomorphology.
The publication o f the study o f ‘nature o f slope
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A few centres o f geom orphology have co m e
up in the c o u n try . T h e A lla h a b a d C e n tr e o f
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GEOMORPHOLOGY
The Calcutta Centre of geomorphology is
given credit for early start in geomorphological
researches. S.P. Chatterjee and K. Bagchi paved the
way forthe initiation and development of geomorphic
researches through their pioneer works in this field.
Presently, the department o f geography, Calcutta
University is widely known for researches in differ­
ent branchesof geomorphology. M.K. Bandopadhyay
is actively eng ag ed in the stud y o f glacial
geomorpthology of the Himalayas and has regularly
monitored the recession of glaciers on the basis of
field studies. S.C. Mukhopadhyay has made signifi­
cant contributions in fluvial geomorphology while
landslides in the eastern Himalaya are regularly
monitored by S.R. Basu.
The geomorphologists o f the Central Arid
Zone Research Institute (CAZRI), Jodhpur,e.g. Bimal
Ghose, Surendra Singh, P.C. Vats and Amalkar have
done outstanding researches in arid geomorphology
and applied geomorphology on the basis o f intensive
field surveys and remotely sensed data. R.K. Rai and
his associates are actively engaged in the field of
fluvial geomorphology, structural geomorphology,
karst geomorphology, etc. at Shillong. Besides,
geomorphological researches are being persued at
Bhagalpur (Anil Kumar and his team), Jaipur (H.S.
Sharma), Delhi (S.K. Pal), Jamm u (M.N. Kaul),
Thanjavur (Victor Raja Manickam), Almora (J.S.
Rawat and R.K. Pandey), Varanasi (K. Prudhvi
Raju), Srinagar-Garhwal (Devidatt) etc. A very out­
standing contribution in the form of development of
a c o m p u te r s o f tw a r e for the i n t e r p r e ta tio n
(geomorphological) of satellite imagery has been
developed by S.R. Jog (Pune).
geomorphology has initiated geomorphological re­
searches since 1971. In the beginning, attention was
focused on the morphometric study of drainage
basins based on topographical maps and limited
field observations. The detailed field studies started
in the decade 1980-90 wherein probably the 1st
d o c to ra l
d is s e r ta tio n
on
environmental
geomorphology was produced by Alok Dubey un­
der the supervision of Savindra Singh in 1985.
Besides fluvial geomorphology, a new branch of
urban geomorphology has been developed by
Savindra Singh and a few doctoral theses have been
produced. The doctoral theses on solution topogra­
phy of Rohtas Plateau by M. S. Singh (1991) and
applied geomorphology of Belan-Son interstream
area by Neera Rastogi (1994) are significant contri­
butions. The geochemistry of cave water and mor­
phogenesis of Guptadham cave (Rohtas plateau,
Bihar) based on laboratory analysis of water, solutes
and rock samples for 36 months was subsequently
published in Zeitschrift fur Geomorphologie by
Savindra Singh, M.S. Singh and Alok Dubey in 1992.
Recently, micro-level study of rill and gully erosion
has been initiated by Savindra Singh and Alok Dubey.
A major research project on 'gully erosion and man­
agement’ of a micro-man-impacted gully basin (about
56.000 m2. area) funded by the DST, New Delhi, has
been completed (1991-95) wherein the meteorologi­
cal, hydrological and geomorphological variables
have been recorded through field instrumentation for
three wet monsoon months of 1991 to 94 and soil
erosion and soil loss, sedimentation, discharge etc.
have been regularly monitored.
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The Poona Centre of geomorphology is char­
a c t e r iz e d by s e r io u s r e s e a rc h e s in flu vial (
1. 5 SYSTEM C O N C EP T
geomorphology. structural geomorphology, river bed
The system concept was adapted in the expla­
morphology, alluvial geomorphology and coastal
nation
of geomorphic problems after the postulation
geomorphology. The gemorphological researches
o f ‘general system theory’ by Von Bertalanffy in
were initiated by K.R. Dikshit. He encouraged young
1950. ‘A system may be defined as a set of objects
geomorphologists for field instrumentation of the
that are considered together by studying their rela­
processes. Consequently, V.S. Kale and S.R. Jog
tionships to each other and their individual attributes’
m a d e s ig n if i c a n t c o n tr ib u tio n s in flu vial
(C.A. M King, 1966). A geomorphic system is an
geomorphology. A number of research projects funded
integrated complex of mosaic of geomorphic fea­
by the U.G.C., D.S.T. and other organizations have
tures and this system functions under definite condi­
been undertaken by K.R. Dikshit, V.S. Kale and S.R.
Jog. S.N. K arlekarand their associates have studied
tions through the input of energy (precipitation,
extensively the western coasts of Maharahtra and
insolation, upliftment etc.) and output o f matter. A
have made significant contributions in coastal
critical balance between the input o f energy and
geomorphology.
output of matter is a prerequisite condition for the
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13
n atu re o f g e o m o r ph o lo g y
successful functioning o f a geomorphic system. In
fact, ‘a geomorphic system is a structure of interact­
ing processes and landforms that function individu­
ally and jointly to form a landscape com plex’ (R.J.
Chorley, S.A. Schumm and D.E. Sudden, 1985).
The system state includes its composition, organi­
zation and flow of energy and matter wherein the
geomorphic system may be in a steady state, dy­
namic equilibrium state or in changing state in terms
of time. Further, a super geomorphic system consists
of several subsystems of different suites of landforms
and these subsystems are interconnected through the
input-output linkages.
effected by any of the external factors (say input
factor) which regulates the equilibrium condition o f
the geomorphic system, is counter-balanced by
changes in other system components, this is called
‘hom eostatis’ or negative feedback mechanism. It
is apparent that closed geomorphic system is regu­
lated through positive feedback mechanism and leads
to progressive changes in landforms through time in
such a way that a featureless plain with minimum
relief (peneplain) is produced in the end while an
open geomorphic system operates according to nega­
tive feedback mechanism and thus the geomorphic
system remains in equilibrium.
Geomorphic systems are divided into closed
and open system s. A closed system has well defined
boundary wherein neither energy nor matter can
cross this boundary. Davisian ‘geographical cycle’
is an example of closed geomorphic system which
begins to function with the input of initial potential
energy through short-period rapid rate-upliftment.
With the march of time both height and energy
decrease progressively due to denudation resulting
into minimum height and energy at the attainment of
peneplain stage. Sometimes there may be temporary
increase in energy due to rejuvenation caused either
by upliftment or by negative change in sea-level but
ultimately the system runs down when the land is
eroded down to peneplain and the sum of available
energy and the work to be done equals zero resulting
into maximum entropy. On the other hand, an open
geomorphic system is characterized by continuous
renewal of energy and removal of matter from the
system which functions in such a way that it attains
steady state. A drainage basin is an example of an
open geomorphic system which receives energy
through insolation and rainfall and releases water
and eroded material from its mouth.
Explanation-A simple example may explain
positive feedback— increased amount o f rainfall (in­
crease in input) causes phenomenal increase in the
overland (low and surface runoff which accelerates
soil erosion leading to removal o f surficial soil
cover and exposure of underlying resistant rock
cover which discourages infiltration ol water and
augments soil erosion resulting in the lowering of
relief. Negative feedback— a profile ol equilibrium
of a stream means equilibrium o f works o f the stream
pertaining to erosion, transportation and deposition.
A graded stream having attained the profile o f equi­
librium is such that there is equilibrium between
transporting capacity of the stream and total load
(sediments) to he transported and thus a graded
stream neither erodes nor deposits in short term.
Suppose, there is sudden increase in the sediment
load of the stream due to accelerated rate of erosion
consequent upon increased rainfall. This situation
disturbs the equilibrium condition because the work
to be done (i.e. sediment load to be transported down
stream) exceeds the transporting capacity (available
energy) of the stream. This change forces the stream
to deposit extra load till the channel gradients (steep­
ening of gradient due to deposition) becomes such
that it provides required velocity and hence required
energy to transport increased sediment load so that
equilibrium condition is re-attained and the stream is
regraded.
The internal structure of a geomorphic sys­
tem is controlled by feedback mechanism. ‘Posi­
tive feedback occurs whenever externally induced
changes of input produce changes in the same direc­
tion as the input changes (i.e. lead to progressivelychanging ‘timebound’ state). Negative feedback
operates when changes in the system input result in
changes in other system components which regulate
the effects of the changed input such as to bring a
new ‘timeless’ equilibrium or steady state’ (R.J.
Chorley, 1967). In other words, when any change
1.6 GEOMORPHIC M ODELS
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Model is generally defined as simplified ap­
proximation of external real world. A model may be
in the form of structured idea to represent real
situation, an hypothesis, a theory or a law (H. Skilling,
1964). ‘It can be a role, a relation or an equation. It
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GEOMOR PHOLOGY
14
(i) Models are selective approxim ation s as
they include some of the relevant and fundamental
aspects ol real world while they ignore detailed
aspects ;
(ii) Modes are s tr u c tu re d ideas about the real
world i.e. the selected relevant and fundamental
aspects are well interconnected in such a way that the
real world may be projected in simple and general­
ized form;
(iii) Models are suggestive in nature i.e. these
incorporate scope for their future extension and
generalization;
(iv) Models are analogies;
(v) Models have the quality of reapplicability
to the real world etc.
The functional role of models includes (i)
psychological aspect which 'enables some groups
of phenomena to be visualized and comprehended
which could otherwise not be because of its magni­
tude or complexity’ (P. Haggett and R.J. Chorley,
1967; (ii) acquisitive aspect which provides scopc
for the acquisition of data, information and ideas for
the formulation of models; ( iii) logical aspect which
enables the geographer (investigator) to explain the
details of data and information; (iv) normative
aspect, which includes provision of comparison of
selected phenomena (not previously known with
precise perfection) with already known situation; (v)
constructional aspect includes provision for for­
mulation of theories and iaws etc.
Models are classified on different bases— (1)
On the basis of familiarity o f situation and existing
reality models arc divided into (i) descriptive m o d ­
els and (ii) norm ative models wherein descriptive
models involve description of real situation having
empirical information whereas normative models
are concerned with description of a less familiar
situation on the basis of more familiar situation. (2)
On the basis of stuff models are classified into (i)
h a rd w a re models, physical models and experi­
mental and (ii) theoretical m odels, sym bolic mod­
els, conceptual m odels ctc. (3) On the basis of
system concept models are divided into 'i) synthetic
system models, (ii) partial system m odels, (iii)
balck box models.
According to R.J. Chorley (1967) the concep­
tual geomorphic model system may be approached
in 3 ways e.g. (i) in terms o f time and space, (ii) in
terms of physical system, and (iii) in terms of general
system. The translation of systematic geomorphic
views in time or space yields natural analogue
system.
Natural Analogue System — A natural ana­
logue system is such wherein geomorphological
phenomena of a geomorphic system arc described
on the basis of such analogous natural system which
is simple and better known and similar to the original
system. The natural analogue system is divided into
(i) historical natural analogue system when time
factor is taken into consideration and (ii) spatial
natural analogue system when space becomes
main consideration. The historical natural analogue
model implies the concept of ‘time-controlled se­
quences’ i.e. many gcomorphic activities are re­
peated through lime and thus the past geomorphic
history has relevance to the present history. Thus,
the past geomorphic history of a given region may be
reconstructed on the basis of present geomorphic
processes and their responses (resultant features).
James Hutton's concept of ‘present is key to the
p ast’ and ‘no vestige o f a beginning : no prospect
of an end’ is a line example of historical natural
analgue model. In the spatial natural analogue model
the geomorphic features of the original region are
described on the basis of identical and contiguous
region which is better known. In other words, the
original area is described on the basis of comparison
ol another area which is similar to original one but is
better understood. Fenneman's physiographic re­
gions' (1914), ‘tectonic or structural provinces’
on the basis of morphotectonics, ‘m orphogenetic
regions’ on the basis of the concept that ‘each
climatic type produces its own characteristic assem­
blage of landforms' etc. are a few examples.
P hysical S ystem involves dissection of
geomorphic problems into several component parts
and the study of operation of each part and intercon­
nections between the parts presents a complete syn­
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can be synthesis of data. Most important from the
geographical view point, it can also include reason­
ing about the real world bv means of translations in
space (to give spatial models) or in time (to give
historical models' (P. Haggett and R.J. Chorley,
1969). The main characteristic features o f a model
are—
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NATURE OF GEOMORPHOLOGY
dom unpredictable effects o f natural processes which
obscure simpler deterministic relationships i.e. cause
and effect relationships. S toch astic m athem atical
m odels remove such w eakness o f deterministic
models. Stochastic models arc, infact, statistical
models wherein besides mathematical variables, pa­
rameters and constants, certain aspects c5f natural
processes are also included so that the simpler deter­
ministic relationships are also revealed.
thesis of the entire physical system comprising all
com ponent parts. Physical system approach of
geomorphological investigations is based on quanti­
tative method. Physical system includes three inter­
related models e.g. (i) hardware model, (ii) math­
ematical model and (iii) experimental design model.
H ardw are m odel involves simulation of natural
geomorphic complexes in the laboratory involving
similar natural conditions but on very smaller spatial
and shorter temporal scales e.g. development of
river meander, development of rills and gullies in the
laboratory etc. The construction of hardware models
in geomorphology has not been very successful
because (i) the natural geomorphic system is very
complex and (ii) this complexity imposes problems
of scale, both spatial and temporal. M athem atical
geom orphic m odels are abstract forms of equations
wherein phenomena, forces, processes, events, fea­
tures etc. o f natural geomorphic systems are re­
placed by mathematical variables, parameters, sym­
bols. letters, constants etc. For example, Davisian
model o f ‘landscape is a function of structure, proc­
ess and tim e’ has been paraphrased into mathemati­
cal model by K.J. Gregory as a geomorphological
equation—
Experim ental design m odels are constructed
on the basic premise that ‘within a given range o f
observational data exist certain meaningful c o m p o ­
nent parts which can be identified by em ploying a
suitable experimental design' (quoted by R.J.Chorley,
1967). T h e design, ,which is derived from past
observation, logical deduction, intuition, or a c o m ­
bination of these provides a structure within w hich
other data are collectcd and then analysed by c o n ­
ventional statistical means to produce some gener­
alization’ (quoted by R.J. Chorley, 1967). The c o n ­
struction of such models very often incorporates the
use of simple and multiple regression analysis, h ar­
monic analysis, spectral analysis ctc. A.N. Strahler's
model of linear relationship between channel slope
and ground slope and linear relationship between
discharge and stream width, depth and velocity
(A.N. Strahler, 1950) etc. are exam ples o f experi­
mental design models.
F
=
f(M P )t
where,
F
=
forms (landforms)
f
=
function of
M
=
maternal (geomaterials)
P
=
process
t
=
time
Mathematical models are classified into (1)
deterministic mathematical models and (ii) stochastic
mathematical models. The deterministic m athem ati­
cal m odels are constructed on the basis of exact
predictable relationships between independent and
dependent geomorphic variables i.e. relationships
between cause and effect. Horton's laws of stream
numbers and stream orders, and stream lengths and
stream orders (exponential function model) are good
examples of such model. Law of allometric crowth
(power function model) stating proportionate growth
in all components o f drainage basins with time is
another exam ple of deterministic mathematical
models. Though nearly all of the variables of com ­
plex natural situation are included in de*erministic
mathematical models yet there a^e certain such ran­
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G eneral system involves the consideration
of groups of geomorphic phenom ena which are
structured into a broad general system wherein ‘e m ­
phasis lies in the organization and operation o f the
system as a whole or as linked com ponents rather
than in detailed study of individual system elem ents’
(Von BcrtalanlTy, quoted by R J . Chorley, 1967).
Within geomorphology a 'geomorphic system’ is
consiuereu as a general system wherein detailed
study of geomorphological processes operating within
the system and their responses (resultant landforms)
provides explanation oflandform characteristics. ‘A
geomorphic system is a structure of interacting proc­
esses and landforms that function individually and
jointly to form a landscape com plex' (Chorley,
Schumm and Sugden. 1985). A fluvially originated
drainage basin may be cited as an example o f a
geomorphic system which operates through input of
energy (solar energy and precipitation) and output of
energy and matter
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16
Geomorphic models should be such that they
can be applied for practical purposes. It may be
mentioned that natural physical system is character­
ised by ‘homeostatis mechanism’ involving nega­
tive feedback which counterbalances any change
effected by natural factors in any component of the
natural physical system and thus regulates the sys­
tem and maintains equilibrium. But the changes and
interventions effected by human activities are some­
times so enormous that they exceed the resilience of
the system and upset the balance. ‘Geomorphologists
should therefore ensure that any intervention in
landform systems is thoroughly regulated so as to
exploit the system successfully, rather than cause its
degradation. Such intervention must therefore be
based on proven geomorphological models which
can accurately predict the likely impact of any planned
intervention in the system’ (A Goudie, 1981).
1.7 METHODS AND A P P R O A C H ES TO THE
STUDY O F LANDFORMS
The main task of a geomorphologist is to
study the evolution and characteristics of erosional
and depositional landforms and geomorphological
processes operating therein. The entire practice and
exercise of landform studies may be grouped into
three closely linked steps e.g. (A) main tasks, (B)
approaches and (C) methods (of data collection and
of analysis). A geomorphologist has three main
tasks o f (i) description, (ii) classification and
(iii) explanation of landforms. The description and
explanation of landforms may be approached in a
variety of ways viz. (i) qualitative Vs. quantitative
(empirical) approach or (ii) systematic Vs. regional
approach while the methods of analysis may be (i)
inductive or (ii) deductive or (iii) analytical. The
landforms may also be analysed by adopting system
approach.
(A) MAIN TA SK S
The first and foremost task of a student o f the
science of landforms is (i) to describe the landform
characteristics either subjectively or objectively on
the basis of detailed information available to him,
(ii) to classify the ladforms either genetically or
quantitatively, and finally (iii) to explain the evolu­
tionary processes of the concerned landforms.
(I) DESCRIPTION OF LANDFORMS
Landform characteristics may be described in
a variety of ways depending on the audience to
which the description is addressed and the nature of
problems needing description and explanation. Gen­
erally, landform description involves (a) subjective
description, (b) genetic description and (c) objective
or quantitative scientific description.
(a) S ubjective d e scrip tio n involves general­
ized and literary presentation of physical landscapes
in a stylish manner by the non-specialist person.
Such description depends upon the thinking of the
individual as how he looks at the problems. Thus, the
description becomes highly subjective and totally
unscientific and hence has no geomorphological
significance.
(b) G enetic description involves besides
general information of landform characteristics, rev­
elation of causes and factors o f origin and develop­
ment of landforms. For example, if the hillslope of
any given region is undergoing the process o f de­
cline or water divide is being narrowed down, then
one must also describe the processes and causes of
slope decline and shifting of interfluves. If the rivers
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General system is divided into (i) synthetic
system , (ii) partial system and (ii) black boxes.
Process-response model is ultimate result of syn­
thetic system. In fact, process-response model is
constructed on the basis of structure and analysis of
geomorphic events and final conclusions. The main
goal of partial system is to establish workable rela­
tionships between sets of factors or subsystems of a
geomorphic system wherein detailed understanding
of internal functioning of the sub-systems is not
considered to be necessary but the information of the
interrelationships between the sets of factors or sub­
systems enables the investigator to determine and
predict the behaviour of the entire system under
different input conditions (R.J. Chorley, 1967,
p. 84). A black box is that wherein no detailed
knowledge of the internal structure of different com­
ponents of the geomorphic system is required. ‘The
black box models are constructed on the basis of
assumptions and not on the basis of detailed knowl­
edge of geomorphological processes. Examples of
such models are ‘dynamic equilibrium model’ of
G.K. Gilbert, ‘climatic geomorphology’ of German
and French geomorphologists (e.g. Budel, Peltier,
Cailleux, Tricart etc.)’ ‘geographical cycle’ of W.M.
Davis, W. Penck's ‘morphological system’ etc...
GEOMORPHOLOGY
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MATURE o f
17
g eo m orph o log y
on deferen t scales) and rela tio n sh ip s b etw ee n
morphometric variables.
have developed meandering courses, then the mode
of development o f meanders should also he de­
scribed. W.M. Davis adopted entirely genetic ap­
proach for describing landform characteristics of
any physiographic region having certain environ­
mental conditions. He described the landforms in
terms of youth, mature and old stages. T h e Davisian
method of genetic description has ...... been very
widely applied in geomorphology but it is unfortu­
nately a decidedly clumsy tool, lacking in any real
precision’ (R.J. Small, 1970). A.N. Strahler (1950)
has also criticised Davisian method of genetic de­
scription of landforms as he remarked ‘a generalized
overall scheme o f landscape evolution stated in
terms of youth, maturity and old age contributes next
to nothing to the understanding of factors determin­
ing the mechanism and intensity of erosion on slopes’
(A.N. Strahler, 1950).
(II) CLASSIFICATION OF LANDFORMS
An investigator after having an observation
of physical landforms and processes and their distri­
bution patterns in the field attempts to classify the
landforms and processes into identifiable catego­
ries. The landforms may be classified on two bases
i.e. (a) quantitative basis (quantitative or non-genetic classification) and (b) genetic basis (genetic
classification).
(a)
Q uantitative (n on -genetic) cla ssifica ­
tion involves numerical data which are obtained
through morphological mapping, field instrum enta­
tion and interpretation of air photographs and satel­
lite imageries and is descriptive in nature as it does
not include the consideration of mode of origin and
nature of development of landforms, which, nodoubt,
(c)
O bjective description also called as quan­
is very important aspect of geomorphology. A hillslope
titative or scientific description involves math­
profile may be classified on the basis o f slope angle
ematical and statistical techniques. The relevant
and slope plan into summital convex, free-face,
data and information required for scientific descrip­
rectilinear and basal concave slope. The m easure­
tion of landscape characteristics of a given region
ment of slope angles of hillslope profiles in the field
arc gathered through precise measurements of
facilitates the geomorphologists to classify slopes
landforms in the field, or data are derived from
into (i) level slope (0°— 0.5°), (ii) almost level slope
topographical maps, air photographs and satellite
(0.5°— 1°), (ii) very gentle slope (1°— 2°), (iv) gentle
imageries and the data so derived are analysed through
slope (2°— 5°), (v) moderate slope (5°— 10°), (vi)
appropriate statistical techniques. Quantification is
moderately steep slope (10°— 18°), (vii) steep slope
applied not only to landscape forms, giving rise to
(18°— 30°), (viii) very steep slope (30°— 45°), (ix)
the branch of modern geomorphology known as
precipitous to vertical slope (45°— 90°) (A Young).
morphom etry, but also to processes such as river
Fluvially originated drainage basins arc divided into
flow, movement of sediments, types and rates of
1st, 2nd, 3rd, 4th............. order basins on the basis of
weathering, soil creep, solifluxion and so on' (R.J.
stream ordering and hierarchical order of the streams.
Small, 1970, p. 4.). Exam ple : An ideal hillslope
On the basis of periodicity of water flow streams are
profile may be quantitatively or objectively de­
divided into ephemeral, seasonal and perennial
scribed as follows— the hillslope is characterized by
streams. ‘Indeed, classifications o f this kind are
limited submittal convexity which is succeeded (down
normally a prelude to the development o f hypoth­
the slope profile) by free face element of more than
eses
of origin, and really represent an organization
70° angle, middle rectilinear element having slope
of the evidence on which such hypotheses are to be
angle of more than 25° and thin vineer of debris and
founded’ (R.J. Small, 1970, p. 6).
basal concave element (pediment section) having
(b)
Genetic classification involves division
o t landform assemblage o f a given geomorphic
region into certain categories on the basis o f their
mode of origin. For example, slopes can be geneti­
cally divided into tectonic slope (due to faulting,
folding, warping etc.), erosional slope, slope of
accumulation (depositional slope) etc. Streams may
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slope angle ranging between 7° and 0.5°. Similarly,
a flu v ia lly o r ig i n a te d d ra in a g e b asin is
morphometrically described on the basis of hierar­
chical position o f different tributary streams (stream
ordering), stream number, stream lengths, basin
areas, bifurcation, length and area ratios (the data for
all aspects are derived from the topographical maps
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18
GEOMORPHOLOGY
la n d fo rm s’. T h o ugh the advocates o f climatic
geomorphology have attempted to relate particular
landform or landform suites to a particular climate
(e.g. pediments to semi-arid climate, tors to periglacial
climate, convexo-concave slope to humid climate
etc.) but they have not succeeded as the so-called
diagnostic landforms o f a particular climate have
been found in more than one climatic regions. For
example, tors are found right from tropical climate
to periglacial climate, pediments have developed in
many climatic regions except glacial and periglacial
climates. Similarly, the presence o f tors in the areas
having granites, sandstones, quartzites and even
limestones has put big question mark before the
advocates of structure-form approach.
be classified into sequent and insequent streams.
Sequent streams (which follow the regional slope)
are further divided into consequent, subsequent,
obsequent and rcsequcnt streams whereas insequent
streams (which flow across the geological structure
and regional slope) are grouped into antecedent and
superimposed streams. Besides individual landforms,
landform assemblage may also be collectively di­
vided e.g. youthful landscape, mature landscape and
old stage landscape as envisaged by W.M. Davis. On
the basis o f cyclic origin of landforms they may be
divided into mono-cyclic landforms, poly-cyclic
la n d fo r m s , r e ju v e n a te d la n d fo r m s , e x h u m ed
landforms etc. Landform assemblages are also clas­
sified morphogenetically on the basis of basic tenet
o f climatic geomorphology that ‘each climatic type
produces its own characteristic assem blage of
landform s’ into (a) humid, sub-humid, arid, semiarid and glacial landscapes (W. Penck), (b) glacial,
periglacial, boreal, maritime, selva, moderate, sa­
vanna, semi-arid and arid landscapes (L.C. Peltier).
The historical or chronological approach of
landform explanation is based on the concept ‘that
there is sequential change in landforms through
time’, and on the ‘principle o f uniformitarianism^
(that ‘all the physical laws and processes that operate
today operated throughout geological periods not
necessarily with same intensity as now ’ and ‘present
is key to the past’), cyclic nature o f earth's history,
'the cunccpt of palimpsest topography' and Davisian
model o f ‘cyclic evolution o f la n d fo rm s'. The
landform development is described in term s o f ev o ­
lutionary stages of youth, mature and old as envis­
aged by W.M. Davis. The main goal of this approach
is to reconstruct the chronological history o f d en u ­
dation of a given region known as denudation
chronology and to 'identify, date and interpret plan­
tation surfaces developed in past cycles and subcycles
of erosion' (R.J. Small, 1970, p. 9). This approach
also suffers from several shortcom ings w hich would
be detailed out in the succeeding subsections.
(Ill) EXPLANATION OF LANDFORMS
The origin and development of landforms are
explained on the basis of available information de­
rived through their description and classification.
The explanation of landscapes may be approached
through (a) establishing relationships between
landforms and climate (clim atic geom orphology
a p p r o a c h ) or between landforms and structure or
rock types ( s tr u c tu r e - fo rm a p p ro a c h ), (b) through
seeking landform origin and development in histori­
cal perspective (chronological or historical a p ­
p r o a c h ) and (c) through establishing relationships
between landforms and processes (process-form
a p p r o a c h ).
The p ro c e ss-fo rm a p p r o a c h o f landform
explanation involves establishment o f relationships
between geomorphological processes and landform s
on the basis ot the concept that ‘each geom orphic
process produces its own assem blage o f landform s.’
This approach further involves detailed study and
m o nito ring o f m ode and rate o f o p e ra tio n o f
geomorphic processes in terns o f w eathering, ero­
sion, transportation and deposition on one hand and
their relationships with individual and groups o f
landforms on the other hand. A few geomorphologists
have also expressed reservations against this ap­
proach. For example, S.W. W ooldridge remarked, ‘I
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The s tr u c t u r e - f o r m a p p r o a c h of landscape
explanation is based on the basic tencnt of structural
geomorphology that ‘geological structure is a dom i­
nant control factor in the evolution of landforms .
Thus, the influences of geological structure and
lithological characteristics on the evolution o f indi­
vidual landforms (e.g. hillslopes, scarps, valleysides, tors) or general landforms and landtorm as­
semblage (e.g karst topography) are studied. C li­
m a te (through processesj-lan dform a p p r o a c h of
landform explanation is based on fundamental co n­
cept o f climatic geomorphology that ‘each climatic
ty p e produces its own characteristic assemblage of
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19
nature o f g eo m o r ph o lo g y
esses in smaller areas during shorter period o f time.
The description o f morphological characteristics o f
larger areas may also be approached in two ways e.g.
(i) historical o r c h ro n o lo g ical a p p r o a c h and (ii)
em pirical a p p ro a c h . Alternatively the explanation
of landform characteristics may be approached ei­
ther through (i) regional a p p r o a c h or through (ii)
system atic a p p ro a c h .
regard it as quite fundamental that geomorphology
is primarily concerned with the interpretation of
forms, not the study of processes’ while A.N. Strahler
cautioned that ‘geographically-trained geom orpho­
logists are not well qualified to work in the field of
process.’
Though process-form approach is more sci­
entific and involves mathematical and statistical
techniques but it also suffers from certain shortcom­
ings. (1) The mechanism of all geomorphological
processes is not the same. Some processes operate so
slowly (e.g. soil creep or chemical weathering) or so
intermittently (e.g. rainwash) that their precise and
careful measurement in the field becomes necessary
so that reliable data my be obtained. (2) The changes
in some landform take place at exceedingly slow rate
over long period of time that it becomes virtually
impossible to measure them within a life-time of the
investigator. (3) It becomes difficult to relate all the
landforms to the present processes as many of the
landforms are in fact ‘relict’ or ‘fossil’ features, the
result of past processes (e.g. granitic tors of Dart­
moor of England). (4) ‘Another fundamental prob­
lem is the sheer difficulty of proving a causal rela­
tionship between process and form. How can it be
demonstrated conclusively that a particular process
results in a particular form?’ (R.J. Small, 1970, pp.
11 -12) because many processes operate together and
thus it becomes difficult to isolate one process from
other processes. For example, most of the weather­
ing processes (physical, chemical and biological)
operate together (physico-biochcmica! weathering).
This approach will be further elaborated and exam­
ined in the succeeding sections.
(I) HISTORICAL APPROACH
Historical approach o f landform studies in­
volves description of landform evolution through
successive stages of geological time or say cyclic
time involving larger spatial and longer temporal
scales. ‘In this type of analysis the em phasis is
placed on the historical development o f the land­
scape, based on the cyclic concept o f Davis, on the
assumption that evidence of the past character o f the
landscape is still apparent in its present form ’ (C.A.M.
King. 1966, pp. 15-16). In fact, historical approach
is based on the concept of ‘p a lim p se st to p o g r a p h y ’
which means such a surface which bears the imprints
of geomorphological processes during past geologi­
cal periods after partially erased initial imprints
(features) in the beginning. Palimpsest refers to that
manuscript which has been written, erased and re­
written several times. Similarly, palimpsest topog­
raphy represents complex topographic features of a
region which have been written (characterized by
topographic features) by geomorphological proc­
esses, erased (previous geomorphological features
partially destroyed by succeeding processes) and re­
written (production of new reliefs on older surfaces)
several times.
An attempt is made to reconstruct (reproduc­
tion) the past geomorphic history of the region
concerned on the basis of present and remnant
landforms following the dictum o f ‘p re s e n t is key to
the past. This method of landform study is popularly
known as d enudation chronology (denudational
history of a given region). ‘The principal objective
(of this method) is to identify, date, and interpret
plantation surfaces developed in past cycles and
sub-cycles of erosion’ (R.J. Small, 1970, p. 9) on the
basis of evidences of drainage development, relic
surfaces and past tectonic events. The degree of
precision of landform analysis rests on deductive
power of the researcher and level of qualitative and
quantitative description of relic features.
(B) A PP R O A C H ES TO GEOM ORPHOLOGICAL
AN ALYSIS
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The explanation of morphological character­
istics of a given region may be approached in a
number of ways depending on spatial and temporal
scales and goals of the geomorphologists. Based on
conceptual bases the geomorphic studies may be
approached in two ways e.g. (i) historical a p p ro ac h
and (ii) functional a p p ro a c h . The historical ap­
proach is adopted when geomorphological evolu­
tion of larger areas is traced through long geological
period while functional approach is adopted when
landform characteristics arc related to present proc­
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GEOMORPHOLOGY
20
This approach suffers from ccrtain perceptiblc weaknesses. This approach is highly deductive
because unknown events and their responses are
described on the basis of very limited known infor­
mation and evidences. In fact, the past geomorphic
history is reconstructed on (he basis of very small
parts ol pre-existing landforms. ‘An important criti­
cism which has been levelled against the denudation
chronology approach is that it succeeds in explain­
ing directly only very small parts of the existing
landscape, namely the fragments of former (erosion)
surfaces which have been dissected and almost to­
tally destroyed in some cases by more recent ero­
sion' (R.J. Small, 1970). Secondly, historical ap­
proach is highly speculative because the old erosion
surfaces and remnant forms have been so greatly
modified by subsequent processes that it becomes
difficult or say impossible to find out their original
forms and initial heights. The dating of erosion
surfaces is also highly speculative as valid geologi­
cal evidences are not available.
drainage density and drainage texture and carto­
graphic presentation of their spatial patterns) ; and
r e l i e f aspect (computation of altimetric, hypsometric
and clinographic variables and determination of
relationships between area and height, height and
slope angles, determination o f altitudinal frequency
ipaxima for the identification o f erosion surfaces,
calculation of hypsometric and erosion integrals for
the determination of stages of cycle of erosion,
computation of relative reliefs, dissection and ruggedness indices, slope angles and measurement of
slope profiles etc.)
This quantitative approach was developed in
the U.S.A. in 1940s and was subsequently adopted
by geomorphologists worldover. It may be pointed
out that the results derived through morphometric
analysis are sometimes misleading and erroneous
and if they are not verified on the basis of field
checks thes,e may lead to wrong conclusions about
the geomorphological problems.
(II) QUANTITATIVE AND EMPIRICAL APPROACH
Regional approach involves study of land­
scape assemblage of a geomorphic region at large
spatial and long tem poral scales e.g. m egageomorphology an d m eso-geom orphology. In fact,
regional approach also involves theoretical studies
of ‘cyclic evolution of landforms and more practical
studies of denudation chronology’ at different spa­
tial scales varying from regional to continental scales.
Similarly, the approaches to the study of the mega­
scale landforms may be grouped into 3 sub-catego­
ries e.g. (i) explanation of present landscape charac­
teristics and their evolution with reference to palaeoprocesses involving spatial scales varying from re­
gional to subscontinental areas and temporal scales
of 108 years to 10s years ; (ii) examination and
explanation of ‘present processes and the dynamic
balance between process and form on sub-continen­
tal and regional scales* (Rita Gardiner and Helen
Scoging, 1983) involving temporal scale of 1 to 100
years; and (iii) examination and explanation of sig­
nificant determinants of geomorphological proc­
esses i.e. climatic and sea-level changes and re­
gional to global tectonics. Thus, regional approach
lor the study of mega-geomorphology aims at, mega­
scale, ‘an accurate understanding o f the nature of the
past environmental conditions and associated proc­
esses; for an appreciation of how and when these
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Quantitative or empirical approach as alter­
native to historical approach is adopted to explain
landform characteristics of such larger areas where
sufficient evidences for historical study are not avail­
able because of destruction of relic forms due to
g r e a te r d eg ree o f dissectio n by subsequent
geomorphological processes. The empirical approach
to study geomorphic problems of the large-scale
geomorphological features involves the measure­
ment of geometry of different aspects of landscape
and their quantitative interpretation. A fluvially origi­
nated drainage basin is selected as an ideal geomorphic
unit for morphometric study wherein measurable
properties of different aspects are measured, com­
puted and tabulated for reasonable explanation e.g.
linear aspect (determination of hierarchical orders
of streams, computation of stream numbers and
bifurcation rattio, measurement of stream lengths
and basin areas and computation of length and area
ratios and establishment of relationships between
these morphometric variables and examination of
morphometric laws of stream numbers, stream lengths
and basin areas based on exponential function mod­
els and law of allometric growth based on power
function model) ; areal aspect (measurement of
basin shapes and computation of stream frequency,
(Ill) REGIONAL APPROACH
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nature o f g e o m o r ph o l o g y
past processes m oulded the surface of the earth; and
for description and models of the present dynamic
interaction between process and form ’ (Rita Gardiner
and Helen Scoging, 1983, p. xi). It is apparent that in
one way or the other the regional approach is analo­
gous to historical approach o f landscape studies. It
may also be pointed out that the historical or regional
approach has been overshadowed by process-form
approach involving micro-geomorphology (spatial
and temporal scales both being very small).
(IV) SYSTEMATIC (FUNCTIONAL ) APPROACH
Systematic approach of landform studies in­
volves the measurement and analysis o f operation of
geomorphological processes which shape different
suites of landforms in varying environmental condi­
tions. The conceptual base o f systematic approach
comprises functional studies o f reasonably contem­
porary processes and the behaviour of earth material
which can be directly observed and which help the
geomorphologist to understand the maintenance and
change of landform s’ (Chorley, Schumm and Sugden,
1985). Functional approach lays more emphasis on
the observation and monitoring of operation of present
day processes at very small spatial and short tempo­
ral scalcs and establishment of causal relationships
between process and form which becomes process
geom orp h ology which aims at prediction of likely
responses (effect) to be produced by causative fac­
tors i.e. independent variables. Systematic approach
is further divided on the basis of major causative
factors of landscape development into (a) processform approach and (b) structure-form approach.
21
evolution of form over tim e’ (Rita G arddiner and
Helen Scoging, 1983). Thus, a gcomorphologist's
task is to (i) have detailed instrumentation and study
o f micro-processes so as to understand the physical
and chemical works performed by them, their co m ­
plex interactions and responses (effects) in the evo ­
lution of morphological features, (ii) reconstruct
chronology of environmental changes w hich might
have occurred during geological past, identify palaeoprocesses and their probable relationships with
landforms, and (iii) ‘analyse m ega-scale (regional
and continental) dynamic systems existing at present
because the independent variables controlling the
development of the landform may change totally as
the scale changes from mega to micro levels. O nce
these aspects of geomorphology have been ev alu ­
ated and combined we will better understand, model,
and predict the morphological developm ent o f the
surface of the earth’ (Rita G ardiner and Helen
Scoging, 1983).
(C) R E S E A R C H METHODS
Explanation of processes and landform s and
building of models require data acquisition from
various sources. R.J. Chorley (1966) has outlined 3
steps and methods of data acquisition which ulti­
mately lead to theoretical work. The integrated
approaches to research methods in geom orphology
include, according to R.J. Chorley, field observa­
tions, laboratory observations, office observations
and theoretical work.
‘O bservation in the field plays a very large
part in geomorphological work, w hatever the aim o f
the particular study or whatever the method o f ap ­
proach’ (C.A.M. King, 1966). Field observation
involves qualitative as well as quantitative methods
of data acquisition depending on the approaches of
landform studies. For example, the geomorphologists
of the school o f denudation chron ology used to
derive information about chronological evolution o f
landscapes and erosion surfaces at regional and
mega scales through qualitative field observations
and through ‘subjective map analysis’ but the emer­
gence of functional and process-from approach to
landform studies dem anded accurate quantitative
data regarding forms, processes and materials (rocks
and soils). Thus, quantitative data are obtained through
numerical measurement o f forms (e.g. slope angles,
The process-from approach envisages that
‘an understanding o f the erosional and depositional
processes that fashion the landforms, their mechan­
ics and their rate o f operation must also be obtained
in order that the past evolution can be explained and
future evolution predicted. The aspects and short­
comings o f process-form approach and structureform approach have already been detailed in the
preceding section.
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It may be concluded that ‘if geomorphology
is to continue to exist as an independent discipline,
and not to be subsumed within earth sciences, geol­
ogy, engineering, hydrology and so on, it must
attempt to explain the relationships between form
and process, both in past and present, as well as the
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GEOMORPHOLOGY
22
absolute and relative reliefs, various properties re­
garding height, dimension etc. of landform compo­
nents) and processes (e.g. measurement of discharge,
infiltration, evaporation, sediment load, rainfall, runoff
etc. in the case of fluvial process) in the field through
appropriate instruments.
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Laboratory observation involves experimen­
tation and numerical measurements of samples col­
lected in the field (e.g. chemical and mechanical
properties of soil and rock samples, chemical prop­
erties of water, grain-size measurement and deriva­
tion of chemical properties of suspended sediment
load and other eroded materials etc.) and measure­
ment derived through controlled experiments in the
laboratories (e.g. nature and rate of rill and gully
development, rate of meander development, rate of
soil erosion and sedimentation, rate of nick point
recession etc.). The examples may be cited from
other processes and related landforms of different
environmental conditions.
Office observation comprises data deriva­
tion from map analysis say map work. These days a
mass data set is being derived through measurement
and computation involving numerous useful tech­
niques from topographical maps, air photographs
and satellite imageries pertaining to different com­
ponents of landforms. The measurement and deriva­
tion of data of geomorphological significance from
air photographs and satellite images at regular time
intervals has enabled the geomorphologists to moni­
tor geomorphic changes.
The recent work by Savindra Singh and Alok
Dubey (1991-1995) on ‘gully erosion and manage­
ment’ in sub-humid tropical riverine alluvial envi­
ronment of India incorporates almost all the steps
referred to above for the study of genesis, micro­
geometry, morpho-cyclcs and integrated manage­
ment of gully watersheds as the gully basin was
surveyed thrice ( 19 9 1, 1992 and 1994) and contours
at the interval of one meter were traced on the ground
for the derivation of morphometric data of gully
basin, the meteorological (rainfall, temperature, rela­
tive humidity, evaporation etc.), hydrological (dis­
charge, runoff, infiltration, hydrological budget etc.)
and geomorphological (rate of erosion, deposition,
suspended sediment load etc.) data were obtained
(hi i ugh field instrumentation during 3 wet monsoon
months of July, August and September for 1991,
1992, 1993 and 1994 and the mass data set, so
derived, were processed in the computer lab. besides
analysis o f w ater and s e d im e n t loads in the
geomorphological laboratory.
The quantitative ap proach gave birth to
morphometric analysis o f linear, areal and relief
aspects of fluvially originated drainage basins which
have been recognized as ideal geomorphic units
since 1945. Detailed data pertaining to linear aspect
(e.g. hierarchical orders of streams, stream number,
stream lengths, sinuosity, m eander properties etc.),
areal aspect (stream frequency, drainage density,
drainage texture etc.) and relief aspect (relative
relief, average slope, dissection index, altimetric,
hypsometric and clinographic properties etc.) are
derived from topographical maps o f different scales.
Theoretical \york involves data processing
and formulation of models and theories e.g. laws o f
stream numberand stream lengths (R.E. H orton)and
calculation of mathematical models.
(D) METHODS O F A N A LYSIS
There are three alternative routes for satis­
factory scientific explanation o f geom orpholoigcal
problems e.g. (i) inductive m ethod, (ii) ded u ctive
method and (iii) analytical m ethod, all o f w hich are
based on data acquisition, their classification, and
analysis so as to come to certain ‘conclusions con­
cerning the nature and genesis of the particular
feature, investigated, whether it be a whole conti­
nent or one small slope or spit’ (C. A.M. King, 1966).
In du ctiv e m e th o d of argum ent and analysis
of geomorphic problems involves, in successive
steps, arrangement ol unordered facts in logical
order on the basis ot correct definition and classifi­
cation of observed facts of the given problem s so that
one (fact) leads to another and then to the final
conclusion (C.A.M. King, 1966), inductive gener­
alization and linal conclusion resulting into formu­
lation ol laws and theories which offer satisfactory
explanation of geomorphic problem. It may be pointed
out that in inductive methods data are collected first,
t ey are defined and classified and final conclusion
about the real world is drawn (i.e. model or theory
Ul, ' ng) ' n
*ast stagc. In other words, inferences
and final conclusions are drawn on the basis of
o served tacts. ‘As a method it is best suited to a
air y simple problem, the solution o f which is based
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NATURE OF GEOMORPHOLOGY
23
only those facts which validate the tentative hypoth­
esis and may ignore those facts which do not favour
his deductions. T h e quality of results will depend
on the nature of deductions and the closeness of
comparisons. When there are many complex or
peculiar deductions then there is a better chance that
the comparison will be valid, and the theory will be
more strongly supported’ (C.A.M. King, 1966).
on a wide field of observation and relevant data, so
that it is not necessary to invoke theoretical reasoning' (C.A.M. King, 1966). This method suffers from
the shortcomings that no generalization about the
real world is made in the beginning and hence a lot
of labour in the collection of data is wasted and since
only one conclusion is derived at the end and hence
such conclusion may be questionable or sometimes
may be even false because some of the facts, which
may be geomorphologically significant but may not
be favourable to the final result, are deliberately or
subconsciously ignored. But, ‘it is sometimes help­
ful to give at least some indication of the final
conclusion nearer the beginning of the argument*
(C.A.M. King, 1966).
The fundamental difference between induc­
tive (the method of ruling hypothesis) and deductive
(the method of working hypothesis) is that in the 1st
method theory is formulated in the last stage on the
basis of observed facts while in the second method
a working hypothesis is deduced in the beginning
and the fieldwork and data collection is accom ­
plished according to the demand o f the deduced
hypothesis.
The analytical m ethod involves deduction
and formulation of more than one alternative hy ­
potheses (multiple hypotheses) and thus data are
collected according to alternative hypotheses and
hence the investigator does not have bias to a par­
ticular hypothesis. The observed facts and deduc­
tions of all the alternative hypotheses are compared
and finally only that hypothesis is approved and
retained which conforms with the greatest number
of observations derived through filed work. Thus, it
is obvious that the analytical method of landform
analysis overcomes the shortcomings of deductive
and inductive methods.
The deductive m ethod of explanation of
geomorphic problems involves formulation of a
tentative hypothesis regarding the real world (i.e.
geomorphic problems under investigation) in the
beginning. After the formulation of tentative hy­
pothesis its consequences are deduced in advance,
facts are collected according to the demand of de­
duced hypothesis, actual field observations are com­
pared with deduced consequences and finally it is
argued whether the hypothesis is approved or re­
jected. In case the tentatively deduced hypothesis is
not approved or it becomes unsuccessful, original
hypothesis is revised and the entire process as re­
ferred to above is repeated but if the hypothesis
becomes successful after comparison of deduced
and observed facts, it leads to the construction of
laws and theory which may offer reasonable expla­
nation o f the real world. This method suffers from
the weakness that there is every likelihood that the
investigator may become biased in the matter of
collection o f data and information as he may retain
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It may be concluded that ‘main essential in all
the other methods discussed is that observations
should be accurate, as far as possible quantitative,
and carried out on a systematic basis, while imagina­
tion and integrity are required in the development
and testing of hypotheses’ (C.A.M. King, 1966).
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
C o n c e p t s r e l a t e d to u n i f o r m i t a r i a n i s m , g e o l o g i c a l s t r u c t u r e ,
g e o m o r p h o l o g ic a l processes, stages o f time, geom orphic scale (time
s c a l e - c y c l i c tim e , g r a d e d tim e and ste a d y tim e, sp a tia l s cale ),
g e o m o r p h o l o g ic a l equation, com plexity o f landforms etc.
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CHAPTER 2
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24-56
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FUNDAMENTAL CONCEPTS IN
GEOMORPHOLOGY
T h e d e v e l o p m e n t o f g e o m o rp h o lo g ic a l
thoughts through different periods of evolution of
geom orphological know ledge and associated re­
searches pertaining to the understanding and expla­
nation o f landform characteristics and geomorphic
processes associated with their genesis and meth­
odological development o f geomorphic research have
enabled the geom orphologists to conceive a few
fundamental concepts which generalize the landform
developm ent. W.D. Thornbury (1959) has presented
a su m m a ry o f a few fundam ental concepts in
geom orphology. It is, thus, desirable that the readers
s h o u ld be a q u a i n te d w ith su ch fu n d a m e n ta l
geom orphic concepts.
CO N CEPT 1
The sam e p h ysica l processes and laws that
operate today, o p era ted throughout geological time,
although no t necessarily always with the same intensity
as now ’
(W.D. Thornbury)
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The present conccpt is fundamental principle
o f modern geology which is very often popularly
known as ‘p rin cip le o f u n ifo rm ita ria n ism ’ which
was first postulated by renow ned Scottish’ geolo­
gist, Jam es H utton, in 1785. This concept was
furthei modified and developed by his disciple Jhon
P layfair i n 1802. S ir C harles L yell popularized this
concept o f uniformitarianism by giving suitable place
to it in his famous book ‘p rin cip les o f g eo lo g y ’. It
may be pointed out that H utton's original concept
was a bit different from the co n ce p t stated above and
suffered from some sh o rtcom ing s. F or example,
Hutton stated that ‘geological processes w ere active
with same intensity during each period o f geological
tim e’ and thus he postulated an o th e r principle on this
concept e.g. ‘the p resen t is k ey to th e p a st’ and ‘no
vestige o f a b eg in n in g an d n o p ro sp ect o f an end.’
It is inferred from his co n cep ts that all the geological
processes affecting the earth's crust, w hich operate
at present, were also active in the geological past and
hence the past geological and g e o m o rp h ic history of
the earth may be reco nstructed on the basis o f present
processes and their topographic expressions (landform
characteristics).
Hutton's concept ‘that physical processes were
always active with sam e intensity throughout geo­
logical periods is erro n eo u s and confusing. For
example, g laciers w ere m ore activ e during Carboniler.ous and P leistocene p erio d s than other pro­
cesses. At the sam e time, they w ere m ore active
during aforesaid periods than the present glaciers.
The temporal variations in the m ag nitude o f opera­
tion ol processes are because o f clim atic changes
and there are definite ev id en ces for several phases of
climatic changes du ring past geological times. Thus,
the distributional p atterns o f d ifferent climatic types
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
25
have registered spatial shiftings during geological
past. For example, some areas, which are presently
characterized by humid climate and dominance of
fluvial process, have been dominated by dry climatic
conditions and aeolian process. Similarly, some of
the present dry desert areas have been humid regions
in the past. For example, the fossils of coal found in
Great Britain are indicative of vegetation commu­
nity of equatorial climate, which forcefully proves
that Great Britain, which enjoys humid temperate
climate at present, was characterized by hot and
humid equatorial climate during Carboniferous pe­
riod when the present-day tropical areas were domi­
nated by glacial climate. For example, ample evi­
dences are available to elucidate several phases of
climatic changes in India. There is presence of
glacial boulders and boulder clay just below the
Talchir coal seams in Orissa. Most of the coal seams
of India were formed during Gondwana period,
which means belore the formation of Gondwana
system o f rocks (sedimentaries including coal), the
regions having coals in India were glaciated. The
coal seams overlying glacial boulder indicate the
prevalence of hot humid climate. Similarly, vulcanicity
was not uniformly active throughout geological pe­
riods. It was more active during Cretaceous period
than today. The Cretaceous lava flow was so wide­
spread that extensive lava plains and plateaus were
formed in almost all o f the continents including
basaltic lava flow over Peninsular India. The m ou n­
tain building was confined to certain periods only
e.g. pre-Cambrian. Caledonian. Variscan(hercynian)
and Tertiary periods of mountain building.
believed in orderliness o f nature i.e. the nature
evolves in orderly course. According to him the_
nature is systematic, orderly, coherent and reasonable i.e. destruction leads to construction while
construction results into destruction. For example,
denudation of uplands (destruction) leads to sedimen­
tation in lowiying areas giving birth to alluvial plains
(construction). Continuous sedimentation leads to
subsidence of ground surface. The nature has inbuilt
self regulatory mechanism known as hom eostatis
mechanism which acts in such a manner that any
chang e effected by natu ra l fa c to rs (w h e th e r
endogenetic or exogenetic) is suitably com pensated
by changes in other components of the natural
system.
Hutton was the first scientist who postulated
the concept of cyclic nature o f earth's history. All
major geological activities are repeated in cyclic
manner. For example, there have been four major
periods of mountain building viz. precam brian,
Caledonian, hercynian and tertiary periods o f m oun­
tain b u i 1d in g a n d ^ a c h jn o u n ta i^ ^
succeeded by a period of quiescence. Similarly,
glacial periods during Pleistocene ice age w'ere sepa­
rated by interglacial periods. There are ample evi­
dences to validate the observations that each geo­
logical process has completed several cycles during
geological past but it becomes difficult to find out as
to when a particular geological process began to
work and it is equally a difficult task to predict as to
when a particular process would cease to work.
Based on this connotation Hutton postulated his
concept, ‘no vestige o f a b eg in n in g : no prospect o f
an end.
It is. thus, obvious that geomorphic and tec­
tonic processes were active in all the geological
periods and their mode o f operation was the same as
today (e.g. rivers formed their valleys through ver­
tical and lateral erosion in the past in the same
manner as they are forming their valleys to day, sea
waves shaped coastal areas in the same manner as
they are doing today, the glacial movement and
erosion was controlled by the same laws and princi­
ples during Carboniferous and Pleistocene periods
as they are controlled today etc.) but the intensity of
erosional and depositional works differed tempo­
rally.
The examples of denudation chronology o f
the Applachians and Peninsular India may dem on­
strate the cyclic nature of earth's history as envis­
aged by Hutton. The Applachian revolution during
Permian period resulted in the 1st upliftment of the
Applachians which was followed by long period of
active denudation culminating into the development
of Schooley peneplain which w'as again uplifted and
then was peneplained to form Shenondoah peneplain.
The third phase of upliftment was again followed by
active denudation resulting in the formation o f
Harrisberg peneplain which was again uplifted in
the recent past and fourth cycle o f erosion is in
operation. Peninsular India has passed through van-
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The processes (mainly endogenetic) which
affect the earth's crust act in a cyclic manner. Hutton
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GEOMORPHOLOGY
26
ous phases o f cyclic development e.g. D h a r w a r
landscape cycle, Cuddapah-Vindhyan landscape
cycle, Cambrian landscape cycle, Gondwana land­
scape cycle, Cenozoic landscape cycle etc. (R.P.
Singh), (see Chapter 17).
CONCEPT 2
‘G eologic structure is a dom inant control
fa c to r in the evolution o f landforms and is reflected
in them. ”
(W.D. Thombury)
The above concept demonstrates imposing
influence of geological structure on primary and
secondary landforms (produced by exogeneticdenudational processes). W.M. Davis included ‘struc­
tu re’ in his ‘t r i o ’ namely structure, process and
time, as important controlling factors of landscape
development through his postulate that ‘landscape is
a function o f structure, process and time’ but he gave
more importance to ‘tim e’. A few usages like ‘rocks
and reliefs’, ‘geological structure and landforms’,
‘geologicalgeomorphology’ (Chorley, Schumm and
Sugden, 1985), ‘structural geomorphology’, ‘vol­
c a n ic la n d f o r m s , ’ ‘a re n a c e o u s la n d fo r m s ’,
‘argillaceous landforms,’ ‘calcareous landforms,’
‘igneous landforms’, ‘metamorphic landforms’ etc.
c le a rly d e m o n s tra te the view s o f a host of
geomorphologists about strong control of geologi­
cal structure and lithological characteristics on mor­
phological characteristics of a region.
Even the modem geomorphologists like J.T.
Hack, R.J. Chorley, S. Schumm, D.E. Sugden etc.
have clearly outlined influences of geological struc­
ture on landforms. ‘Exposed rocks are immediately
acted upon by exogenetic weatheripg and erosional
processes to form secondary landforms, which re­
flect geologic controls at both global and local
scales (p. 7 8 )............ The distinctive characteristics
o f landscape are commonly a complex response to
variations in rock type (lithology), to primary struc­
tures within the rock units, to secondary structures
involving groups of rocks units mainly due to
diastrophic processes, to the effects of different
exogenetic processes and to the geomorphic history’
(Chorley, Schumm and Sugden, 1985, p. 150).
domi n a n t that they overshadow the control of geokaL
cal structure. Som e um es geological structure
- ^ ^ T j S v e factor in the evolution o f landforms.
‘There is tendency to regard structure as the domi­
nant control of surface form and no doubt this is true
in many instances. But structure is not always the
principal control and never the only one’ (E.H.
Brown) and thus ‘thejandform s_cannpt be
rn nne cause, but are the result o f a complex inter.
several factors and processes, both
^ ^ i d r T o r T g i n a t i n g from within the earth's
m i r t nnH i " H a tin g structure and rock-type) and
^ r r l r w i r (originating from the atmosphere and'
T n du din gw eath ering , transportation and erosion’
(R.J. Small, 1970).
If structure is used in narrow sense of the term
then it includes only deformation and arrangement
of rocks due to earth-movements (endogenetic forces)
but if this term is used in w ider sense then structure
includes (i) nature o f rocks (lithology, meaning
rock types), (ii) arran gem en t o f rocks (widely
known as structure) and (iii) rock characteristics.
Here, ‘structure’ is used in w ider sense o f the term so
as to demonstrate influences o f all the aforesaid
aspects of geological structure and landforms.
1. Lithology or Nature of R o cks
Lithological aspect o f geological structure
includes types of rocks (e.g. igneous, sedimentary
and metamorphic groups o f rocks). Lithological
c h a r a c te r i s t ic s h a v e g r e a t e r s ig n if ic a n c e in
geomorphology because these determ ine and con­
trol the evolution o f landform s and nature of land­
scape. Considering this fact S.W. Wooldridge and
R.S. Morgan aptly remarked, ‘rocks whether igne­
ous or sedimentary, constitute on the one hand the
manuscripts of the past earth-history, on the other,
the basis for contem porary scen ery ’. In fact, differ­
ent types of rocks differ considerably as regards their
composition and chemical characteristics and hencc
weathering and erosional processes act upon them at
varying rates thus giving birth to variations in landform
characteristics. ‘Lithological controls over landforms
produce a large num ber o f variations and, more
important, these variations may be associated with a
wide range o f discrete regions varying in size from
a distinctive outcrop o f a few square metres to areas
of uniform rock type extending over hundreds of
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This does not mean that geological structure
is always and only dominant control factor in the
evolution o f landforms as sometimes exogenetic
(denudational) processes become so effective and
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27
FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
near Khandala (between Bombay and Pune). The
Yellowstone river has dug out a large canyon in the
Columbian lava plateau o f the U.S.A.
square kilometres’ (Chorley, Schumm and Sugden,
1985).
The relatively hard rocks (most o f igneous
and metamorphic rocks) give birth to bold topogra­
phy. Sometimes, the influence of some rocks on
geomorphic features is so dominant that the resul­
tant landscape is named after the rock group or
individual rock e.g. granitic landforms, karst or
limestone landforms, chalk landforms etc. The associa­
tion of few rocks and their topographic expressions
(landforms) may be examined to elucidate the con­
cept in question.
If the sills are intruded in the tilted or inclined
sedimentary layers and if they are more resistant
than the surrounding sedimentary' rocks, the latter
are enoded more than the former and thus resistant
sills project above the general ground surface as
cuestas and h o g b ack s (fig. 2.1 >.
Granitic rocks when subjected to exfoliation
or onion w e a th e rin g give birth to dom eshaped
landforms known as exfoliation d o m e s. Several
exfoliation domes o f granite-gneisses are seen over
the Ranchi plateau, for example. Kanke Dome near
Ranchi city, a group of gneissic dom es near Buti
village (near Ranchi city).
Igneous Topography
Variations in structure and composition of
igneous rocks o f a particular area exert strong influ­
ence on the genesis, development and nature of
landscape. Further, intrusive (e.g. granites) and ex­
trusive (e.g. basalt) igneous rocks influence land­
form characteristics differently depending on their
degree o f relative hardness.
M assive lava flows over extensive areas re­
sult, after cooling and consolidation, in the forma­
tion o f lava plateaus the surfaces of which are least
affected by fluvial erosion because ‘the drainage is
conducted underground by the joint systems, perme­
able ash and flow cavities, but deep weathering of
basalt (especially where closely jointed in the humid
tropics) and areas o f poorly welded tuffs may lead to
considerable piecemeal reduction of volcanic pla­
teau by ero sio n ’ (Chorley et. al. 1985) but the rivers,
which develop over the basaltic plateaus and are
subsequently fully established, resort to vigorous
valley deepening through active downcutting with
the result the extensive basaltic plateau is seg­
mented into num erous smaller plateaus character­
ized by flat tops and steep slopes on all sides. Such
features are called as m e sas and bu ttes. Basaltic
plateaus and plains give birth to picturesque land­
scapes after continued weathering and erosion. Very
deep and long gorges and canyons have been formed
by the source segm ents o f the Saraswati (draining
towards Arabian Sea) and the Krishna rivers (drain­
ing towards the Bay o f Bengal) through their vigor­
ous vertical erosion in the massive and thick basaltic
covers o f M ahabaleshw ar plateau (about 100 km
south-west o f Pune). Similarly, the Ullahas river has
entrenched a very deep gorge in the basaltic plateau
Fig. 2.1 : Landforms resulting from differential erosion
o f sills and surrounding rocks.
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Massive granitic batholiths. when exposed to
the earth's surface due to removal o f superincum bent
load of overlying rocks through continued erosion,
become interesting landforms. These dom e-shaped
hills project above the general surface. Such ex­
posed granite-gneissic domes are very often found
on Ranchi Plateau. The granitic batholiths were
intruded in the Dharwarian sediinentaries during
Archaean period. After a long period o f prolonged
subaerial erosion the Dharwarian sedimentaries have
been removed and the batholiths, regionally known
as Ranchi Batholiths, have been exposed well above
the ground surface (50 to 100m from the ground
surface). M urha Pahar near Pithauria village, lo­
cated to the north-west o f Ranchi city, is a typical
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GEOMORPHOLOQy
28
exam ple of exposed grantic-gneissic batholithic
domes. These exposed batholithic domes have suf­
fered intense fracture because of the removal of
superincumbent load o f Dharwarian sedimentaries
and hence resultant massive joints have been re­
sponsible for the development of different types of
‘t o r s ’. Extensive granitic domes of Yosemite P a rk ,
Sierra Nevada, S to ne M o u n ta in of Georgia (U.S.A.)
and S u g a r L o a f of Rio de Janeiro (Brazil) are other
exam ples o f such granitic domes which have been
formed due to unloading of superincumbent load
(sedimentaries) consequent upon prolonged erosion.
Fit’. 2.3 : An example o f volcanic butte.
The differential erosion of the basaltic ‘cap
r o c k s ’ (fig. 2.2) produces interesting features like
m e sas and buttes. Mesa is a Spanish word meaning
thereby a table. Mesa, in fact, is such a hill which is
characterized by almost flat and regular top-surface
but by very steep slopes (wall-like) from all sides.
When mesas are reduced in size due to continuous
weathering and erosion, they are called buttes. Messas
are locally called as ‘P a ts ’ or ‘P a tla n d ’ 011 the
Chotanagpur plateau of south Bihar. Jamira pat,
Netarhat Pat, Bagru pat, Khamar pat, Raldami pat,
Lota pat etc. are typical examples of lava-capped
messas of the western Chotanagpur High Lands.
Mahabaleshwar plateau and Panchgani plateau (of
the Western Ghats, Maharashtra) are characteristic
representatives of well developed basaltic mesas.
Grand Mesa and Raton Mesa of the state of Colo­
rado, USA, are typical examples of extensive mesas.
Grand Mesa rises more than 1500m (5,000 feet)
higher than the surrounding ground surface.
Sometimes magma is injected in a vertical
columnar form in the sedim entary rocks. The upper
portion of vertical column of magma appears as
butte when the overlying rocks arc eroded down.
Such butte is called as ‘volcanic b u t t e ’ (fig. 2.3).
The grantic rocks having rectangular joint
patterns are weathered and eroded along the inter­
faces of their joints and thus smaller tables or blocks
are separated by the eroded narrow clefts developed
along the joints. Such granitic topography develops
rectangular drainage pattern (fig. 2.4).
Fig. 2.4 : Development ofrectangular topographicfea­
tures on granitic rocks having rectangular
joint pattern.
The igneous rocks having columnar joints
give birth to hexagonal landforms after weathering
and erosion (fig. 2.5).
Scoria and ash cones when subjected to fluvial
erosion develop radiating rills and gullies whereas
strato-valcanic cones, after prolonged erosion, are
c aracterized by n u m e ro u s ra d ia tin g valleys
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Fig. 2.2 : Lava-capped mesa and butte.
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
1
Fig. 2.5 : Development o f hexagonal landforms on
igenous rocks having columnar joints.
known as ‘b a r r a n c a s ’. The valcanic pipe filled with
breccia is exposed after prolonged erosion above the
ground surface and is called d ia tre m e . Shiprock
(fig. 12.8) o f New; M exico (USA) is fine example of
diatreme which projects 515m above the surround­
ing surface composed of sedimentary rocks. If magma
is intruded as sills into inclined sedimentary beds of
weak resistance then the sedimentary beds are eroded
and the sills being resistant project above the ground
surface.
Fig. 2.6 : Formation o f tors.
Similarly, mesas and butles are co n fin ed not
only to basaltic plateau but these have also been
found over sandstone rocks where these overlie
weak shales and siltstones. M orchapahar(H azaribagh
plateau, Bihar, India) is a fine exam ple o f sandstonecapped mesa. Similarly, B hander plateau (M.P.,
India) having Vindhyan sandstones over w eak shales
and siltstones is an example of extensive m esa. It
may, thus, be concluded that the d ev elo pm ent o f
mesas and buttes is no doubt lithologically co ntrol­
led but these are not confined to a particular rock
type. They may be formed through active fluvial
erosion in humid and subhum id climate w henever
relatively resistant rock overlies weak rock.
Well jo inted granitic rocks give birth to very
peculiar landform s such as to rs which ‘are piles of
broken and exposed masses o f hard rocks particu­
larly granites having a crown o f rock blocks of
different sizes on the top and clitters (trains of
blocks) on the sides. The rock-blocks, the main
com ponents o f tors, may be cuboidal, rounded, an­
gular etc. in shape. They may be posted at the top of
the hills, on the flanks o f the hills facing a river
valley or on flat basal p la tfo rm ’ (Savindra Singh,
1977, p. 93, N ational G eographer, Vol. 12(1) (fig.
2.6). A few alternative hypotheses o f tor formation
have been put forth e.g. pediplanation theory o f L.C.
King, deep basal w eathering theory o f D.L. Linton,
Sedimentary Landforms
The landform s developed over different sedi­
mentary rocks (e.g. arenaceous— siltstones, m u d­
stones, sandstones; argillaceous— clay and shale;
calcareous— limestones, dolom ites etc. rocks) are
called sedimentary landforms. Som etim e, the co n­
trol of a particular sedim entary rock on landform
characteristics is so dom inant that particular rock is
p refix e d w ith g e o m o rp h o lo g y e.g. ‘lim e sto n e
g eom orp h ology 9 (Stephen Trudgill, 1 9 $ 5 )o rk a rst
geom orp h ology etc. Sandstones having silica ce­
mentation are resistant to chem ical w eathering and
hence give birth to bold topography and developm ent
o f low drainage density while sandstones cem ented
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periglacial theory o f J. P alm er and R.A. Neilson,
two-stage theory o f J. D einek, glacial theory ot R.
Dalh etc. but there is no unanim ity am ong the exp o­
nents becausc tors are not confined to a particular
rock type and clim ate as tors have been found over
granites (even basalt), sandstones, limestones etc.
right from hum id tropical to periglacial climate.
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30
GEOMORPHOLOGY
by ferrous contents are subjected to rapid rate of
oxidation and fluvial erosion and hence give birth to
undulating and rolling terrain. The argillaceous rocks
e.g. clay and shale are less resistant to erosion and
thus low relief is associated with them. Argillaceous
rocks respond differently in humid, arid and semiarid environment e.g. in humid regions these are
characterized by low relief, low to gentle slope
angles (less than 8°), moderate drainage density,
dendritic drainage pattern, convexo-concave hills ;
subhumid and semi-arid regions having clay-shale
rocks are characterized by the development of badland
topography with high drainage density (due to
numerous rills and gullies) and subdued reliefs, the
gully valleys having steep valley sides (30°-60° and
sometime 70°-80°) are separated by narrow ridges.
Calcareous rocks (e.g. limestones, dolomites
and chalk) are subjected to solution under humid
conditions and give birth to solution holes and de­
pressions of varying shapes and dimensions (e.g.
sink holes, swallow holes, dolines, polje, uvala etc.),
underground solution networks (caves and associated
features), disorganized and poor surface drainage
etc. The landforms developed on carbonate rocks are
collectively called as k a rs t topography. In humid
tropics two special types of karstic topography have
been identified e.g. cone karst, in the ‘cockpit
country* of Jamaica and Cuba, characterized by
steep sided rounded hills, and tower karst, in
monsoon land of China and Vietnam, characterized
by isolated very steep sided (almost vertical ) narrow
but high pillars (upto 300m). Wherever sandstones
overlie shales and siltstones majestic mesa and butte
are formed and escarpments are crowned by stupen­
dous steep scarps (e.g. Rewa escarpments, Bhander
escarpments, Rohtas plateau escarpments etc. where
Vindhyan sandstones lie over shales and siltstones).
Metamorphic Landforms
2. Arrangement of Rocks
Arrangement of rocks means disposition of
rock beds mainly of sedimentary rocks due to de­
formation processes. Sedimentary rocks are gener­
ally deformed due to isostatic, tectonic and orogenetic mechanisms into folded, faulted, domed, homoclinal (uniclinal) structures etc. Horizontal dis­
position of sedimentary beds denotes least deforma­
tion but these may be subjected to upwarping. Such
geological structures exert strong influence on land­
form characteristics.
(I) FOLDED STRUCTURE AND LANDFORMS
Sedimentary rock beds are sqeezed and buck­
led and folded into anticlines and synclines due to
lateral compressive forces. The folded structure ranges
from simple folds to complex folds (i.e. recumbent
folds depending on intensity of compressive forces).
Simple folded structure is characterized by sequence
of anticlines and synclines and in the initial stage
trellis drainage pattern evolves over such structure.
Such drainage pattern is characterized by the devel­
opment ot consequent, subsequent, obsequent and
resequent streams. The Fegion of folded structure
when subjected to continued fluvial erosion for
longer period experiences the process of inversion
of relief wherein original anticlines (due to more
erosion) are eroded down and become anticlinal
valleys where as synclines (due to less erosion)
become synclinal ridges (fig. 2.7). For details see
chapter 10 and figs. 10.9, 10.10 (chapter 10).
examples ot inverted reliefs are found in Jura moun­
tains and southern Applachians.
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Unlike sedimentary and igneous rocks meta­
morphic rocks are not pronounced in the develop­
ment of landforms because these (e.g. quartzite,
slate, schist, gneiss etc.) have uniform resistance to
erosional processes though the process of meta­
morphism-‘coverts rocks of lower resistance (e.g.
shale and sandstone) to those of higher resistance
(e.g. slate and quartzite). Although metamorphic
rocks generally present more resistance to erosion
than do their sedimentary counterparts, it is not easy
to identi fy a separate class of distinctly metamorphic
landforms' (Chorley, Schumn and Sugden, 1985).
Quartzitic sandstones when lie over shales and
siltstones give birth to stupendous escarpment char­
acterized by upper free face and rectilinear segment
and basal concave pediment section (last two devel­
oped on shale and siltstone). Quartzite^ are on an
average resistant to mechanical and chemical weath­
ering and produce bold topography having very high
reliefs. Slates are more succeptible to erosion and
are associated with subdued reliefs while resistant
schist rocks produce highland topography. Gneissic
rocks form domes and tors.
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
faultline s carp is formed due to renewed downward
erosion caused by further fall in base-level of ero­
sion. In fact, resesequent scarps result from the
reversal of obsequent scarp and it is oriented in the
direction of the original normal or consequent scarps
but is much older than the latter (fig. 2.8(4)).
Fig. 2.7: Development o f landforms over folded structure.
(II) FAULTED STRUCTURE AND LANDFORMS
A fault is a fracture in the crustal rocks wherein
th* rocks are displaced along a plane called as fault
plane. In other words, when the crustal rocks are
displaced due to tensional movement caused by the
endogenetic forces along a plane, the resultant struc­
ture is called a fault. Different types of faults are
created due to varying directions of motion along the
fault plane e.g. normal faults, reverse faults, lateral
or strike-slip faults, step faults, transform faults etc.
Differentfaulttypesproduce,aftererosion. landforms
of varying characteristics. Take the case of normal
fault where downthrown block is displaced down­
ward along the fault plane giving birth to fault scarp
which is, without doubt, structural in genesis. Such
fault scarps after prolonged erosion produce differ­
ent types of erosional landforms e.g. (a) consequent
faultline scarp is formed due to erosion of weak
rocks of downthrown blocks. Such fault scarps are
oriented towards the direction of original fault scarp
(fig. 2.8 (1) ; (b) reverse o r obsequent faultline
s c a rp developes in opposite direction to the original
fault scarp due to erosion of weaker strata of the
upthrown block of the fault. Such fault line scarps
are formed at much later date at relatively lower
height (fig. 2.8 (3)). ‘An obsequent fault-line scarp
will normally represent a later stage o f development
than a consequent scarp, though this is not invariably
the case__ the reversal of the fault line scarp is
possible only because a Iall in base-level has ex­
posed to denudation the weak rocks on the upthrown
side of the fault* (R.J. Small, 1970). (c) Resequent
Fig. 2.8 : Developmen t ofdifferent types o f fault line scarps
over normalfaults e.g. 1. consequent or normal,
2. obliteration o f scarps by erosion^ 3. ob­
sequent and 4. resesquent fault-line scarps.
(Ill) DOMED STRUCTURE
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Domed structure results either due to upw'arping of crustal surface effected by diastrophic force or
due to intrusion of magma into surficial rocks. The
superincumbent material is removed due to pro­
longed erosion and the underlying structure is ex­
posed to the surface and few typical features like
cuesta, hogback and ridges are formed. Domqs
formed due to upwarping are characterized by the
development of radial or centrifugal drainage
p a tte rn having a set of sequent streams which fol­
low the slope gradient e.g. consequent, subsequent,
obsequent and resequent streams (fig. 2.9). For de­
tails, see ‘fluvial cycle of erosion on domal struc­
ture’ (chapter 10).
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32
GEOMORPHOLOGY
Fig. 2.9: Development o f erosional landforms over
domed structure.
(IV) UNICLINAL/HOMOCLINAL STRUCTURE
Homoclinal structures are those which repre­
sent inclined rock strata at uniform dip angle caused
by general regional tilt. ‘These structures are formed
in two main ways, either by the uplift of a sequence
of off-lapping coastal plain sediments or as part of
one limb of a large dome or fold' (Chorley, Schumm
and Sugden, 1985). Such structure^ involve both
hard and soft rocks and sometimes there are alter­
nate bands of soft and resistant rocks and hence these
are subjected to differential erosion with the result
rivers form their valleys along soft rocks giving birth
to the formation of strike vales while resistant rock
beds arc less eroded and hence become lines of
asymmetrical hills known as cuesta having one side
of steeper scarp slopes while other side represents
gentle slope. Homoclinal structure formed due to
general tilting of sedimentary beds of coastal plains
and retreat of sea water presents ideal condition for
the development o f consequent and subsequent
streams. The consequent streams drain seaward across
resistant and weak rock beds alike but the lateral
subsequent streams develop on the less resistan
rocks. Thus, lines of asymmetrical cuesta features
having steeper landward facing scarp slopes and
gentler seaward facing dipslopes are formed parallel
to the coast lines (fig. 2.10).
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Fig. 2.10 Development o f trellis drainage and cuesta on uniclinal strata of coastal plain, after Von Engeln. 1948.
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
(V) HORIZONTAL STRUCTURE AND LANDFORMS
33
3. Rock Characteristics
If the regional sedimentary formation has
developed well defined horizontal beds o f resistant
rocks, say sandstones, then after fluvial erosion
tabular topography is formed. The uplifted hori­
zontal thick beds of relatively resistant rocks (e.g.
sandstones) lying over shales and siltstones, when
subjected to erosion from all sides, produce isolated
flat-topped hills known as m esa (ot large size) and
butte (of smaller size). Such numerous features
have developed over Rew a and Bhander plateau
(M.P.). In fact, Bhander plateau having massive
sandstone capping over shales and siltstones of
Vindhyan formation is itself an example of very
extensive mesa while a few smaller mesas have
developed around Bhander plateau (fig. 3.8). Look
hill in Jawa block of R ew a district (M.P.) is fine
example o f mesa capped with Vindhyan sandstone
overlying shales. The horizontal structures having
alternate bands of sandstones and shales or sand­
stone - limestone - shale, are sub jected to differential
erosion and give birth to step -lik e scarps and bench
topography (stru ctu ra l ben ch es). The Grand
Canyon (Colorado, U.S.A.) having horizontal beds
of alternate bands o f sandstone, limestone and shale
presents a picturesque view of well pronounced
structural benches flanking the deeply entrenched
canyon of the Colorado river. Even horizontally dis­
posed basaltic beds of different phases of lava flow
sometimes are of varying resistance and after vigorous
erosion produce picturesque stepped topography
(e.g. source tributaries of the Savitn and the Krishna
rivers have produced Grand-Cany on - like topography
around M ah ab alesh w ar plateau in Maharashtra).
T ooth -lik e top ograp h y develops over resistant
quartzitic sandstones whereas impervious and insolu­
ble resistant rock produces rounded topography.
The rock characteristics include chemical and
mechanical composition of rocks, permeability and
impermeability, joint patterns, rock resistance etc.
Chemical composition determines nature o f chem i­
cal weathering of rocks which in turn determines
resultant landforms. For example, limestone co m ­
posed of calcium carbonate is very much prone to
intense chcmical weathering under humid condition
and hence running and groundwater, when acts on
carbonate rocks, produces picturesque limestone
landscape (karst topography). Dolomite having m ag­
nesium carbonate as principal constituent is also
readily attacked by acidulated water. Some sandstones
having calcareous or ferrous cements undergo the
process of chemical erosion under warm and humid
climatic conditions. The prolonged chemical action
on some common minerals and rocks produces dif­
ferent kinds of clay (e.g. terra-rosa on limestone and
dolomite, kaolinite on granite and gneiss, clay on.
chalk etc.) the thick accumulation o f which on sur­
face causes soil crecp and slumping resulting in
gentle rounding of the existing landscape. The re­
sultant soil creep produces convex slope.
Rock joints are considered to be significant
attribute of rock characteristics which influence
landform characteristics both at macro-and microscalcs because rock joints determine permeability of
rocks, their weathering and erosion and detailed
shape of some landforms. A well jointed rock being
more permeable is subjected to intense chemical
weathering because it allows dow nward movement
of corroding agent (solvent water). Similarly, rocks
having well developed joint pattern are vulnerable to
mechanical disintegration into big rock blocks. A
permeable rock having well developed joint system
reduces surface drainage by allowing efficient dow n­
ward movement of water and hence fluvial erosion
and transportation at the surface is remarkably mini­
mized. Joint pattern also influences development of
drainage pattern at least on well jointed rocks. Widely
jointed granites after weathering produces ‘tors’
while poorly jointed rocks like besalt are chemically
decomposed enmass.
‘Perm eability refers to the capacity of a rock
for allowing water to pass through it. A prime factor
determining the degree of permeability is the pres­
ence of bedding planes and joints, but in some
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Fig. 2.11 : Development ofstripped and structural plains
on horizontal structure, after W.M. Davis and
C.A. Cotton (in Chorley et. al, 1985).
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34
GEOMORPHOLOGY
instances porosity can promote and enhance perme­
ability. Porosity refers to the presence of small gaps
between the constituent mineral particles of a rock’
(R.J. Small, 1976). Highly permeable rocks disfa­
vour erosion as these allow more efficient perco­
lation of water and hence form high relief topogra­
phy e.g. high plateaus, escarpments and ridges (for
example, sandstones and limestones) while imper­
meable rocks (e.g. clay and shale), which are me­
chanically weak, discourage percolation of water
and hence are more readily eroded and produce
undulating vales and lowlands.
Rock h a rd n e ss is always considered in rel­
ative sense because a particular rock may be resis­
tant to weathering and erosion in certain environ­
mental condition while the same rock may be less
resistant or weak in other environmental conditions.
For example, limestone becomes weak rock in hu­
mid climatic conditions because of active dissolution
of rock but the same rock becomes relatively resist­
ant in hot and dry climate due to absence of water.
Normally, less resistant rocks (e.g. clay, shale) are
more rapidly eroded and give birth to lowland while
resistant rocks produce bold topography due to less
erosion. It may be mentioned that ‘however, the
relationship between rock strength and erosive proc­
esses is by no means straightforward’ (R.J. Small,
1970).
It may be concluded that geological structure
and lithological characteristics no doubt are impor­
tant factors in influencing landform characteristics
in different environmental conditions but it is not the
only factor controlling landscape development and
landform characteristics.
CO N CEPT 3
‘Geomorphic processes leave their distinctive
imprints upon landforms and each geomorphic proc­
ess develops its own characteristic assemblage o f
landforms. ”
’
W.D. Thornbury
Meaning
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Geomorphic process and geomorphic agent
are considered separately for different meaning by a
few geomorphologists. According to W.D. Thornbury
geomorphic processes include all those physical and
chemical changes which affect earth s surface and
are involved in the evolution and development of
landforms of varying sizes and magnitudes, while
geomorphic age it is medium through which eroded
materials are transported from the place of erosion to
the place of deposition. On an average, geomorphic
process and geomorphic agent should be considered
as synonym. In fact, geomorphic processes include
those physical processes which operate on the earth s
surface both internally and externally (Savindra Singh,
1991, p. 277). ‘In geomorphology the word process
is a noun used to define dynamic actions or events in
geomorphological systems which involve the appli­
cations of forces over gradients. Such actions are
caused by agents such as wind and falling rain,
waves and tides, river and soil water solution (J.B.
Thorns, 1979).
Types of Processes
On the basis of source-place geomorphic pro­
cesses are divided into two broad categories e.g.
endogenetic and exogenetic processes. The inter­
nal or endogenetic processes originating from within
the earth fostered by diastrophic and sudden forces,
caused by thermal conditions of the interior of the
earth and varying physical and chemical properties
of the materials of which the earth’s interior has been
composed of, introduce vertical irregularities on the
earth's surface and create various suites of habitats
for biotic communities. The significant endogenetic
or hypogenous processes include diastrophic, seis­
mic and volcanic activities. The external or exogenous
(epigene) processes originating from the atmos­
phere driven by solar energy change the face of the
earths surface through erosional and depositional
activities. Exogenetic processes include running water
(rivers— fluvial process), groundwater, sea waves
(marine process), wind (aeolian process), glacier
(glacial process), periglacial process etc. Besides,
weathering and mass translocation of rockwaste are
also included in this category. There are certain
extraterrestrial processes (e.g. fall of meteorites)
which are neither related to the interior of the earth
nor to the atmospheric conditions.
The endogenetic and exogenetic processes
are considered competing forces which are engaged
in continual conflict. Thus, the interactions between
endogenetic and exogenetic processes produce com­
plex sets of physical landscapes. Endogenetic pro­
cesses are considered as constructional processes
as these produce surface irregularities in the form of
mountains, plateaus, faults, folds, volcanic cones,
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
35
Mechanism of Processes
depressions etc. on the earth’s surface. On the other
hand, exogenetic processes are called as grada­
tional or planation processes because these are
continuously engaged in removing vertical irregu­
larities created by endogenetic processes through
denudational mechanism (including both weather­
ing and erosion) and depositional activities. The
planation work o f the earth s surface irregularities is
accomplished through (i) degradation (e.g. weath­
ering and erosion wherein upstanding landmass is
lowered dow n by weathering (disintegration and
decomposition and consequent downslope transfer
of weathered materials) and erosional activities (this
mechanism o f planation is called as level down) and
(ii) aggradation (deposition, this mechanism of
planation is termed as level up).
Exogenetic processes are generally called as
erosional processes which perform three-phase
work i.e. erosion, transportation and deposition.
These external processes are also known as
destructional processes because these are con­
tinuously engaged in the destruction o f relief fea­
tures created by the endogenetic forces through
weathering, erosional and depositional activities.
The erosional work by differrent processes
is performed through the mechanism o f chem ical
erosion (corrosion or solution), corrasion or ab ra­
sion, attrition, hydraulic action, deflation, plucking, polishing, crvoturbation etc.
1. Erosional Work
(l) The mechanism o f corrosion involves
dissolution of the soluble materials (carbonate rocks)
through the process of disintegration and decom po­
sition of carbonate rocks. Solution refers to dissolu­
tion of soluble particles and minerals from the rocks
with the help of water (having dissolved carbon
dioxide in it) in motion. Solution o f rocks depends
on the nature of rocks, solubility o f solids, ratio
between the volume of solvent (water) and the solids
and contact time of solvent and solids. Running
water (streams), groundwater and sea waves effec­
tively corrode carbonate rocks. Streams remove
soluble materials from the parent rocks and the
chemically eroded sediments are suspended in the
running water of the streams. Most o f the salts are
removed from the bedrocks through the process o f
carbonation and are suspended in river water. A c­
cording to the estimate of Murray every cubic mile
water of the river contains about 7,62,587 tons o f
suspended minerals of which about 50 per cent is
calcium carbonate. On an average, the world rivers
discharge about 6,500 cubic miles o f water into the
oceans ever)' year. On the basis of Murray’s estimate
it may be inferred that about 5 billion tons o f miner­
als are removed from the bedrocks by the world
rivers every year. Groundwater is the most effective
efficient process of corrosion o f carbonate rocks.
Rainwater mixed with atmospheric and organic car­
bon dioxide (C O J becomes active solvent agent and
disintegrates and dissolves carbonate rocks at the
sunface and below the surface to form numerous
types of solutional landforms. It may be pointed out
that amount of dissolution o f carbonate rocks by
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A. E pigene or Exogenous Processes
(gradational/planation/denudational processes)
1. D egradational work
(i) weathering
(ii) massmovement of rockwaste
(iii) erosion
fa) running water (rivers)
(b) groundwater
(c) marine process (sea waves)
(d) aeolian process (wind)
(e) glaciers
( f ) periglacial process
2. Aggradational work
Deposition of weathered and eroded
sediments
(a) running water (rivers)
(b) groundwater
(c) Sea waves
(d) wind
(e) glaciers
B. H ypogene o r E n do g en o u s Processes
(constructional forces;
1. D iastrophic movements
(i) Epeirogenetic force
(a) emergence
(b) submergence
(ii) Orogenetic force
fa) faulting
(b) folding
(c) warping
C. Extra-terrestrial Process
.D. Anthropogenous G e o m o r p h o lo g ic a l Processes
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36
g eo m orphology
lateral abrasion leading to erosion of valley walls
Lateral abrasion causes valley widening while the
vertical abrasion leads to valley incision wherein the
erosion tools drill the valley floor through the mecha­
nism of pot hole drilling resulting into the forma­
tion of pot holes (cylindrical depressions in the
valley floors). Vertical abrasion becomes most ef­
fective when the erosion tools are of large size
(boulders and cobbles), and of high angularity (high
calibre)
and the channel gradient is steep causing
(2)
Abrasion or corrasion involves the re­
high
velocity
of running water. Vertical abrasion
moval ot loosened materials of the rocks by different
and
valley
incision
(downcutting) becomes more
erosional processes in different manner. The degree
effective during juvenile (youthful) stage of river
o f abrasion depends on a host of variables, e.g.
and valley development when channel gradient and
nature of erosion tools, nature of erosional processes
velocity are very high. A b rasion by groundwater
(e.g. rivers, groundwater, seawaves, glacier, wind
is not effective because of exceedingly slow move­
etc.), nature of geomaterials (rocks), force of ero­
ment
of water and very fine sediments, that too in
sional processes, nature of ground surface, gradient
solution
form. A brasion by sea waves is very effec­
etc. Erosion tools refer to all those solid materials
tive
because
high-energy storm waves charged with
(boulders, cobbles, pebbles, sands etc.) with the help
large cobbles drill out circular pot-holes and abrade
of which erosional agents attack and abrade the
the
standing bedrocks. W ind armed with entrained
rocks. The efficiency of abrasion depends on size,
sand
grains as tools of erosion attacks the rocks and
amount and calibre of erosion tools. Calibre of
erodes
them through the mechanism of abrasion,
erosion tools means shape and angularity of eroding
pitting, grooving and polishing (collectively called
materials (e.g. whether rounded or angular in shape).
as
sandblasting). Aeolian abrasion is minimum at
Generally speaking, large-size and quantity and high
ground-level
because wind velocity is retarded by
calibre (more angular) of erosion tools make the
friction. Similarly, wind ceases to become an ero­
erosional processes most effective abrading agents.
sive agent beyond the height of 182 cm frcm the
Nearly all of the erosional processes resort to abra­
ground surface level because normal wind cannot
sion work but the mode of abrasion differs from
lift and carry particles of average size. Thus, maxi­
process to process.
mum abrasion occurs at the height between 20-25
Abrasion by running water (rivers) refers to
cm
from the ground surface. A b rasion by glaciers
the breakdown of rocks and removal of loosened
depends
on the rate o f movement of glaciers, gradi­
materials of rocks of valley walls and valley floors
ent and nature of erosion tools. Normally, glacier
with the help o f erosion tools as referred to above.
erodes
its bed and valley walls with the help of
The erosional tools or river loads move down the
erosion tools (coarse debris) through the m echanism
channel gradient along with water and thus strike
of abrasion.
against the rocks which come in contact with them.
The repetition of this mechanism weakens the rocks
(3)
Hydraulic action involves the break
which are ultimately loosened, broken down and
down ot rocks due to pressure exerted by water
dislodged. The nature and magnitude of abrasion by
currents ot the rivers and sea waves. In fact, hydrau­
rivers depends on the nature, size and calibre (angu­
lic action is the mechanical loosening and rem o v al
larity) of erosion tools, channel gradient and How
ol materials of rocks by water alone (without the
velocity. Boulders, cobbles and pebbles of various
help of erosion tools). It may be pointed out that
sizes and angularity are by far the most important
chemical erosion (corrosion), abrasion and hydrau
tools of erosion which arc generally called as drill­
lie action are so intimately interrelated that it ,s
ing tool*. The erosional mechanism of abrasion
unwise to think of exclusively pure
action
operates in two ways e.g. (i) vertical erosion leading
without chemical erosion and abrasion. The rivers
to erosion and deepening of valley floors and (ii)
erode their valley walls through hydraulic action-
groundwater depends on temperature, partial pres­
sure ot atmospheric carbon dioxide, organic carbon
dioxide, chemical composition of carbonate rocks
(e.g. calcium carbonate - limestone, magnesium
\.arbonate - dolomite etc.). rock joints, nature and
velocity of flow o f groundwater, contact time of
groundw ater with the rock etc. Sea waves also resort
to corrosion o f coastal rocks and form numerous
coves and caves of varying dimensions.
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h y d r a u l i c
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
37
Sea waves are more powerful agents of hydraulic
action which refers to impact of gushing water on the
coastal rocks. Powerful storm sea waves attack the
coastal rocks with enormous hammer-blows amounting.to 50 kg f per square centimeter (gravity force (f)
is 9.81 and hence sea waves normally hurl a force of
50 kgf per square centimeter of the coastal rocks).
Repeated blows of striking sea waves enlarge the
incipient joints, fracture patterns and thus help in
breaking the rocks into smaller joint-bounded blocks.
The waves are capable of dislodging larger fragment
of rocks weighing several tonnes in weight. This
process of displacement of rock fragments is also
called as quarrying and sulcking.
tion (frost weathering), congelffluctlon (soil creep),
frost heave (bulging and subs'dence), nlvationfsnow
patch erosion) etc. are significant weathering and
transportation rnecahnisms performed by periglacial
processes. T he mechanism o f erosion, though very
slow and insignificant, by periglacial processes is
cryoturbation.
2. Transportational Work
T he tra n sp o rta tio n w o rk by different
gcmorphic processes is accomplished through flota­
tion, suspension, traction, saltation , solution etc.
Running water (rivers) transports sedim ents through
traction, saltation, suspension and solution. G.K.
Gilbert has propounded a law o f stream tran sp or­
tation based on the relationship between stream
velocity and its transporting power. T he law is
known as Gilbert's Sixth Pow er L aw according to
which the transportation power o f the streams is
proportional to the sixth power o f their velocity
(transportation power a stream velocity*). The mecha­
nism ofsaltation by streams involves the transport
of load with water currents wherein coarse load
moves downward by leaping and jum ping through
valley floors. This mechanism is extremely slow.
The downstream movement of loose materials on
the valley floor is called traction. The bed-load
being transported by traction method consists o f
gravels, pebbles, cobbles and boulders. The m ateri­
als of medium size are suspended in water (called as
suspended load) due to buoyancy. The transporta­
tion by streams is unidirectional (downstream).
The soluble materials are dissolved in water and
become invisible and are transported downstream in
solution. The groundw ater transports dissolved
materials in suspended form.
(4) A ttrition refers to mechanical tear and
wear of erosion tools suffered by themselves. The
boulders, cobbles, pebbles etc. while moving down­
stream with water collide against each other and thus
are fragmented into smaller and finer pieces in the
transit. The rock pieces are so broken down that
ultimately they are comminuted into coarse to fine
sands which are transported down the channel in
suspension. Attrition by marine process involves
mechanical tear and wear and consequential break­
down of rock fragments due to their mutual collision
effected by backwash and rip currents which remove
the fragments from the cliff base and transport them
towards the sea. A ttrition by wind involves me­
chanical breakdown o f rock particles while they arc
transported by wind through the processes ofsaltation
and surface creep. Saltating grains frequently rise to
a height of 50 centimeters over a sand bed and upto
2 meters over pebbly surface by combined action of
aerodynamic lift and the impact of other saltating
grains which return back to the ground surface.
Thus, the particles, while they are moving, collide
against each other and are further comminuted in
finer particles.
The transportational w ork o f sea w aves
varies significantly from other agents o f erosion and
transportation. For example, the backw ash or un ­
dertow currents (moving from the sea coasts and
beaches towards the sea) pick up the eroded materi­
als and transport them seaward but the uprushing
breaker waves or su rf curents pick up these mate­
rials and bring them again to the coasts and beaches.
Thus, the transportation o f materials takes place
from the coastland towards the sea and from sea
towards the coast (i.e. to and fro transportation).
Longshore curents transport the materials parallel to
the coast and shorelines. The materials involved in
(5) D eflation, the process of removing, lift­
ing and blowing away dry and loose particles of
sands and dusts by winds, is called deflation (de­
rived from Latin word deflatus, which means blow­
ing away). Long continued deflation removes most
of loose materials and thus depressions or hollows
known as ‘b low ou ts’ are formed and bedrocks are
exposed to wind abrasion.
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(6) The mechanism o f periglacial processes is
quite different to other processes i.e. congelifrac-
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g e o m o r ph o l o g y
the transportation by sea waves includc sands, silts,
gravels, pebbles, cobbles, and some times boulders.
The transportation by sea waves is bi-directional.
The tran sportation al w ork o f wind differs
significantly trom other agents of erosion because
the direction ot wind is highly variable and hence
wind-lransportation is m ulti-directional. Wind trans­
port involves cntrainm cnt of loosened grains of
sands and dust in the air and their movement to new
locations. Very tine materials with a diameter of less
than 0.2 m m are kept in suspension by upward
m oving air. Such materials kept in suspension are
called dusts and extremely fine particulate matters
arc called haze or snioke. The materials larger than
0.2 m m in diam eter are transported through the
m echanism o f bouncing, leaping or jumping, which
is know n as saltation whereas the loosened materi­
als transported through surface creep or traction
alw ays touch the ground. A very significant aspect
o f wind transport is that materials are transported at
the ground surface and above the ground surface.
Only very fine materials are transported to greater
distances in one step while coarser materials are
transported in stages and steps by rolling, leaping
and jum ping.
G lacial sedim ents (glacial drifts) are trans­
ported along the sides and floors of the glacial
valleys and snouts o f the glaciers. The debris falling
directly into the galcier is transported without touch­
ing the bottom of the glacier while the debris falling
on to the surface of a glacier is transported downslope
with the moving ice mass. The materials derived
from the bed by subglacial erosion are transported
by touching the bottom.
The mechanism o f transportation of materials
in periglacial areas has been described variously e.g.
con geliflu ction , congeliturbatityi (it is also used
for erosion) and gelifluction etc. Solifluction or
co ng elifluction involves only soil-flow in the
periglacial areas having permafrost below activc
layer. According to K. Bryan (1946) cryoturbation
includes all types o f massmovement of regolith in
periglacial environment. Recently, gelifluction is
used in place o f congelifluction.
3. Depositional Work
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The deposition o f load carricd by the streams
is effected by a variety of factors e.g. (i) decrease in
channel gradient, (ii) spreading o f river water oVer
large areas, (iii) obstruction in channel flow, (iV)
decrease in the volum e and discharge o f water, (V)
decrease in stream velocity, (vi) increase in sedi­
ment load etc. The decrease in stream velocity re­
duces the transporting pow er o f the streams which
are forced to leave additional sedim ent load to settle
down. Sedimentation takes place in the river beds,
flood plains and at the river m ouths (to form deltas).
Depositional work by groundw ater takes
place when solvent (water) becomes oversturated.
As the chemical erosion o f carbonate rocks contin­
ues, the groundwater or say solvent receives more
and more solutes and becomes saturated with dis­
solved sediments. Since the m ovem ent of ground­
water is exceedingly slow it cannot transport enough
sediments. Thus, chemical erosion (dissolution) and
sedimentation (deposition) take place together. Largesized sediments immediately settle down whereas
suspended fine sediments kept in supended form are
deposited due to following factors— (i) due to ob­
struction in the flow path of groundwater and conse­
quent decrease in the flow velocity of solvent, (ii)
due to evaporation of water because of increase in
temperature and consequent decrease in the volume
of groundwater and increase in solute-water ratio,
(iii) due todecrease in solution capacity of groundwater
etc. Deposition of sediment takes place at various
places in different forms e.g. (i) at the floor of caves,
(ii) along the ceiling o f caves, (iii) in the rock joints
etc. All the deposits in the caverns are collectively
called speleothem s of which calcite is the common
constituent. Banded calcareous deposits are called
travertines whereas the calcareous deposits, softer
than travertines, at the cave mouths are called tufa
or calc-tufa. The calcareous deposits from dripping
water in dry caves are called dripstones.
Deposition by m arine processes (sea waves)
is most variable and temporary in character because
surl currents or breakers abrade the coasts and back-'
wash or undertow curents and rip currents bring
them seaward and deposit at the lower segments of
wave-cut platforms but these sediments are again
picked up by surf curents and breakers and are
brought to the coasts. Thus, marine sediments are
reworked by sea waves again and again. When there
isequilibrium between incoming supplies of sediments
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PUNDAMBNTAI, r'ONCHKrS IN OliOMOKl'HOUKiY
39
Procaaa-Raaponaa (Landform*)
by backwash on the wave-cut platforms, a profile of
equilibrium it achieved If the wavc-cui rock plat­
form i» characterised by steep slope towards the
oceanic slope, Ihc destructive waves become very
aciive and thus resultant powerful backwash re­
moves sediments from the landward side so lhal Ihc
slope ol the platform is lessened, On the other hand,
if the slope of wave-cut platform is less sleep,
constructive waves become more effective as they
favoui sedimentation and beach deposition on Ihc
landward side so that the slope of the platform
becomcs steeper. Beaches, cusps, bars and associ­
ated features arc formed due to marine sedimenta­
tion but since the depositional work depends on a
variety of factors and fiencc these features are sel­
dom permanent as they are built and depleted and
rebuilt.
fl is evident from the aforesaid analysis o f the
mechanism o f the operation (erosional and depositional work) of exogenetic processes that the mode
of operation of each geomorphic process is different
from the other process and hence the landforms
produced by each process may be differentiated if
wc accept the m ono-process concept e.g. dissected
by streams, abraded by wind, glaciated by glaciers
etc. Before the emergence o f process geomorphol­
ogy, landscape characteristics o f a gi ven region were
studied as a response of com bined actions o f all
processes operating in that region (p oly-p rocess
approach) but now operational mechanism (ero­
sional, transportational and depositional works) o f
each gcomorphic proccss and resultant landforms
(erosional, depositional and relict) are studied sepa­
rately. Bccausc of distinctive characteristics the
landforms produced by one particular process may
be dif ferentiated from those produced by other proc­
esses. For example, alluvial cones and fans, flood
plains, gorges and canyons, natural levees, river
meanders, and deltas arc indicative o f the work o f
fluvial process (streams) while solutional holes and
depressions (sink and swallow holes, dolines, polje,
uvalas etc.), limestone caves, stallectites and stalag­
mites arc the products of the erosional and d e­
positional works of groundwater on carbonate rocks.
Sand dunes indicate the depositional work by winds,
moraines, drumlins, eskers etc. and U-shaped valley
with hanging valley, cirque, aretes etc. denote the
product of glacial proccss whereas patterned ground
t.stone circles, stone nets, stone polygons etc.), pingo,
thermokarst, solifluctatc lobes and terraces, stone
glacier, blockfields, altiplanation terraces.nivation
hollows etc. arc the exclusive responses o f periglacial
processes.
D e p o s itio n a l w ork by w in d
is gcornorphologically very important because significant
features like sand dunes and loess arc formed.
Deposition of wind blown sediments occurs due to
marked reduction in wind speed and obstructions
caused by bushes, forests, marshes and swamps,
lakes, big rivers, walls etc. Sands arc deposited on
both windward and leeward sides ol fixed obstruc­
tions. '/b e accum ulated sand mounds on cither side
of the obstructions arc called sand shadow s whereas
accumulations o f sands between obstacles arccallcd
sand drifts.
'I hc rock debris carried by glaciers arc collcctively callcd as glacial drifts which include (i) till,
(n) ice-con tact stratified drift, (iii) outw ash etc.
'Ibe unsorted arid non-stratificd glacial drifts arc
called tills which arc further divided into ( IJ basal or
lodgem ent till and (ii) ablation till. I he basal or
lodgement tills are com pact, tough, dense and rich in
clay. These arc deposited at the base of the glaciers.
'Ihc ablation tills are poorly consolidated and lack in
fine grain *ize. The ice-contact stratified drifts are
modified glacial debris by inellwater. Till is also
known as b oulder clay. Glacial debris arc divided
into 3 type* on the basis of location e.g. (i)en glacial
d ebris, which is transported within the glaciers, (ii)
supraglacial d ebris, which exists on the surface of
the glacier and (in* su bglacial d eb ris, which is
found at the base o f the glacier. The glacial deposi­
tion it generally called m oraine.
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On the basis o f landform assem blage having
d istin ctiv e ch a ra cteristics produced by each
geomorphic proccss the landforms may be classified
genetically as initiated by W .M. Davis. The genetic
classification o f landforms enables us to understand
the mode of origin o f particular landform, sequence
o f developm ent and gcomorphic history. Generally,
a few terms arc used to indicate certain sets o f
general landforms which do not give any clue for
their genesis e.g. ridge, gorge, scarp, column, mound,
table, hole, depression, valley, trough, cave, dune,
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40
GEOMORPHOLOGY
terrace, bench, cone, fan, creek, plain, hummocks,
cliff, polygon etc. If these and many more forms are
associated with the processes which have formed
them, then we may have knowledge of their genesis
and developmental mechanism. For example, plain
is formed by several processes e.g. flood deposition
(flood plain), peneplanation (peneplain, all by flu­
vial process), karst plain (by groundwater), pediplain
(by scarp retreat and pedimentation in semi-arid
climate), panplain (by coalescence of flood plains
caused by lateral erosion by fluvial process), etchplain
(by etching and washing of debris by streams in
savan na region), alluvial plain (deposition by
streams), outw ash plain (due to fluvio-glacial ac­
tion), cryoplain (due to cryoplanation) etc. The
following additional examples support genetic as­
pect of landforms and processes responsible for their
formation.
thermokarst (frost thaw, periglacial process); hum.
mocks -earth hummock (frost weathering, periglacial
process), turf hummock (frost weather ring, periglacial
process); polygon - frost polygon (frost weathering,
periglacial process), stone polygons (frost heave’
periglacial process), cliffs - river cliff (fluvial), sea
cliff ( erosional, sea waves) ; platform - wave-cut
platform (erosional, sea waves), wave-built plat­
form (depositional-sea waves) etc.
Ridg e— anticlinal ridge (tectonic), synclinal
ridge (erosional, streams), hogback ridge (tectonic
and erosional), beach ridges (depositional, sea waves),
morainic ridge (deposition, glacier), nivation ridge
(depositional, periglacial process) etc.; gorge-river
g o rg e ; scarp— faultscarp (tectonic), fault-line scarp
(erosional, fluvial process), normal, obsequent and
resequent fault-line scarps (erosional, fluvial proc­
ess), resurrected scarp (erosional, fluvial) e t c .; val­
leys- (V-shaped valley-fluvial), rift valley (tectonic),
hanging valley (both fluvial and glacial), karst val­
ley, blind valley, solution valley (solution by
groundwater), glacial valley (U-shaped, glacial ero­
sion), dry valley (periglacial process) etc.
(e.g.) in v o lu tio n s , h u m m o c k s , pingo,
thermokarst, frost cliffs, frost polygons etc.).
(1) CONGELIFRACTATE LANDFORMS
(due to frost weatehring and frost-heave)
(2) PATTERNED GROUND
(due to frost heave and solifluction)
(e.g. stone circles, stone nets, stone polygons,
stone garlands, stone stripes)
(3) CONTORTED SURFACE
(due to frost heave and congelifraction)
(4) SOUFLUCTATE/CONGELWLUCTATE LANDFORMS
(due to differences in the movement of so­
lifluction)
(e.g. solifluction terraces, solifluction lobes,
talus, stratified scree).
(5) ALTIPLANATION LANDFORMS
(e.g. altiplanation terraces, altiplanation cliffs,
tors, frost-riven cliffs, blockfields, stone streams)
(6) NIVATION LANDFORMS
(e.g. nivation hollows, nivation platforms,
nivation ridge, nivation fans)
(7) PERIGLACIO-FLUVIAL LANDFORMS
(e.g. thaw gullies, thaw ravines— thaw badland)
It may be pointed out that it is easier, theo­
retically, to associate a particular landform with *
particular process but very few landforms are of
mono-process origin because most o f the land­
forms have been developed by more than one pro­
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>
Holes — sink hole, swallow hole (solutional
work by groundwater), thaw sink (periglacial proc­
ess), pot-hole (fluvial process, erosional) etc.; moundmima mound (congelifluctate, periglacial process);
dunes-sand dunes (aeolian, depositional), bencheswave-cut benches (erosional, seawaves), structural
benches (tectonic and structural), giant benches (ero­
sional, glacier); terraces-river terraces, paired ter­
races, fluvial terraces (both erosional and depositional,
streams), marine terraces (erosional, sea waves),
solifluction terraces( soil creep, periglacial process),
altiplanation terraces (frost action, periglacial pro­
cess), nivation tcrraces (depositional, periglacial
process); cone - alluvial cone (depositional, streams),
volcanic cone (depositional, vulcanicity); karst
(solution al,. g r o u n d w a te r, c a rb o n a te rocks),
Savindra Singh's genetic classification of
periglacial landforms (1974) presents an ideal ex­
ample of process-related and mechanism-related
(weathering, erosion, transportation and deposition)
landforms developed in periglacial areas as fol­
lows—
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41
FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
c
It has been accepted that geomorphic pro­
cesses play significant role in the evolution and
changes in the form of hillslopes but there is con­
trasting opinion about the evolution of slopes in
terms of mono-process or poly-process origin. Con­
vexity and concavity have been related to soil creep
and rainwash respectively. Fenneman (1908) ex­
plained the evolution of convexo-concave slope
through rainwash alone. H. Baulig (1950) postulated
the concept of poly-process origin and development
of hillslope wherein soil creep and rainwash were
accepted as the most important processes. The
summital convexity o f a convexo-concave hillslope
in humid temperate region results due to soil creep as
it becomes more active than rainwash due to less
volume of rainwater while basal concavity is formed
by rill and gully erosion because soil creep becomes
less effective due to abundance of surface water
(coming from upslope). A few geomorphologists are
of the view that soil creep and rainwash instead of
working separately work together to form different
slope forms.
The advocates o f climatic geomorphology
have pleaded for the study of landforms association
of a climatic region together involving all the pro­
cesses active therein and have suggested to divide
the world into morphogenetic regions e.g. L.C. Peltier
(1950) has divided the world into glacial, periglacial,
boreal, maritime, selva, moderate, savanna, arid and
semi-arid morphogenetic regions (see chapter 4).
CONCEPT 4
“As the differen t erosional agencies act on the
earth’s surface there is p ro d u ced a sequence o f
landform s having distinctive characteristics at the
successive stages o f their developm ent. ”
— W.D. Thornbury
The present concept is related to one o f ‘trio
o f D avis’ (landscape is a function o f structure,
process and tim e ) which was given more impotance
rather was overemphasised by Davis. The stage
concept is based on the concept o f ‘cyclic tim e’
which involves long geological period o f millions o f
years and larger spatial areas. It may be pointed out
that Davis used ‘tim e’ as a p ro cess’ rather than ‘an
attribute’ of landscape developm ent wherein he
envisaged sequential changes in landform s through
tim e.’ ‘For Davis, the concept o f evolution implied
an inevitable, continuous and broadly irreversible
process of change producing an orderly sequence o f
landform transformation, w'herein earlier forms could
be considered as stages in a progression leading to
later forms. By this model, time becam e not a tem ­
poral frame work within which events could occur,
but a process itself leading to an inevitable p rogres­
sion of change’ (Chorley, Schum m and Sugden,
1985, p. 17).
Thus, following Davis there is progressive
change in landform characteristics with the passage
of time. Davis’ model o f cycle o f erosion is based on
the conccpt of ‘low -entropy closed sy ste m ’ w herein
initial potential' energy in the closed system is p ro ­
vided by initial rapid rate short-period upliftm ent o f
landscape. With the passage o f time and continuous
erosion there is equal distribution o f energy in the
geomorphic system so that all com ponents o f the
system are characterized by equal energy levels and
hence in the absence o f difference in the energy
levels of different com ponents of the system, the
state of m axim um disord er and hence m axim u m
entropy is achieved wherein no further w ork is
performed because there is no energy flow and the
ultimate result is the developm ent o f peneplain.
Though this concept o f Davis (closed geom orphic
system characterized by evolutionary changes in the
landform geom etry) is subject to severe criticism but
‘for Davis, each stage or his cycle w as associated
with declining potential energy as the relief was
worn down, and each stage was characterized by an
assemblage o f landforms (i.e. valley-side slopes,
drainage patterns etc.) having geom etries appropri­
ate to the local potential energy expressed by the
difference in level between the land surface (ridge
crest or top o f w ater divides) and some, low er
elevation (base level, valley floor) tow ards which
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cesses i.e. they are of poly-process origin as dif­
ferent geomorphic processes seldom operate in iso­
lation. For example, even in periglacial environment
(as referred to above) different geometrical patterns
(very commor.iy called as patterned ground having
definite geometrical patterns such as circle, net,
polygon, stripe etc.) are formed due to combined
actions of frost heave and solifluction whereas invo­
lutions, h um m ocks and pingo are formed by
congelifraction (frost weathering) and altiplanation
landforms (as referred to above) are the result of
combined actions of solifluction, ni vation, frost heave
and congelifraction.
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GEOMORPHOLOGY
42
mum clue to high velocity o f flow rate and high
kinetic energy because o f very steep channel gradient High transporting capacity enables the rivers to
carry big boulders (tools o f erosion) o f fairly good
degradation was directed’ (Chorley, Schumm and
Sugden, 1985).
W.M. Davis divided the whole time span of
geographical cycle of erosion (fig. 3.1) into three
distinct stages of varying landform geometries on
the basis of time span of human life e.g. (i) youthful
stage characterized by higher energy landform s,
(ii) m ature stage of m edium -energy landform s
and (iii) old or penultim ate stage of low but equal
energy-landform s. Based on further variations in
landform characteristics he further divided each
stage into early, m iddle and late e.g. (i) early youth,
middle youth and late youth, (ii) early mature, mid­
dle mature and late mature and (iii) early old, middle
old and late old stages. Based on Davisian model of
normal cycle of erosion in humid temperate regions
the following sequences of landform evolution through
successive stages of youth, mature and old stages
may be presented in the support of the above con­
cept.
size (large size) and calibre (angular boulders) which
help in the p othole d rillin g o f the river beds. It may
be mentioned that pothole drilling is the mostactivc
and powerful process o f vertical erosion (valley
deepening) in the juvenile stage o f the normal cycle
1. Youthful stage
The region experiences rapid short-period
upliftment resulting into m axim um potential en­
ergy and m inim um entropy. ‘The potential energy
of landform o f initial uplift is the dominant source of
energy input (potential energy) and that, thereafter,
there is an irreversible equalization of energy levels
throughout the landform assemblage, leading ulti­
mately to a spatially uniform terrain-the peneplain
or peneplane’ (at the end of the cycle i.e. old stage)
(Chorley, Schumm and Sugden, 1985).
River capture is the m ost characteristic fea­
ture of the juvenile stage o f the normal cycle of
erosion. Main rivers having steeper channel gradi­
ents and more volume o f w ater capture smaller
streams of relatively low channel gradient through
headward erosion.
2. Mature stage
Marked valley deepening through vertical
erosion uring youthful stage results in pronounced
ecrease in channel gradient and consequent de­
crease in flow velocity with the result the arrival of
y maturity is heralded by marked decrease in
ey eepening due to (i) decrease in channel
gra lent, (ii) decrease in the velocity o f river flow,
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Consequent streams (which follow the re­
gional slope) are originated with the upliftment of
land area due to endogenetic forces. In the begin­
ning, the streams are less in number and short in
length. Very few tributaries of the master conse­
quent streams are originated. The slopes are domi­
nated by numerous rills and gullies rather than big
streams. These rills and gullies lengthen their lon­
gitudinal profiles (increase their lengths) through
headw ard erosion. Gradually and gradually the
main streams deepen their valleys. The origin and
evolution of tributaries of master streams give birth
to the development of dendritic drainage pattern.
The rivers are continuously engaged in rapid rate o f
downcutting o f their valleys (valley incision) be­
cause the transporting capacity o f the rivers is maxi­
of erosion.
The valley becom es very narrow and deep
with almost vertical side walls due to continuous
active downcutting o f the valley floors at exceed­
ingly fast rate. The valley side slopes are convex in
plan. Thus, the resultant ju v e n ile valleys are Vshaped and are called gorges and canyons. The
valley floors are studded with num erous pot holes
which are the result of pothole drilling. The inter­
stream areas or w ater d iv id es (land area between
the valleys of two major stream s) are extensive and
wide and these are least affected by denudational
processes because valley w id en in g by lateral ero­
sion is less effective in the early and middle youth
stages. The valley thalw egs (longitudinal profiles of
the rivers) are characterized by num erous rapids and
waterfalls which always recede upstream. Most of
the waterfalls and knick points disappear by late
youth. The rivers are underloaded (not having the
required amount ol sedim ent load according to their
transporting capacity) and thus available energy is
more than the work to be done. The rivers are well
integrated by the end o f youth when maximum
relative reliefs are formed.
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
(iii) decrease in the transporting capacity etc. Conse­
quently, valley w idening through active lateral
erosion dominates over valley incision through
43
downcutting. The convex slope o f valley sides is
progressively transformed into u niform or recti­
linear slope and the gorges and canyons charac­
terized by deep and narrow valleys are replaced by
broad and flat valleys.
The rivers deposit big boulders at the foothill
zones due to sudden decrease in channel gradient
and hence marked decrease in the transporting ca­
pacity of the rivers. These materials form alluvial
fans and alluvial cones. The gradual expansion o f
these fans and cones due to their continuous grow th
result in the formation of extensive p ied m o n t plains
through the coalescence o f several fans and cones.
Interstream areas or water divides are continuously
narrowed due to backw asting caused by active
lateral erosion and valley widening. Thus, inter­
stream areas are transformed into narrow ridges. T he
major river erodes down to its base level (sea level)
and becomes ‘graded’. Thus, the longitudinal pro ­
file of the master river becomes the p rofile o f eq u i­
librium wherein there is balance between available
energy and the work to be done i.e. balance betw een
the transporting capacity and total sedim ent load to
be transported. Because of marked decrease in
channel gradient rivers adopt sinuous courses and
develop numerous m eanders and loop s in their
courses. Extensive flood plains are formed due to
sedimentation o f alluvia. Rivers frequently change
their courses because o f gentle to level slopes o f the
flood plains. Numerous ox-bow lakes are formed
due to straightening o f highly m eandering loops.
Deposition of sediments on either side of the river
valleys leads to the formation o f natural levees.
3. Old Stage
The old stage is characterized by further de­
crease in channel gradient, almost total absence o f
valley deepening, decrease in the num ber of tribu­
tary streams and flattening o f valleys. Tributary
streams also attain the base level of erosion and are
graded. Lateral erosion and consequent backwasting
eliminates most of interstream areas. Valleys be­
come broad and flat characterized by concave slopes
o f valley sides. Downcutting o f the valleys is totally
absent. Weathering processes are most active. Thus,
lateral erosion, downwasting and weathering con­
tinuously degrade the land resulting into gradual
lowering of absolute altitude and water divides.
Interstream areas and water divides are remarkably
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Fig. 2.12 : Stages o f landform development - 1. initial
stage. 2. early youth. 3. late youth. 4. early
m aturity. 5. m aturity and 6. old stage
(peneplain).
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kinetic energy (through precipitation and channel
flow), o f thermal energy (through insolation from
the sun) and o f chem ical energy (through disintegra­
tion and decom position o f rocks) and there is con­
tinuous export o f energy and m atter out o f the system
and hence the geom orphic system tends to be in
equilibrium condition. Thus, the Davisian concept
o f sequential changes o f landform s through succes­
reduced in height and are changed to lowland but
they still rise above the surrounding areas. Trans­
porting capacity o f the rivers becomes minimum
because o f very low channel gradient and thus the
rivers becom e overloaded. Consequently, sedi­
m entation becom es m ost active during this stage.
The rivers adopt highly meandering courses. The
extensive flood plains with level to gentle slopes (2°5°) and very low channel gradient make the river
flow so sluggish that the main channel of the river is
divided into num erous distributaries and thus the
river becom es braided. Valley sides are bordered by
extensive natural levees which are also known as
bluffs which denote the farthest limit of recurrent
floods o f the concerned rivers. Rivers deposit and
form extensive deltas at their mouths if other envi­
ronm ental conditions remain favourable for delta
formation.
sive stages is not tenable.
Moreover, it is argued that the life cycle of
landform development cannot be equated with hu­
man life cycle because the time span o f three stages
o f the latter (youth, mature and old) is almost fixed
and one stage changes to the next stage after certain
time period but this is not possible in the case of
landscapes because a region having weak and less
resistant rocks is quickly eroded dow n and youth
stage advances to mature stage within shorter period
of time but if the region is characterized by hard and
resistant rocks then the period o f youth stage is
lengthened and change from youth to mature stage is
much delayed. This is why W. Penck pleaded for the
rejection of Davis’ concept, ‘landscape is a function
of structure, process and time (stage)’, and postu­
lated the concept that, ‘landforms reflect the ratio
between the intensity of endogenetic processes (i.e.
rate of upliftment) and the magnitude o f displace­
ment of materials by exogenetic processes (the rate
of erosion and removal o f weathered and eroded
materials)’. Inspite o f some inherent weaknesses in
Davisian model the stage concept cannot be alto­
gether discarded. Even Penck is supposed to have
deliberately avoided the use o f stage concept in his
model of landscape development either to under­
mine the cyclic concept o f W.M. Davis or to present
a new model. According to Von Engeln (1960)
Penck found escape from the concept o f cyclic
change marked by the stages youth, maturity and old
age . In the place o f stage he used the term
entwickelung meani ng thereby d ev elo p m en t Thus,
in place of youth, mature and old stages he used the
terms aufsteigende entw ickelung (waxing or ac­
celerated rate of developm ent), g le ic h f o r m ig *
entwickelung (uniform rate o f developm ent) and
absteigende enlw ickeluge (waning or decelerating
rate of development). In fact, stage does not mean
specified absolute period o f time rather it denotes
the phase of landform development and hence ‘stage’
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The entire landscape is converted into exten­
sive flat plain o f undulating surface except a few
residual convexo-concave hills which project above
the general flat surface and thus break the monotony
o f reliefless flat plain, called as peneplain. These
residual hills, the result of differential erosion, are
called m onadnocks on the basis of monadnock hills
o f the North-East Applachians in New England
region (USA). The whole landscape is dominated by
concave slope, minimum available energy, both
potential (because o f very low height) and kinetic
energy (due to very low channel gradient) and m axi­
m um entropy (means maximum d i s o r d e r ^ relief,
as the whole area is characterized by featureless
peneplain).
The Davisian model of sequential changes in
landforms through youth (maximum relief, maxi­
m um potential and kinetic energy, narrow and deep
valleys with convex valley sides and minimum en­
tropy), maturity (graded stream profile, broad valley
with rectilinear valleysides) to old stage (equally
distributed energy, broad and flat valleys with con­
cave slope, featureless plains-peneplain, minimum
potential and kinetic energy and maximum entropy)
is possible only in low-entropy closed geomorphic
system but the geomorphic systems having different
landform assemblages are open systems wherein
t h e r e is continuous input of potential energy (through
upliftment of landscape, plate tectonic theory has
demonstrated continuous tectonic activities), of
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLtXJY
45
should be used in relative sense and not in absolute
sense.
It may be pointed out that time and space are
no longer passive factors rather they are active
independent variables which influence both proc­
esses and landforms at micro-meso and macro ncale
resolution levels. ‘At different scale resolution lev­
els, which are mapped out according to our aims and
abilities, different problems arc identified; different
types of explanation arc rele v an t; different levels o f
organization arc appropriate: different variables are
dominant; and different roles o f casue and effect are
assigned’ (Chorley, Schumm and Sugden, 1985).
It may be further argued that each stage of
geomorphic cycle docs not have same time-period.
Further, if the landscape development in different
regions is passing through similar stage (say youth
stage) it does not mean that the time-period of
similar stage is the same in all regions. If two regions
are characterized by same stage of landscape devel­
opment the landform characteristics in both the
regions may be similar but not the same.
The geomorphic scales, very often used in
geomorphological investigations, arc o f two types
e.g. (i) time scale and (ii) spatial scale. The scale
level resolutions depend on the objectives o f study.
For example, if the evolutionary phases of landscape
development ever long period of time involving
larger areas are to be reconstructed, the model o f
Davisian cycle of erosion involving cyclic time
(millions of years) may be more apropriate but if a
component of landform assemblage is to be studied,
a shorter time scale would be more appropriate. It
may be mentioned that conclusions derived about
landform development and processes at one spatial
and temporal scale may not be applicable to other
scales because the influence of dominant variables
changes from one scale to another scale.
CO N CEPT 5
1. ‘G eom orphic scale is a significant param ­
eter in the interpretation o f landform development
and landform characteristics o f geomorphic sys­
tems. '
2. ‘Landscape is function o f time and space \
The geomorphic investigation requires study
of different geomorphic processes (both mode and
rate of operation ) and related landforms of a spatial
unit over definite time-span for having ‘postdiction
(extrapolation from the present to the past of con­
temporary ‘process-form interrelationships) and
prediction’ (future development of landforms). Both
gemorphological processes and landforms are con­
sidered at various levels of spatial and temporal
resolutions. The detailed study of processes through
field instrumentation in small areas over small time
span has revealed significant results regarding their
mode and rate of operation and their influences on
landform characteristics under varying time-intervals. ‘Certainly one major result of process study has
been the relegation of time to the position of a
parameter to be measured rather than a process (as
envisaged by W.M. Davis) in its own right. Another
major result of the change in gemorphological em­
phasis has been a reduction in the spatial and tempo­
ral scales within which landforms are now consid­
ered’ (M.G. Anderson and T.P Burt, 1981). In 1965
an important contribution to the development ol
landform as a function of lime and space (area)
was made by Schumm and Lichty. rhcy argued that
the kind of model we construct for the study of
landform development depends upon the length ol
the time-span wc have in mind (P. McC ullagh,
1978).
Time Scales
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Generally, temporal scales are considered at
three resolution levels e.g. macro-temproal scale
involving millions of years for the study o f mega­
geomorphology, meso-temporal scale involving
thousands of years and micro-temporal scale in­
volving shorter time-span involving tens and hun­
dreds of years. For geomorphic evolution and inter­
pretation temporal scales are, alternatively, consid­
ered at three resolution levels e.g. cy d ic time, graded
time and steady time. Time scale assumes greater
significance in the study o f the rates of operation of
processes and changes occurring in landscapes.
Generally, no perceptible change may occur in the
morphological features during short period of time
because either the force exerted by the processes
may not be enough to introduce significant change
or the processes might have not operated for desired
sufficient length of nine. Any changc in the rate of
the operation of geomorphic process is supposed to
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GEOMORPHOLOGY
46
limc-span involves progressive but slow change in
both process rate and landforms. In a cyclic time
•landforms slowly lose energy and mass as agents of
denudation reduce altitude’ (P. M cCullagh, 1978).
Davisian model o f cycle o f erosion .s based on cyclic
time wherein there is progressive sequential change
in landforms through time i.e. as the erosion begins
with the completion o f upliftm ent there is continu­
ous lowering of reliefs and loss o f energy in such a
way that there is equal distribution o f energy in
geomorphic system so that all co m po nents of the
system are characterized by equal energy levels and
hence in the absence o f difference in the energy
levels o f different co m ponents o f the system the
slate of m axim um d isord er and h ence m axim um
en tro p y is achieved wherein no further w ork is
performed because there is no energy flow and the
ultimate result is the dev elopm ent o f peneplain.
Cyclic time is punctuated by g ra d ed tim e (fig. 2.13
A) having a time-span o f 100 to 1000 years.
bring corresponding changc in the landforms. ‘Some
times the response is instantaneous, as when a large
flood passes through a channel. At other times, the
response may be quite slow or there may be ‘dead
tim e’ when nothing happens to land forms to reveal
the change in process. The time taken for the system
to respond to externally imposed changes is called
its 'reaction tim e’ (J.B. Thornes, 1979).
Cyclic Time
Cyclic time involves longer geological pe­
riod ol time measuring millions o f years (say
10,000,000 years) and very larger spatial areal unit
measuring thousands of square kilometers of arca.This
t
®
■e
C y c l i-c T i m
( 10,000 000 Y e a r s )
h2
S. A. Schumm and R.W. Lichty (1965) have
identified ten drainage basin variables (10) and their
relative importance in term s o f cyclic, graded and
steady time-scales.As regards the cyclic develop­
ment of landforms, tim e, in itial r e li e f (representing
difference of height between ridge crest and valley
doors or between highest and low est parts created by
tectonic events-upliftment and subsidence, vulcanicity
or sea-level changes), geology (both structure-folds,
faults ctc. and lithology-rock types) and clim ate
(precipitation and insolation) are in dep en den t vari­
ables which control landform d e v e lo p m e n t involv­
ing cyclic time-span (long geological period o f time
ranging in millions o f years), w h ereas vegetation
(type and density, d e p e n d in g on p recip ita tio n ,
insolation and geological characteristics), re lie f or
volume o f landmass above base level, h yd rology
(runoff and sedim ent yield p er unit area within the
system-drainage basin), d ra in a g e n etw o rk m orP ° ogy (diainage density ex p essed as total stream
Gr aded
T i me
LU
TIME
CYCLIC
Gr aded Time
( 100 -1000 Y e a r s )
r y
Steady Time
t & f ---- ---------
2:
<
i
(j
GRADED
TIME
©
S t e a d y Time
C 10 Y e a r s . )
V
,nn!f K
I*0 *1 ^aSm area)’ h illslo p e m o rp h ology,
f
.I r°
(discharge o f water and sediment
0,11' . f ^ ? tCir^ 3re ^e Pendent variables which are
n ro e
y aforesaid four independent variables
ime, initia relief, geology and clim ate) but time is
I n s t a n t a n e o u s Ti me
( O n e day )
S TE ADY
T I ME
concenKnnrCant,1|nuC^en^Cnt varia^*e - There are three
2 14A» t h e r e |U br, Um * d e c a * “ l^ i b r iu m (figis progressive but slow rate o f decline
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Fig. 2.13 : Timcscales-(A) cyclic tune, (/ij graded tii
and (C) steady time.
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
47
*
energy level wherein erosional processes act in epi­
sodic manner as envisaged by S.A. Schumm and
R.W. Lichty (1965). Based on the concepts of
geom orphic thresholds and com plex system re­
sponse Schumm postulated that some changes in the
fluvial system are not effected by external factors
(isostatic upliftment) rather these are caused by
inherent geomorphic controls in the'eroding system
e.g. due to erosional and depositional activities.
According to him effective erosion is not a continu­
ous process rather it is episodic in nature and thus the
valley floors are not continuously deepened but are
reduced in discontinuous manner as periods o f ero­
sion are separated by periods of deposition of
sediments to an unstable condition. In other words,
the period of erosion (period o f instability) is fol­
lowed by period of deposition of sediments.W hen
the sediment storage in the valley crosses the thresh­
old value and channel gradient is steepened then the
system becomes unstable and active erosion is initi­
ated resulting in the downcutting (excavation o f
deposited sediments and valley floor) of valley floor.
The process continues till the sediments are flushed
out and again period of deposition is initiated due to
lessening of channel gradient. Thus, the valley floor
becomes stepped. It is apparent that there is period o f
dynamic equilibrium between periods of instability
occasioned by episodic erosion (see chapter 3, and
fig. 3.7). The result is stepped valley floor (fig. 2.14
D=a, b. c, d indicate steps in the valleyfloor). T h i s
dynamic metastable equilibrium model of eipsodic
erosion shows, in addition, that many of the details
of the landscape (e.g. small terraces and recent
alluvial fills) do not need to be explained by the
influence of external variables because they devleop
as an integral part of system evolution’ (Chorley,
Schumm and Sugden, 1985, p. 40).
in form through time leading to establishment of
equilibrium condition in the penultimate stage-old
stage-ot Davisian cycle of erosion), dynamic equi­
librium (tig. 2.14 C) (indicating a condition of
forms oscillating around a moving average value but
also characterized by continuous decline in form
through time e.g. a river’s long pofile characteized
by alternate actions of erosion and deposition) and
dynamic metastable equilibrium (fig. 2 . 14D) (rep-,
resenting ‘a condition of oscillation about a mean
value of form which is trending through time and, at
the same time, is subjected to step-like discontinuities
as a threshold effect appears to promote a sudden
change of form' (Chorley, Schumm and Sugden,
1985) i.e. a condition of equilibrium at insufficient
*
,r4v/Va / I(/V
a aVA Y A V*, v ay I
St eady S ta t e
aa
«
Equ i l ib r iu m
*-
H
Graded Time
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rig. 2.14 : Equilibrium types : A—decay equilibrium.
B—steady stale equilibrium. C—dynamic
equilibrium, D—dynamic mestastable
equilibrium (based on A\ J. Chorley and R. B.
Beckinsale, /980 and SA. Schumm, 1975).
a, b, c and d indicate stepped valley floor.
The time-scale having shorter period (say 100
to 1000 years), during which smaller streams or parts
ot big streams and individual hillslopes in adrainage
network achieve graded stage of steady state equi­
librium (where geomorphic forms of a system, say
drainage basin, oscillate around a stable average
value) due to self regulatory mechanism (i.e. nega­
tive feedback mechanism), is called graded time.
As the timc-span of landscape development is re­
duced the number of controlling (of landforms)
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48
GEOMORPHOLOGY
factors (i.e. independent variables) increases and
number o f dependent variables decreases. For exam­
ple, in a drainage basin time, initial relief, geology
and climate are independent (controlling) variables
in cyclic time but in graded time besides these four
variables, vegetation (type and denisty), relief (above
base level) and hydrology (runoff and sediment
yield per unit area within the system) also become
independent variables (which are dependent vari­
ables in cyclic time). It may be mentioned that time
and initial reliefs, which are very significant control­
ling variables (of landforms) in cyclic time become
insignificant in the development of landforms in
graded time while drainage network morphology,
hillslope morphology and discharge of water and
transport o f sediment out of the system remain
dependent variables e.g. they are controlled by afore­
said independent variables.
may be studied in terms of graded or steady timescale while larger area should be studied in terms of
cyclic time-scale.
Spatial Scales
There has always been shift in the selection of
ideal geomorphic unit having specific areal cover­
age for the study of landforms and geomorphic
processes with varying view points and objectives.
If we go in historical perspective, spatial scales have
varied considerably i.e. from ‘physiographic re­
gions’ of N.M. Fenneman (1914) through Hortons
(1945) ‘drainage basin’ as ideal geomorphic unit to
J.F. Gellert's ‘m orphotops’ or ‘m orphofacies’
(1982). Fenneman's physiographic regions of N.
America on the basis of chronology and uniformity
of geological history and structural geology repre­
sent large spatial scale i.e. macro or mega scale and
further subdivisions of major physiographic regions
into smaller units involved small spatial scale i.e.
meso and micro scales. Bourne ( 1932) based on his
concept of ‘characteristics-site-assem blage’ rec­
ognized morphological regions at two levels e.g. (i)
‘regions of first level were distinguished on the
basis of morphological features produced by ero­
sional and depositional features’ and (ii) regions of
second level were identified on the basis of areal
units having similar environmental conditions for
the development of pedogenic processes, vegetation
etc. R.E. Horton ( 1945) recognized ‘erosional drain­
age basin’ as ideal spatial geomorphic unit for the
study of drainage basin processes and forms. R e­
cently, J.F. Gellert( 1982) recognized ‘m orphotops’
or ‘morphofacies’ as basic units for morphological
regionalization and ‘suggested a uniform shape (mor­
phology, morphometry), homogeneous lithological
structure, uniform origin and d e v e lo p m e n t
(morphogenesis, morphochronology) and uniform
present-day processes (morphodynamics) as the
characteristic features for the identification of
geomorphological regional units’ (Mamta Dubey,
1993). It is apparent that spatial scales have changed
from macro or mega-scale (of earl ier gemorphologists
dealing with the cyclic development o f landforms
and denudation chronology) through m eso-spatial
scale to present - day m icro-spatial scale (in the
case of process geomorphology).
Steady Time
Still shorter time-span (10 to 100 years),
during which a very short reach of the stream or a
single slope segment (e.g. convex or rectilinear or
concave segment) involving very small area reaches
steady state, is called steady time in which there is
balance between erosion, transport and deposition.
The aforesaid seven variables (e.g. time, initial re­
lief, geology, climate, vegetation, volume of relief
above base-level, runoff and sediment yield per unit
a re a w ithin the s y s t e m , d r ai na ge , which are
indipendent variables in cyclic and graded time plus
drainage network morphology and hillslope mor­
phology (which are dependent variables in graded
time) becom e independent variables and only one
variable (i.e. discharge o f water and sediment out of
the geom orphic system (say drainage basin) be­
com es dependent variable in steady time. The in­
s ta n ta n e o u s tim e re fe rs to the condition of form
at a single day.
‘It will be seen that time can be considered as
the most significant independent variable in landform
studies, or regarded as o f relatively little signifi­
cance, depending upon the time-span involved (and
the size o f spatial unit-areal coverage). It is generally
true to say that most modern geomorphological
emphasis is upon studies concerned within graded or
steady tim e’ (P. McCullagh, 1978, p. 11). The
geomorphic system having smaller areal coverage
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It may be mentioned that spatial scale has
much significance in controlling the rate and mecha­
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
nism of operation of processes and their responses
(landforms) as the areal coverages of study areas
change. For example, if a small area (less than one
square kilometer) of gullied zone is selected for the
study of behaviour of runoff, discharge, soil erosion,
sediment transport etc. during strong rainstorms
associated with thunderstorm, the fluvial process is
highly accelerated and the rate of erosion becomes
very high becuase of maximum runoff and discharge
but if the study area is a large drainage basin then the
effect o f strong rainstorm of short duration is ob­
scured as only the part of the basin is affected by high
intensity rainstorms. The post- 1950 geomorphology
lays more emphasis on the study of different aspects
of processes on the basis of field instrumentation and
laboratory experiments. This requires shorter tem­
poral scale (time scale) and smaller spatial unit. It
may be concluded that ‘at different scales of space
different variables become dominant, different lev­
els of generalization may be employed and even
different problems identified’ (Chorley, Schumm
and Sugden, 1985).
C O N C EP T
6
A sim ple geom orphological equation may be
envisaged as a vehicle for the explanation o f landforms
as fo llo w s —
F = f (PM ) dt
K.J. Gregory, 1977
This geomorphological equation envisages
that ‘the landform (F) is the function of process (P),
material (M, geomaterial) and change through time
(dt)’. Gregory stated (1977) that ‘morphology (F) =
function o f processes (P) on materials (M) over
tim e ( t) \ According to him ‘morphology refers to
the form of earth's surface or landform; processes
include the geomorphological processes associated
with weathering, wind, water, ice and massmovement,
and materials connote the rock, soil and superficial
deposits upon which processes operate (Gregory,
1977, p. 137). He has identified four aspects of
interest wherein the equation may be studied at four
the equation (e.g. between form, proccss
and materials) at specific time.
Level 3 : Differentiating the equation, involving
the investigation o f the way in which
some relationships between form, proc­
ess and materials vary over time.
Level 4 : Applying the equation i.e. to apply the
results drawn through aforesaid three
levels of investigation for solving the
environmental problems.
Study of Elements of the Equation
It is necessary to study detailed aspects of
forms (landforms), geom aterials (of which the
landforms have been formed) and processes (which
shape the la n d fo rm s th r o u g h e r o s io n a l and
depositional activities) independently so that the
landscape of a particular geomorphic unit o f a spe­
cific spatial scale may be studied in right perspec­
tive.
Different aspects of forms (landforms) have
been widely studied and given more attention right
from the beginning of geomorphological investiga­
tions to the development of the branch of landform
geography (B. Zakrzewska, 1967). Morphometric
techniques have enabled geomorphologists to study
different morphometric aspects (shapes, amplitude
and dimension) of landforms produced by various
denudational processes. Information derived from
aerial photographs and satellite imageries have also
enriched landform geography. ‘Although the study
of form is a necessary p re-req u isite to later
geomorphological analysis it has been argued that it
should not be an end in itself because it is very
difficult to understand the past development o f form,
the present significance or future character, from
morphology alone’ (E. Derbishire, K.J. Gregory and
J.R. H ails, 1979) and h e n c e m a te r ia ls and
geomorphological processes should also be studied
with equal emphasis.
Geomaterials, of which the landforms are
composed, have not been studied in right perspec­
tive inspite of the fact that geological structure plays
an important role in the evolution of landforms (see
concept 2). Generally, geomaterials include rock
types, geological structure (disposition of rock beds
e.g. folded, faulted, uniclinal, domal etc. structures),
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levels—
Level 1 : Study of elements of the equation, i.e.
investigation of three elements of the
equation (e.g. form, process and materi­
als) independently.
Level 2 : Balancing the equation i.e. tu obtain
relationships between ihe elements ot
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GEOMORPHOLOGY
rock characteristics (mechanical and chcmical com­
position), weathered materials, surficial deposits
and soils. Traditionally, geological structure includes
three aspects viz. lithology or nature of rocks (igne­
ous, sedimentary and metamorphic rocks), arrange­
ment and disposition of rock beds (folded, faulted,
uniclinal, domal etc.) and rock characteristics (chemi­
cal and mechanical composition, permeability and
impermeability, joint patterns, rock resistance etc.).
esses (driving force- operation of processes) and
materials (resisting force) leading to the attainment
of equilibrium when driving force equals the resist­
ing force (see chapter 3, Gilbert’s model).
•
Differentiating the Equation
Differentiating the equation requires to find
out ‘the way in which geomorphological systems
change or adjust over time’. In fact, the geomorphic
investigation requires study of different geomorphic
processes (both mode and rate of operation) and
related landforms o f a spatial unit over definite timespan for having ‘postdiction (extrapolation from the
present to the past of contemporary process-form
interrelationships) and prediction’ (future develop­
ment of landforms). ‘Inclusion of time dimension is
necessary because periods of time may be necessary
for a certain process or assemblage of processes
acting upon particulate materials to produce a spe­
cific form’ (Gregory, 1977). The changes in landforms
may be studied through varying temporal scales e.g.
macro-time scale (cyclic time, involving millions of
years), meso-time scale (involving thousands of
years) and micro-time scale (involving tens of years).
Alternatively, landform changes may be investi­
gated through cyclic time (involving long-term pe­
riod of millions of years), graded time (hundreds of
years) or steady time (tens of years). Time scale
assumes greater significance in the rates of opera­
tion of processes and changes occurring in land­
scapes. Generally, no perceptible change may occur
in the morphological features during short period
because either the force exerted by the processes
may not be enough to introduce significant change
or the processes might have not operated for desired
sufficient length of time. Any change in the rate of
operation of geomorphic process is supposed to
bring corresponding change in the landforms, ‘som e­
times the response is instantaneous, as when a large
flood passes through a channel. At other times the
response may be quite slow or there may be ‘dead
time when nothing happens to landforms to reveal
the change in the process’ (J.B. Thornes, 1979).
Processes (see concept 3) constitute third
element o f the equation and include those physical
processes which operate on the earth's surface both
internally and externally (i.e. endogenetic and
exogenetic processes). A detailed investigation re­
garding three-phase work of geomorphic processes
(i.e. erosion, transportation and deposition) is needed
to understand the mode of origin and development of
landforms of varying scales. The detailed study of
exogenetic geomorphic processes (denudational proc­
esses e.g. fluvial, coastal, glacial, aeolian, periglacial,
groundwater etc.) through field observation and
instrumentation and laboratory experimentation has
gained currency since 1950.
Balancing the Equation
After the detailed investigation of form
(landforms), materials and processes individually
and independently, attempt is made to produce a
general model o f ‘form -processes-m aterials rela­
tionships.9 In other words, an attempt is made to
establish relationships between landform and mate­
rials (structure, see concept 2), between form
(landforms) and processes (see concept 3) and be­
tween form, materials and processes leading to for­
mulation of functional theories of landscape devel­
opment. ‘The system approach is ideally suited to
the identification o f the relationships between the
elements o f the equation and has been instrumental
in clarifying the diverse ways in which indices of
materials, o f process, and of form are related’ (Der­
byshire, Gregory and Hails, 1979). It may be men­
tioned that not only perceptible relationships be­
tween form, processes and materials in any specific
area having definite climatic conditions are investi­
gated but spatial contrasts of the elements of equa­
tion and interrelationships are also studied. The
introduction of equilibrium concept has enabled the
geomorphologists to envisage the landscape devel­
opment on the basis o f relationship between proc­
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Recently, the role o f man (through his eco­
nomic activities) as geomorphic agent has increased
significantly and thus it has becom e necessary to
study the influences of man on geomorphological
processes and their responses (forms) in a particular
area at different stages. For example, the rate o f soil
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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
the measurement o f contemporary environmental
(geomorphological) processes since 1950 ushered
in a new era o f realization o f significance o f human
a c tiv itie s
a ffe c tin g
the
en v iron m en tal
(geomorphological) processes (Savindra Singh, 1991).
erosion in man-impacted gully basins has increased
alarmingly (Savindra Singh, e l at 1995). Similarly,
the impact of human activities on hydrological,
fluvial, coastal, periglacial processes etc. has .in­
creased many fold (see chapter 30). ‘It is possible to
envisage several geomorphological equations each
pertaining to a particular time in an area and each
relating to a particular degree of m an’s influence'
(Gregory, 1977).
CONCEPT 7
4C om plexity o f geom orphic evolution is m ore
common than sim p lic ity .9
W .D. Thornbury
Generally, landform characteristics are ex ­
plained on the basis o f most dom inant controlling
factor on the basic premise that majority o f landforms
are simple and have less com plex geom orphic ev o ­
lution but in reality most o f the landform s are the
result of poly-factor rather than m ono-factor. S ec­
ondly, mono-process evolution o f landform s o f
topofunction or of lithofunction or o f tectonofunction
or o f pedofunction or of clim o-function etc. has been
recently refuted by majority o f geom orphologists
and they have been considered to be the o u tc o m e o f
poly-process evolution. In fact, 'the crux o f the
problems o f landform evolution as to w heth er there
is sequential change in landscape ecology w ith the
march of time (time-dependent approach-cyclic ev o ­
lution of landforms). or an individual process is
competent enough to evolve its own characteristic
landforms (process- form approach), or steady state
of operation o f processes leads to tim e-independent
series of landform (dynamic equilibrium — non-cyclic evolution of landforms), or geologic structure is
the most dominant controlling factor in the evolu ­
tion of landforms (structure-form approach, litho­
function), or each climatic type produces its own
distinctive assemblage of landforms (clim ate-process-form approach— climo-function) etc. still re­
main unresolved' because o f the fact that ‘the basic
factors controlling the genesis and developm en t o f
landforms based on the param eters o f geologic struc­
ture (lith o-fu n ction ). tectonics (tecto n o -fu n ction ),
climatic elements (clim o-function), processes (process-resp o n se), vegetal cov ers iflo ro -fu n ctio n K
pedological characteristics (p ed o-fu n ctioii), human
interference with physical environm ent ( a n th r o p o function), and topographic factors (top o-fan ction )
su bstantially vary both spatially and temporally*
(Savindra Singh. 1985).
Applying the Equation
The knowledge derived through the analysis
of geomorphological equation at three levels viz. (i)
study o f forms, materials and processes individually
and independently, (ii) establishing relationships
between form and materials, between form and proc­
esses and between form-processes-materials, and
(iii) investigation of changes in geomorphic system
and landforms over time (cyclic time, graded time
and steady time) is utilized for 'estimation of the
behaviour of geomorphological systems either in
locations where processes have not been measured
(spatial prediction) or in the future (temporal pre­
diction)' (Derbyshire, Gregory and Hails, 1979).
This becomes the field o f applied geomorphology
having varying dim ensions e.g. environmental
geomorphology, urban geomorphology, geomorphic
engineering etc (see chapters 29 and 30).
‘We can think o f environment as a machine
which we need to control. However, such control
can be achieved only if we fully understand how the
geomorphological machine w orks’ (Derbyshire et.
al., 1979).
T h e equation outline is tentatively offered as
a basis for synthesising contemporary approaches to
geom orphology and it could be extended to physical
geography as a w hole’ (Gregory, 1977). The useful­
ness of geomorphic investigations depends on the
successful application of geomorphic knowledge in
ameliorating different environmental problems cre­
ated by hum an a ctiv ities and acceleration of
geomorphological processes by man as a potent
geomorphic agent. T h e under-emphasis on the study
o f m an's role in c h a n g in g the en v iro n m en ta l
(geomorphological) processes till 1950 was because
of lesser attention paid towards the measurement of
contemporary geomorphological processes and quali­
tative assessment o f the reconstruction o f the effects
of palaeoprocesscs. Increased enthusiam towards
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It may be mentioned that landscape mosaic of
any physiographic region or morphogenetic region
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GEOMORPHOLOGY
52
causc interruptions in cycles o f erosion w hich co m ­
plicate the landform s through rejuvenation and ini­
tiation o f new cyclcs o f erosion,
having a distinctive clim atic regim e is the result o f a
variety o f factors but it may be that one o f the factors
may be most dom inant in shaping the landforms.
The variations and com plexity in landform s arc
introduced due to follow ing reasons—
(v)
C h anges in base-lcvcls o f erosion cause
by negative or positive changes (fall and rise) in seaIcvcls either due to tectonic ev ents (rise o f sea-floor
or subsidence of coastal land— rise in sea level
positive changc or fall in sea-lev el— e ith er due to
subsidence of sea-floor or d u e to uplift o f coastal
land - negative changc in sca-level)or climatic changes
(fall in sea-level or negative ch an g c d u e to glacial ice
(i) T h e p r e s e n t la n d s c a p e s o f d iffe re n t
physiographic regions at least at macro-spatial scale
(m egageom orp h ology) are exam ples o f p alim p sest
top ograp h y ('lik e surfacc which has been written
on m any times after previous incriptions have been
only partially erased; G reek : palin -'‘a g ain ’, psegma‘rubbed o ff’-Chorley et a i , 1985) because these
regions have experienced several phases o f cycles of
erosion and the landform s have evolved very slowly
over long period o f geological time and thus the
landscapes having superim posed effects of climate
and tectonic factors show evidences o f poly-cyclic
evolution and com plexity in their general character­
istics. In fact, successive cycles o f erosion introduce
com plexity in landforms. Fo rex am p le, most parts of
peninsular India exhibit a fine exam ple o f palimpsest
topography having polycyclic reliefs characterized
by different erosion (planation) surfaces at different
elevations.
age or rise in sea-level due to intcrglacial period) are
responsible for the initiation o f successive cy cles o f
erosion and hence polycyclic landform s.
On the basis o f variations in landform ch a ra c ­
teristics H orberg (1952) divided the la n d sca p es o f
the globe into five principal categories viz. (1) sim ­
ple landscapes, (2) co m pou nd lan dscapes, (3) m onocyclic landscapes, (4) m ulti-cyclic landscapes, and
(5) exhum ed or resurrected landscapes.
Sim ple La n d scap e s
Simple landscapes are those w hich are gener­
ally devoid o f com plexity and are the result o f m ono­
process acting during a single cy cle o f erosion. For
exam ple, if we take the case o f a region having
sedim entary rocks consisting o f alternate bands o f
relatively resistant (sandstones) and soft rock beds
(shales) and river as agent o f erosion, the differential
fluvial erosion will give birth to step p ed landscapes.
It may be adm itted that even sim ple la n d sca p e is not
the resulf o f a single g eom orph ic process but for
simplification and generalization the m o st d o m in an t
process is given due im portance and landscape d e ­
velopm ent is studied in term s o f m ost d o m in an t
process (e.g. fluvial landscapes, glaciated landscapes,
periglacial landscapes, aeolian or arid landscapes
etc.). For exam ple, if the landscape o f a given region
is evolved due to the work o f ru nn ing water (river),
the fluvial process undoubtedly is the m ost effective
geom orphic agent but w eathering process (corrasion)
and m assw asting and m asstranslocation (slum ping,
soil creep, mud flow etc.) also play significant role.
Similarly, the solution (corrosion) m echanism is
m o s t d o m i n a n t d e n u d a t i o n a l m ech a n ism by
groundw ater in the areas o f carbonate rocks but
surface w ater (surface ru n o ff resulting form rainfall)
also helps in the evolution o f landforms. In fact, the
(ii) The operation of several geom orphic proc­
esses even during a single cycle o f erosion intro­
duces com plexity in landforms. Forexam ple, though
wind is the most dom inant geom orphic process in
warm and hot arid regions but fluvial process be­
com es occasionally very active when there is occa­
sional heavy rainfall through strong rainstorm (though
very rarely). Consequently, besides aeolian landforms
{e.g. inselbergs, yardang, zeugen, sand dunes etc.),
very interesting fluvial landforms (pediments, bajadas,
playas and badland) are also formed. Similarly,
besides the developm ent o f pure glacial landforms
in glaciated regions, fluvio-glacial landforms (e.g.
kame, eskers, outw ash plains etc.) are also evolved.
(iii) The spatial variations in landform-controlling factors (e.g. lithology, geological structure,
climatic parameters mainly temperature and pre­
cipitation, vegetation, soils, human activities etc.)
within a physiographic or m orphogenetic region
introduce complexity in the landforms,
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(iv )
T e c to n ic
ev en ts
(u p w arp in g ,
downwarping, upliftment, subsidence, folding, fault­
ing etc.) are very important factors for creating
variations in landform characteristics. Tectonic events
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53
FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY
Compound Land scap es
The landscapes, produced by more than one
geomorphic processes and landform controlling fac­
tors, are called as com pound landscapes. In fact,
com pound landscapes are more common in reality
than simple landscapes. The landscapes produced
during Pleistocene glaciation present examples of
com pound landscapes as glacial geomorphic fea­
tures (both erosional and depositional) are found at
higher altitudes while fluvial landforms (produced
by rivers) are found at lower levels. Besides, aeolian
features mainly depositional forms have also devel­
oped. Several exam ples o f compound landscapes are
seen in Utah, New Mexico, Arizona, Nevada etc. of
the U.S.A. where volcanic cones and related vol­
canic landform s and lava-flow related features have
developed in the fluvially originated river valleys.
Tectonic events also introduce complexity in the
landscapes. Com posite fault-line scarps are such
examples. Such features bear the characteristics of
fault plane as well as erosional surface. Such co m ­
posite fault-line scarps are formed when fault scarp
is originated due to faulting resulting in the dow n­
ward m ovem ent o f down thrown block along the
fault plane and subsequent erosion of lower segment
o f fault scarp. Thus, the upper segment is technically
formed (due to faulting) while the lower segment is
erosional.
Mono-cyclic La n d scap e s
The landform s produced in a physiographic
region during a single cycle of erosion are called
m onocylic landform s. Like sim ple landscapes,
monocyclic landscapes are less com m on in reality.
Monocyclic landscapes may be possible along coastal
plains provided
that the coastal plains are not
affected by several phases o f em ergence and sub­
mergence. Monocyclic landforms generally develop
over volcanic cones, lava plains and lava plateaus,
newly formed domes etc. It may be pointed out that
monocyclic landscapes may be both sim ple and
compound.
Poly-cyclic Landscapes
Landscapes produced due to com pletion o f
several cycles of erosion (successive cycles o f e ro ­
sion) in a region are called as poly (multi) cyclic
landscapes (example of palimpsest topography). M ost
of the present-day landscapes are the exam ples o f
multicyclic landscapes which have developed d u r­
ing more than one cycles o f erosion. It m ay be
mentioned that landsforms o f older cycles are not
found in their original forms because they are m o d i­
fied by succeeding phases o f cycle o f erosion and
hence only relic features of older cycles are p r e ­
served. Polycyclic landscapes are identified on the
basis of a few diagnostic and representative landforms
e.g. valley in valley topography (multi-storyed
valleys, topographic discordance), rejuvenated river
valleys, uplifted peneplains, incised m eanders, nick
points or heads of rejuvenation etc.). T he m u lti­
cyclic landscapes are evolved due to rejuvenation
consequent upon lowering o f base level o f erosion
cither due to upliftment or negative ch ange in sealevel (fall in sea-level). Applachian highlands o f the
USA present fine exam ple o f polycyclic landscapes
which have developed because o f three successive
cycles of erosion (viz-Schooley, H arrisberg and
Sommerville cycles o f erosion). T h e D am o d ar river
valley at Rajroppa in Hazaribagh (Bihar, India) and
the N armada valley at B heraghat (near Jabalpur,
M.P.) present ideal exam ples o f rejuvenated valleys
having three-tier te rra c e s on e ith e r side. T he
Chotanagpur region in general and Ranchi plateau in
particular represents exam ples o f polycyclic land­
scapes. Hundrughagh falls on the Subam asekha river,
Johna or G autam dhara falls at the confluence d f the
G unga and the Raru rivers, D assam ghagh falls on
the Kanchi river (a tributary o f the Subarnarekha)
etc. indicate heads o f rejuvenation along the junction
of the central and eastern Ranchi plateau (Bihar).
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concept o f ‘m ono-process landform 9 is related to
the concepts o f clim a tic g eom orp h ology and
m orphogenetic regions wherein it is envisaged that
‘each c lim atic typ e (and hence the resu ltan t
geomorphic process) produces its own characteris­
tic assemblage of landforms’. L.C. Peltier’s classifi­
cation of climatogenetic landforms into nine catego­
ries and division of world landscapes into nine
morphogenetic regions (e.g. glacial, periglacial,
boreal, maritime, selva, moderate, savanna, semiarid and arid morphogenetic regions) is based on the
concept o f climatic geomorphology but it may be
p o in te d o u t that the a d v o c a te s o f c lim a tic
geomorphology have not succeeded in presenting
ample convincing evidences in support of their argu­
ments through diagnostic landforms (e.g. lateritic
feature, inselbergs, pediments, tors etc.).
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54
GEOMORPHOLOOY
Resurrected Landscapes
The resurrected or exhum ed landscapes are
those which were covered with either lava flow
(volcanic eruption) or sedimentation (mainly on the
coastal plains) after their formation but were uncov­
ered at a later date due to denudational processes.
Majority o f landscapes were covered with thick ice
sheets during Pleistocene ice age in North America
and Eurasia but these reappeared after deglaciation
o f ice sheets. M any of the landscapes were buried
under lava sheets in Peninsular India during Creta­
ceous vulcanicity and a few o f them have r^ow been
exhum ed due to erosion of lava cover.
CONCEPT 8
‘Little o f the earth's topography is older than
Tertiary and m ost o f it no older than Pleistocene. ’
W.D. Thornbury
It is argued by majority of geomorphologists
that most o f the present-day landforms are the result
o f geomorphic processes which operated in the T er­
tiary and Quaternary times as the landforms older
than Tertiary have been either obliterated by the
dynamic wheels o f denudational processes or have
been so greatly modified that they have lost their
original shapes and cannot be properly and accu­
ra te ly id e n tif i e d . On the o th e r han d , som e
geomorphologists also argue that the present-day
landform assemblages are the examples of palimpsest
topography and are the result o f past (palaeo) and
present processes.
Though the Himalayan orogeny began either
during late Cretaceous period (M esozoic era) or
Eocene period (Tertiary) but it was not complete
until Pleistocene period but most o f the topographic
details were carved out during Quaternary epoch by
the fluvial processes. The H imalayas are character­
ized by young and rejuvenated landforms e.g. deep,
long and narrow valleys (gorges and canyons), three
paired terraces, waterfalls and rapids etc. The side
effects of the Himalayan orogeny are well observed
in the present-day topographic features o f the
Chotanagpur (Bihar, India). Tertiary epoch regis­
tered three phases o f upliftment and hence interrup­
tions in fluvial cycles of erosion occurred several
times mainly in Palamau uplands and Ranchi Pla­
teau. The marginal areas o f the Ranchi plateau
(including ‘paltands’) characterized by waterfalls
(Hundrughagh falls, G autam dhara or Johna falls,
Dassamghagh falls, Pheruaghagh falls etc.), nick
points and breaks in slopes and juvenile characters
of the rivers where these descend from the escarp­
ments, tell the story of Tertiary upliftments. The
formation of the Gangetic trough consequent upon
the Himalayan orogeny rejuvenated the foreland of
Indian peninsula which is evidenced by the presence
of a series of waterfalls on the northward flowing
rivers which after descending through the foreland
meet the Yamuna and the G anga rivers right from the
extreme western point of the Rewa plateau (M.P.) to
Rohtas plateau in the east (Bihar) e.g. Tons or Purwa
falls (70 m), Chachai falls (127m), Kevti falls (98m),
Odda falls (148 m, all in M.P.), Devdari falls (58 m),
Telharkund falls (80m), Sura falls (120m ), Durgawati
falls (80 m), Dhuan Kund falls, Rahim Kund falls
(168 m) etc. (all in Rohtas plateau, Bihar).
‘It is now clear that an understanding of the
new geology ahd o f tectonics is essential to under­
stand landforms, and not only first order landforms......
and there is an increasing concern with the older
landscapes’ (C.D. Oilier, 1981).
It may be mentioned that erosional and weath­
ering processes, responsible for the creation o f most
of the third order landforms are largely determined
by climatic conditions and hence climatic changes
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The advocates o f this concept (aforesaid)
argue that pre-existing earth's surface was greatly
affected and modified by global Tertiary orogeny
(formation of Alpine-Himalayan chains, Rockies,
Andes, Atlas, Island arcs and festoons of east Asia
etc.) and related rejuvenation o f existing cycles of
erosion and initiation o f new cycles resulting in the
origin of new sets o f landforms world over. The
Quaternary epoch experienced global climatic change
and Pleistocene ice age comprising four glacial
periods (Gunz, Mindel, Riss, Wurm in Europe and
Nebraskan, Kansan, Illinoin and Wisconcin in. N.
America) and alternated by four interglacial periods
(warm period) obliterated and modified nearly all of
the pre-existing landscapes in most o f the regions of
North America and northern Eurasia as the advanc­
ing ice sheets filled up the lowlying areas and low­
ered and rounded sharp peaks and hills. The retreat­
ing ice sheets left morainic deposits behind and thus
numerous morainic ridges and glacial lakes were
formed in North America and Europe.
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55
f u n d a m e n t a l c o n c e p t s in g e o m o r p h o l o g y
through
g e o lo g ic a l
tim e s
h av e
g re a te r
geomorphological significance. ‘Weathering fea­
tures preserved in rocks show that climate and weath­
ering havcchangcd not only in Quaternary times, but
through all geological time’ (C.D. Oilier, 1969). For
example, latcrite profiles of Tertiary period have
been covered by lava sheets in Ireland. It is iilso well
known fact that latcrites arc formed under warm and
humid climate and hence the latcritcs of Ireland
cannot be attributed to present climatic conditions.
‘In Triassic time England was largely a desert, as
was Scotland in Torridonian (Prccambrian) lime. In
contrast, South Africa, India and Australia had gla­
cial climates in Permo-Carboniferous time’ (C.D.
Oilier, 1969).
Some of the relics of landforms resulting
from weathering and erosional processes as a conse­
quence of climatic changes through geological times
have been preserved. For example, most of the
southern hemisphere (S. America, Africa, India,
Australia etc.) were glaciated during upper Carbon­
iferous time. ‘In Mesozoic times the whole world
experienced a warm phase, and glaciation was com­
pletely absent. World climates in the Jurassic were
particularly uniform, but in the upper Jurassic and
Cretaceous climatic variations once again became
important......... at the start o f the Tertiary the world
was still considerably warmer than it is now. There
were no ice caps, trees grew in polar regions, and the
climate was more uniform over the earth’ (C.D.
Oilier, 1969).
"Ifie earth's surface contains many relics of
former gcomorphic processes— landforms that were
created long ago, and remain at the earth's surface.
So in the thinking of time-scale we are concerned
not only with the formation, but also the preserva­
tion of landforms. There are places where actual
landforms, such as river valley systems, have been
preserved for hundreds o f millions o f years' (C. D.
Oilier, 1981). It is pertinent to point out that time
scale is also o f paramount significance in the evolu­
tion of landf orms. For example, some landforms are
created instantaneously following tectonic activity
(e.g. faults and fissures due to tensional forces or due
to seismic events), some features are formed in
weeks and months e.g. due to vulcanicity (volcanic
cones such as ash or cinder cones), erosional activity
(e.g. sand dunes by wind, gullies by storm rains etc.)
while the evolution o f some landforms takes m il­
lions of years such as the formation of planation
surfaces. Plate tectonics have demonstrated that
earth movements leading to upliftment are not sud­
den and rapid rather they are slow and continuous.
It may be concluded that, no doubt, the cli­
matic oscillations and tectonic activities since Terti­
ary and mainly during Quaternary have so greatly
modified (Pleistocene glaciation) pre-existing m or­
phological features that they have lost their original
characteristics at least in North A m erica and north­
ern Europe but many relic geomorphic features o f
longer geological histories are indicative of their
palaeo-genesis. ‘Indeed wherever geomorphic his­
tories are long there seems to be evidence that things
were different in the past’ (C.D. Oilier, 1981).
CO N CEPT 9
'Each clim atic type produces its ow n charac­
teristic assem blage o f landform s \
This concept is based on the basic tenet o f
clim atic geom orphology based on the work o f Von
Richthofen (in China), Passarge, Jenson, Walther,
and Thorbecke (in Africa) and Sapper (in Central
America and Malanesia) and advocated by J. Budel
(1948, 1982), L.C. Peltier (1950), C. Troll (1958),
W.F. Tanner (1961). P. Birot (1968). D.R. Stoddart
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G.H. Ashlay has forcefully pleaded for the
very young nature of landforms at global level and
maintained that ‘most of the word's scenery, its
mountains, valleys, shores, lakes, rivers, waterfalls,
cliffs and canyons are post-Miocene, that nearly all
details have been carved since the emergence of
man, and that few if any land surfaces to day have
any close relation to pre-Miocene surfaces’ (G.H.
Ashlay, 1931). It may be mentioned following C.D.
Oilier that since major parts of N.America and
northern Europe were affected by Pleistocene gla­
ciation and the impact of glaciation on landscapes
was great and perceptible that most of the writers of
geomorphology text books were guided by Pleistocene
bias and subscribed to the above view points. But
there are also many geomorphologists who do not
subscribe to this view point because, according to
them, there are numerous relic (fossil) landforms
which arc not the result o f present-day processes or
to those o f Quaternary times, rather they are quite
old.
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GEOMORPHOLOGY
56
(1969), L. Wilson (1969, 1973),J. Tricart and A.
Cailleaux (1972) etc. The concept envisages that
geomorphic processes, which shape the landscapes,
are determined and controlled by climate which thus
produces distinctive landscapes through processes.
The advocates o f climatic geomorphology
have attempted to validate the influences o f climatic
conditions on the evolution and characteristics of
landforms on the basis of certain diagnostic landforms
such as duricrusts (such as laterites, silcrete, calcrete
etc.), inselbergs, pedim ents, tors etc.
The climatic geomorphologists (Budel, Peltier,
Tricart and Cailleux) have divided the world into
definite morphogenetic (climatogenetic) regions on
the basis o f dom inant weathering and erosional
processes generated by a particular suite o f climatic
parameters.
This concept is further elaborated in chapter
4 (clim atic geom orphology) o f this book separately
in o r d e r to in c lu d e all a s p e c t s o f c l im a t ic
geomorphology and morphogenetic regions.
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It is argued that climatic parameters control
landscape development directly and indirectly. Cer­
tain climatic parameters such as temperature and
and erosional processes while indirect influence o f
climate on landforms is through vegetation and
soils.
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THEORIES OF LANDFORM DEVELOPMENT
L a c k o f c o m m o n l y a c c e p ta b le th e o ry ; s ig n if ic a n c e a n d g o a l s o f
g e o m o r p h i c th e o rie s ; h is to ric a l p e r s p e c tiv e ; b a s e s a n d ty p e s o f
g e o m o r p h i c th e o rie s (teleo lo g ical theory, im m a n e n t th e o ry , h is to ric a l
th e o r y , ta x o n o m ic th e o ry , fu n ctio n al theory, realist th e o ry , c o n v e n t i o n ­
a lis t t h e o r y ) ; m a jo r g e o m o rp h ic th eories o f G. K. G ilb e rt, W .M . D a v is ,
W . P e n c k , L. C. K ing , J. T. H ack, M . M o r is a w a an d S. A . S c h u m m ;
g e o m o r p h i c th e o rie s in In d ian co ntext.
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CHAPTER 3
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57-88
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3
THEORIES OF LANDFORM DEVELOPMENT
This chapter deals with a few aspects of
geomorphic theories viz. lack of commonly accept­
able general theory, significance and goals of
geom orphic theories, historical perspective of
geomorphic theories, bases and types of geomorphic
theories and evaluation of important theories.
explain the landscapes of the earth's surface in all
environments on the basis of a single theory. The
conceptual vacuum created by the rejection o f Davisian
cyclic model of landforms could not be filled up as
yet inspite of postulation of non-cyclic model of
landform development (dynamic equilibrium theory).
3.1 LA C K O F COMMONLY A CC EP TA B LE THEORY
Question arises as to why no such common
theory could be postulated which can be acceptable
to majority of geomorphologists and can be applied
in different environmental conditions. C.G. Higgins
has opined that ‘it would seem that one reason we
lack an acceptable theory of landscape development
is that there is as much diversity of opinion about
structure, process and form as there is diversity
among structure, process and landforms themselves.’
It is, thus, obvious that there is spatial and temporal
variation in the factors controlling the genesis and
development o f landforms e.g. geologic structure,
tectonic events, climatic elements, geomorphic proc­
esses, vegetal covers, pedological characteristics
and human interference with physical environment
through his economic activities, and landscapes are
more complex than simple. In spite of the fact that
complexity of geomorphic evolution is more com ­
mon than simplicity, landform development has
been related to single causative factor by individual
geomorphologist. According to C.G. Higgins the
controversy regarding the theories of landform de­
velopment has surfaced because of the fact that the
theories have been oversimplified. He further cat-
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The crux o f the problems of landform evolu­
tion as to whether there is sequential change in
landform development with the march of time (cy­
clic evolution of landform s, time-dependent series
of landform development), or landform develop­
ment is time-independent and there is dynamic equi­
librium (tim e-independent series of landform de­
v e lo p m e n t or n o n -c y c lic d ev elo p m en t of
landform), or each geomorphic process produces its
own characteristic assemblage of landforms (process-geom orphology), or geological structure is the
most dominant control factor in the evolution of
landforms (structural or geological geomorphology ), or each climatic type produces its own charac­
teristic assemblage of landforms (climate-processfonm approach, clim atic geomorphology), or tec­
tonics play important role in the evolution of landforms
(tectonic geom orphology or tectono-geomorphic
concept), or episodic erosion model is most appro­
priate to explain fluvially originated landforms etc.
still remains unresolved because of the fact that the
postulator has always attempted to describe and
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58
OriOMORFHOIXKJY
cgorically stated that ‘there m ay be no definitive
theory or geom orphic system that can fit all
lan d scap es.’ S. Schumm (1975) has also corrobo­
rated the idea o f Higgins as he has aptly remarked,
‘most models o f geomorphic evolution arc oversim ­
plified and therefore they arc unsatisfactory for
short-term interpretation of landform changc. T here­
fore, a very complcx denudational history of a
landscape may be gcomorphologically norm al.’
more than one theories may be applicable in a region
having uniform environmental conditions e.g. paral­
lel retreat and slope decline may be applicable side
by side. For example, the hillslope having sandstone
capping above weak shales in Bhander plateau (M.F*,)
near Maihar is characterized by all the four elements
of ideal hillslope profile (e.g. summital convexity,
free face, rectilincarity and basal concavity) and is
*undergoing the process of parallel retreat of free facc
clement and slope replacem ent at the basal segment
(Penckian model o f parallel retreat and slope re­
placement) while the con vexo-cogcave hills, girdling
the Bhander plateau (fig. 3.8), which have lost sand­
stone capping because o f prolonged backwasting
and parallel retreat, are undergoing the process of
activc downwasting and slope dcclinc (e.g. Sharda
Pole hill very close to Sharda T em ple hill (fig. 3.8),
popularly known as Maihardevi hill, is experiencing
the process o f slope decline Davisian model of slope
decline.
It may be pointed out that majority o f theo­
rists have postulated their respective geomorphic
theories on the basis o f limited study of landforms in
a small area and thus the results so derived may not
be universal and may not be acceptable to all. It
may not be out ot context to emphasize that there is
so much diversity, variability and complexity in the
landform characteristics and their mode of forma­
tion and their controlling factors (as mentioned
above) that the problems of landscape development
in all parts of the earth's surface and in all environ­
mental conditions cannot be solved on the basis of a
single geomorphic theory rather these can be tackled
on the basis of composite or multiple theories. Thus,
according to C.G. Higgins, ‘we need multiple theo­
ries or different theories for different purposes........
as scientists we may all be seeking a correct or
complete rational answer to landform origins, but if
the natural world is irrational, no internally com ­
plete and substantive theory or system would work.’
It may be concluded that the most compelling
reason for the lack o f com m only acceptable general
geomorphic theory has been the lack o f proper and
meaningful investigation o f processes and landforms
and establishment and explanation of relationships
between geom orphic processes and landforms in
different physiographic regions in correct perspec­
tive. Many of the geom orphologists have related the
present-day geom orphic features Df the earth’s sur­
face to the geom orphic processes operating pres­
ently whereas many o f these landforms are relic
features and the result o f past processes (older than
Quaternary.
Further, all of the geomorphic theories, pos­
tulated so far, lack in elastacity and broader perspec­
tives and are unable to accommodate all aspects and
view points related to genesis and development of
landforms in different environmental conditions in­
volving a host o f landform controlling parameters.
But it may also be pointed out that because of
complexity in landforms and parameters controlling
their evolution no single theory can incorporate all
aspects o f landform development. It is also not
desirable that wc should seek solution of all prob­
lems of landform development from a single theory
or geomorphic system. In fact, wc need multiple
solutions instead of single solution of landformrelated problem. For example, the evolution and
development o f hillslope in varied environmental
conditions may be explained separately involving
alternative theories e.g. slope decline theory, paral­
lel retreat theory, slope replacement theory etc. Even
3.2 S IG N IF IC A N C E AND G O A L S
M ORPHIC T H E O R IE S
OF GEO ­
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In any branch o f science a theory plays an
important role for the developm ent of new concepts
and approaches to the study of scientific problems
and hence the formulation of theories is necessary
for the furthcrencc o f scientific knowledge. Thus,
general theory is also required in geomorphology for
the understanding o f mode o f formation and devel­
opment o f landforms. The main role o f a geomorphic
theory is to in teg rate th ree m a jo r aspccts of
geom orphology e.g. decription (descriptive aspeci).
classification (taxanomic aspect) and genesis and
explanation (genetic/evolution aspect) of landforms
in different environmental conditions. A geomorphic
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59
t h e o r ie s o f l a n d f o r m d e v e l o p m e n t
theory may be formulated on the basis of empirical
generalization, deductions or on the basis of inter­
pretation of observed facts related to landforms and
related geomorphic processes, or on models. Only
that theory becomes most significant and commonly
acceptable which is most general, simple and elastic
so that it can accommodate and explain nearly all
aspects of landforms e.g. right from their mode of
genesis through development to the present form.
In fact, the main problem of landform study
(genesis and development) may be conveniently and
logically, if not unambiguously, tackled if three
lines of geomorphic inquiry viz. precise way of
description of landforms. their classification and
mode of genesis and evolution through time and
space and process-form relationship together with
the mode of operation of processes are taken into
account because ‘a future for geomorphic theory
seems assured by the needs of geologists for a sound
basis for historical interpretation of landscape, of
environmentalists and planner for a sound basis for
predicting man’s effects on the landscapes and of the
science itself for a means of maintaining communi­
cation between its perspective, genetic-historical
and process-oriented linesof inquiry’ (Higgins, 1975).
A c c o r d in g to C.G. H iggin s (1975) a
geomorphic theory must seek the solution of the
following three lines of inquiry related to landforms
and landscapes—
(i) How the landforms can best be described ?
(ii) How these have been formed and how
these have changed through time ?
(iii) Which processes have formed them and
how these processes operate ?
It means a sound and forceful geomorphic
theory must be competent enough to decribe the
landforms, to explain the mode of formation and
historical evolution o f landforms and to identify and
reveal the mode of operation of geomorphic proc­
esses.
According to C.G. Higgins an ideal geomorphic
theory must include the following properties
(i) Simple and easily understandable terms
should be used to describe the landforms.
changes.
3.3 GEOM ORPHIC T H E O R IE S : H ISTO RICA L
P ER SP EC T IV E
Though a well organized and general theory
of landscape development was propounded by W .M.
Davis in 1889 (com plete cycle o f river life) and
1899 (geographical cycle) but a few theories and
concepts related to genesis, evolution and decay of
geomorphic features appeared before Davis e.g.
concept o f catastrophism and James Hutton s con ­
cept of uniform itarianism . In fact, the formulation
of real geomorphic theory began with G.K. Gilbert
though he did not admit himself to be called as a
theorist rather he preferred to be an ‘investigator*
and postulated a set of principles based on broad
generalization regarding the genesis and develop­
ment of landforms in different parts o f the U.S.A.
e.g. law of uniform slope, law o f structure, law of
divides or law of increasing acclivity, law of ten­
dency of equality of action, law of interdependence
of parts etc.
The first real and general geomorphic theory
was postulated by W.M. Davis in the form of ‘geo­
graphical cycle’ in 1899. In the beginning Davis
formulated his model of geographical cycle for the
explanation of landscape development in humid
temperate regions of the world but later on he ap­
plied hiscyclic model fortheexplanation of landforms
in arid regions (arid cycle o f erosion, 1903, 1905,
1930), glaciated regions (glacial cycle o f erosion,
1900, 1906), coastal regions (1912, also by D.W.
Johnson in 1919)etc. D avis’ cyclic model became so
popular that it was applied to explain nearly all of the
landscapes produced by different geomorphic proc­
esses by geomorphologists all over the world even in
Germany where his model was severely criticised
and most of the geomorphologists pleaded for out­
right rejection of Davisian model. Karst cycle of
erosion by Beede (1911) and Cvijic (1948) and
periglacial cycle of erosion by L.C. Peltier (1950, a
German geomorphologist) etc. are such examples of
application of Davisian cyclic model.
It may be pointed out that universal applica­
tion of Davisian model (e.g. fluvial cycle o f erosion,
marine cycle of erosion, karst cyclc o f erosion, arid
cycle oferosion, glacial cycle of erosion and periglacial
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(ii) Theory should be based on contemporary
geological and geomorphic ideologies and thoughts.
(iii)
Theory should present bases for histori­
cal interpretation and future prediction for landform
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GEOMORPHOLOGY
60
tutes to fill the conceptual vacuum created by the
rejection o f Davisian model o f cyclic evolution of
landforms.
cycle o f erosion) w eakened the theory to such an
extent that not only the model was severely criticised
and modifications were suggested but siren was
raised for the total rejection o f the model. Subse­
q u e n tly , W . P e n c k p o s tu la te d his m o d e l o f
‘geom orphic sy stem ’ or ‘m orphological an aly­
sis’— ‘m orp h ologisch c a n a ly se’ in 1924 (posthu­
mous publication o f his work) wherein he rejected
D avis’ evolutionary model involving sequential
changes in landforms and pleaded for time-independent developm ent o f landfom is (dynamic equi­
librium model). C.H. Crickmay's ‘panplanation
cy cle’ (1933) and ‘concept o f unequal a cclivity’
(1975), L.C. King's ‘pediplanation cy cle’ (1948),
‘h illslop e cy cle’ (1953), ‘river cy cle’ (1951) and
‘landscape cy cle’ (1962), J.C. Pugh's ‘savanna
cycle o f erosion ’ (1966), S. A. Schumm's ‘episodic
erosion m od el’ (1975) etc. came as a result of
modifications in Davisian model o f geographical
cycle.
3.3
B A S E S A N D T Y P E S O F G E O M O R P H IC T H E O R IES
If we look into the history o f geom orphic
thoughts for the last two hundred years, it appears
that the bases o f geom orphic theories have been
greatly influenced by the contem porary geological,
scientific and philosophical concepts and ideologies
such as teleological, im m anent, historical, taxo­
nomic, functional, realist, conventionalist etc. con­
cepts and view points w hich b eca m e bases of
geomorphic theories in historical perspective. R.J.
Chorley (1978) has elaborated the bases o f geomorphic
theories in historical perspective and has also Out­
lined the c h a ra c te ristic s o f d if fe r e n t ty p e s o f
geomorphic theories.
(1) T E L E O L O G IC A L TH EO R Y
The teleological base o f geom orph ic theory
in the beginning o f the dev elopm en t o f geom orphic
thoughts was influenced by religious orthodoxy
wherein all the natural events were taken as the
result of God's creation. ‘In som e senses it m ight be
argued until the later part o f the eighteenth century
the true object o f geom orphological study was not
the landform itself but the m ind o f the A lm ighty, o f
which the landform was held to be an outw ard and
visible m a n i f e s t i n ’ (R. J. Chorley, 1978). T hus, it is
obvious that landforms were co nsidered as G o d ’s
creation. Theory o f catastrophism , w hich envisaged
quick and sudden origin and evolution o f all anim ate
and inanimate objects in a very short period o f time,
may be cited as a typical exam ple o f teleological
geom orphic theories. It m ay be m entioned that quick
and widespread events o f larger m agnitude, both in
temporal and spatial contexts (like valcanic erup­
tions, seismic events etc.) formed the basis o f tele­
ological geom orphic theories. Even the earth's age
was calculated to be only a few thousand years.
Events o f sm aller m agnitude (both in spatial and
temporal context) were ignored. T he concept of
sudden change and evolution also sw ep t the biolo­
gists and naturalists (e.g. Cuvier) w ho believed in
abrupt evolution and destruction o f all the Jiving
organisms. R.J. Chorley has aptly rem arked ( l 978)
that ‘the decline o f old teleology was due to break­
Geomorphic theory was given a new turn and
direction in the decades. 1930-40 and 1940-50 when
Krumbein and R.E. Horton (1932 and 1945) intro­
duced quantitative techniques in the interpretation
of geomorphic processes and landforms resulting
therefrom. The introduction of quantification in
geom orphology was further strengthened by A.N.
Strahler (1950, 1952 and 1958). It may be m en­
tioned that after 1950 geomorphologists were least
interested in the formulation or search of geomorphic
theories as they became more interested in the study
o f geomorphic processes (mode of operation) through
field instrumentation and experimentation in the
laboratories and interpretation of landforms result­
ing from these processes. This is the reason that L.C.
King's popular work ‘Canons of Landscape Devel­
o p m e n t’ (1953) and models such as ‘landscape cy­
c le ’, ‘epigene cy cle’ and ‘pediplanation cycle’ could
not draw proper attention rather went unnoticed by
the geomorphologists.
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The forceful rejection o f Davisian evolution­
ary model (cyclic evolution) o f landscape develop­
ment resulted in the postulation o f ‘dynam ic eq u i­
librium th eory’ (A.N. Strahler, 1950, 1952, J.T.
Hack, 1960, 1965, 1975, R.J. Chorley, 1962). The
‘geom orphic threshold th eory’, ‘tectono-landform
th eory’ (M. Morisawa, 1975) and ‘episodic erosion
th eory’ (S.A. Schum m , 1975) appeared as substi­
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61
THEORIES OF LANDFORM DEVELOPMENT
singular past events and description o f landforms in
evolutionary manner. The main goal of geomorphic
theories is retrodiction or reconstruction o f past
events and not prediction of future events and changes
in landforms and processes. Thus, historical theories
are essentially based on the ‘law o f evolution’ or
‘law of historical succession’. Models o f cycle of
erosion, denudation chronology and tectonic theory
fall under the category of historical theories. Scien­
tifically speaking, these theories are not considered
as scientific theories because these are based on
singular events whereas scientific laws are not based
on individual events rather these are based on a host
of events and their recurrence whereas history is
based on unique events and non-repeatable proc­
esses.
Davis' ‘geographical cycle’ is considered to
be the first successful attempt for the formulation o f
theoretical model in geomorphology. This model
aimed at the genetic classification and description o f
landforms on the basis of regional spatial and geo­
logical temporal scales. The model o f denudation
chronology was based on the ‘concept o f historical
succession9. Though both the models (cycles of
erosion in the USA and denudation chronology in
U.K.) were initially framed separately but later on
they merged together. The model of denudation
chronology aims at the reconstruction of successive
stages of the earth's history. Though the main goal of
study is landforms but in reality it remained to be the
study of geological history of a given region. It is
argued that Davisian model of geographical cycle
begins on ttte basis of initial conclusion drawn from
the study of maps of the region concerned and then
attempts to validate the initial conclusion on the
basis of logical arguments and ‘carefully selected
field observation’ which may justify the initial con­
clusion.
down in confidence regarding the magnitude and
frequency of events which it presupposed. It was
natural that it should be replaced by a causal basis of
theory founded upon events of smaller magnitude
both in space and time.’
(2) IMMANENT THEORY
The significance of events of smaller magni­
tude in both space and time, inherent features of
endogenetic and cxogenetic processes, interpreta­
tion of landform characteristics on the basis of their
inherent features and causal basis formed the bases
of immanent theories which became dominant dur­
ing eighteenth and nineteenth centuries as a conse­
quence of rejection of teleological theories. Theory
of uniformitarianism of James Hutton and John
Playfair is a typical example of immanent geomorphic
theories. They believed that spatial patterns of ero­
sion and deposition were auto-correlated. Thus, sci­
entists began to conceptualize inherent relationships
between erosion and deposition, upliftment and sub­
sidence, form and process. The further manifestion
of immanent theories in the nineteenth century was
the development of ideas regarding relationship be­
tween landform and geology and between rocks and
relief. J.P. Lesley, W. Smith and J.W. Powell studied
the relationships between geology and landforms in
much detail and postulated that there was clearcut
expression of structure in landforms. It may be
pointed out that intimate relationship between
lithology and structure and landforms was so deeply
conceived that it was not needed to study the causal
relationships between rocks and reliefs ‘in terms of
detailed studies of the manner by which certain
differences in rock types support the recurring dif­
ferences observed to exist in terrain’ (R.J. Chorley,
1978). At a later date detailed studies of lithology
and structure at smaller spatial scale revealed re­
markable variations in geological structure and thus
immanent theory was further modified and strength­
ened. The micro-level studies and results coming
therefrom convinced the en vestigators that very close
relationship between rocks and relief was possible
only at a larger spatial scale and no profound rela­
tionship between these variables couid be possible at
smaller spatial scale.
Though tectonic theory of W. Penck is more
or less theoretically similar to denudation chronol­
ogy but it could not acclaim as much popularity as
was in the case of the latter because of ‘language,
political and personal considerations on the one
hand, and less technical assumptions on the other’
(R.J. Chorley, 1978).
(3) HISTORICAL THEORY
The historical theories started losing their
ground and popularity after 1950 because these
involved very long temporal (geological time scale
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The base of historical geomorphic theories
has been the historical succession of individual or
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GEOMORPHOLOGY
62
involving hundreds o f m illions o f years) and very
large spatial scales. ‘They (historical theories) broke
down because their time scales were so large and
unsignposted that they becam e the playground for
unbridled and untestable speculation. T he field b e ­
cam e dominated by the spinners o f ingenious his­
torical sagas, following themes that were traditional
both in development and outcom e’ (R.J. Chorley,
1978).
(4) TAXONOMIC THEORY
The availability of huge dataset regarding
landforms after 1890 necessitated the classification
of these data and landform assemblages resulting in
the g row th o f reg io n al ta x o n o m ic s tu d ie s in
g e o m o r p h o l o g y . L ik e h u m a n g e o g r a p h y ,
geomorphology was also armed with dualism wherein
‘the theoretical binality of taxonomy has caused it to
assume the gloss of more challenging theory and
thus in geomorphology we find historical/cyclic,
fu n ctional/clim atic and in teractive/ecological
developments of regional taxonomy, not to mention
the social/utilitarian ones upon which present land
classifications rest’ (R.J. Chorley, 1978). The base
of taxonomic theories was provided by two major
geomorphic concepts of clim atic geom orphology
and m orphological geom orphology which devel­
oped in the beginning of the 20th century mainly in
Germany and France. Considering the paramount
influence of climatic parameters mainly humidity
(precipitation) and temperature on geomorphological
processes and landforms resulting therefrom the
concept of m orphogenetic/m orphoclim atic region
was developed and the division o f the globe into
morphogenetic regions (by J. Budel, 1948, L.C.
Peltier, 1950, W.F. Tanner, 1961, D.R. Stoddart,
1969, L. Wilson, 1969, J. Tricart and A. Cailleux,
1972 etc.) became the major manifestion o f taxo­
nomic theory.
(5) FUNCTIONAL THEORY
(6) R E A L IS T T H E O R Y
Realist theory, in fact, is the extended and
modified form o f functional theory. T he basis of
realist theory is the study o f the structure (geomaterials)
of which the landform s have been form ed and the
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The main basis o f functional theories is func­
tional relationships between forms (landforms) and
processes i.e. cause and effect relationship. The
major methodological shift in geomorphology after
Second World W ar was characterized by the appear­
a n ce o f ‘n ew g e o m o r p h o lo g y ’ , ‘ s c ie n t if ic
geom orphology,’ and ‘qu antitative geom orphology’ as a consequence o f application o f statistical
and m athem atical m e th o d s to the study o f landform s
and processes. T h e p rim ary goal o f the em erg e n c e o f
functional theory was to relate m o rp h o lo g ic a l forms
to their controlling factors. It m a y be m e n tio n e d that
a few geo m o rp h o lo g ists (e.g. G .K . G ilb e rt) used
functional basis for the in terp reta tio n o f landform s
and processes and their in terrela tio n sh ip s even b e ­
fo re th e f o r m a l e m e r g e n c e o f q u a n t i t a t i v e
geom orphology i.e. before the classical w o rk o f R.E.
Horton in 1945 w ho e m p h a siz e d the stu d y o f rela­
tionship betw een erosional la n d fo rm s and gross
hydrological transfers and the d etaile d study of
erosional processes but he could not s u ccee d in
developing ‘a genetic m odel for the d e v e lo p m e n t of
large-scale drainage n e tw o r k ’. T h e e m e rg e n c e of
‘classic f u n c tio n a l s c ie n c e ’ in the d e c a d e 1950-60
augm ented the study o f m e so -s c a le la n d fo rm s which
were taken as the function o f g e o m o rp h ic processes.
Further, the relationship betw een fo rm s and proc­
esses was: substantiated w ith the h elp o f statistical
correlation techniques. T h e study o f fu n ctio n al rela­
tionship betw een the fo rm s (la n d fo rm s) o f m edium
tosmall spatial scale involving rapid tem poral changes
and geom orphic p rocesses and o th e r la n d fo rm c o n ­
trolling factors becam e the focal th e m e o f functional
theory but the required in fo rm atio n o f rap id te m p o ­
ral changc to validate functional re la tio n sh ip s w'as
not forthcoming. T h u s, the fun ction al theory d e­
pended on the co m p cten c e o f statistical and m ath­
ematical m ethods. T h e functional theory faces a
form idable problem o f re la tin g the present-d ay
landforms to the present processes. It m ay be m en­
tioned that most o f the la n d fo rm s o f the earth's
surface are considered to be relict an d the landform
assem blages are e x am p les.o f ‘p a lim p se st top ogra­
phy . The real functional rela tio n sh ip betw een forms
and processes m ay be estab lish ed o nly w hen the rate
of changes o f form s and the rate o f operation of
processes is properly u nd ersto od. T h is necessitates
m easurem ent o f the rate o f o p eratio n o f processes in
the field so that ord ered inform ation m ay be avail­
able but the absence o f su ch d ata b ecam e major
im pedim ent in the validation o f functional theory.
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THEORIES OF LANDFORM DEVELOPMENT
63
physical and chemical processes which are respon­
sible for the developm ent and sustenance o f external
form (o f landforms). In other words, the study of the
detailed causal mechanisms and materials of landforms
on one hand, and the study of their (of processes and
materials) interrelationships forms the basis of real­
ist theory. Thus, realist theory emphasizes the de­
tailed and minute investigations of physical and
chemical mechanisms operating in the geomaterials
and geological structure within the external forms
because these mechanisms are responsible for the
creation, changes and maintenance of geomorphic
features. The realization of the importance of the
aforesaid theme blossomed in the form of the emer­
gence o f realist theory and a significant shift in
m ethodology o f geomorphic investigation appeared
after 1960 wherein micro-scale process study was
preferred to meso-scale form study. Though the seed
o f ‘p ro c e s s r e a lis m ’ was sown by G.K. Gilbert
(1909, 1914), A.K. Sundborg (1956), R.E. Horton
(1945), S.A. Schumm (1956) etc. but this concept
blossomed with the work o f A.E. Scheidegger (1961)
and G.H. Dury (1972). It may be mentioned that a
few geom orphologists became so much engrossed
with ‘process realism ’ that they concentrated on the
study o f the mechanisms o f physical and chemical
weathering processes at very micro-spatial and tem­
poral scales. Here, the geomorphologists face two
major problem s viz. (i) the study of physical and
chemical processes at very micro-spatial and tempo­
ral scales requires specially trained geoscientists in
general and biochemists in particular and this may
not be possible for the geomorphologists, and (ii) the
results draw n through the investigation of processes
at micro-scales may not be applicable for the gener­
alization o f mechanisms o f processes at meso-scale.
vation and accumulation of data regarding landforms
and related geomorphic processes. It may be empha­
sized that neither theory can be formulated without
observation and aquisition o f data nor external real­
ity may be properly understood without theory. The
study o f gully erosion and management in Deoghat
area of Allahabad district (U.P., India) at microspatial (about 56,000 m2 area) and temporal scales
(1991— 1994) by Savindra Singh and Alok Dubey
(1996) is suitable example o f such approach as they
have studied the causal mechanisms o f soil erosion
and gully development in man-impacted (cultivated)
gully basins and have suggested management of
fragile gully basins.
3.4 MAJOR GEOM ORPHIC T H EO R IES
Various theories of landform development
have been formulated by different geomorphologists
from time to time on the basis of contemporary
thoughts prevalent in the field of science o f landforms
(geomorphology). It may be pointed out that most of
the geomorphic theories revolved around two basic
concepts of landform development e.g., ‘sequential
change of landform through time’ (i.e. progressive
and irreversible change involving positive feedback
mechanism) and ‘compensatory change or oscilla­
tory change’ (involving steady state and equilibrium
and governed by negative feedback mechanism).
The significant geomorphic theories include those
of G.K. Gilbert, W.M. Davis, W. Penck, J.T. Hack,
L.C. King, Marie Morisawa, S.A. Schumm etc.
1. Geomorphic Theory of G.K. Gilbert
It may be pointed out at the very outset that
Grove Karl Gilbert did not propound any definite
theory of landform development. He did not prefer
to be called as theorist rather he opted to be an
investigator. According to him theorists are seldom
able to prove their theories while investigators are
always in search of collecting information and data,
through field observation and instrumentation, about
landform characteristics and processes which shape
the landforms. Tentative theories of landform devel­
opment are seldom proved on the basis o f field data.
This is the reason that Gilbert devoted most of his
time in the investigation of landforms and landform
making processes in different parts o f the U.S.A.
(e.g. Great Basin, Bonneville Lake, artesian wells o f
Great Plains, Alaska, Basin Range, Henry M oun­
(7) CONVENTIONALIST TH EO RY
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Conventionalist theory is, in fact, admixture
o f different geom orphic theories. The study of
geom orphic processes and forms at micro-spatial
and temporal scales (base o f realist theory) leading
to human welfare and blending o f utilitarian consid­
erations form the base o f conventionalist theory. The
philosophical base o f such theory is the concept that
no appreciable distinction may be made between
theory and observation because theory is constructed
on the basis o f observation. In other words, the
construction o f geom orphic theory precedes obser­
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GEOMORPHOLOGY
64
Though Gilbert did not specifically claim to
have framed any definite geomorphic theory but on
the basis of his writings and interpretation of landforms
and processes his geomorphic theory may be stated
as follows—
4Landscapes remain in equilibrium condition,
their history/ is rhythm ic punctuated by oscillatoty
changes and their fo rm s are punctuated by frictional,
rhythms arising out o f the m echanism o f driving and
resisting forces. ’
According to Gilbert the identification and
quantification o f fric tio n a l rhythm s (processes) and
determ ination o f their ( o f processes) dynam ic com ­
petition is the m ajor geom orphic problem and the
m ain task before the geom orphologists is to solve
this problem .
The geomorphic principles of G.K. Gilbert
revolve around three major components of his pos­
tulates viz. ‘concept o f quantification’, ‘concept
o f tim e’ and ‘concept o f equilibrium ’.
Gilbert used scientific methods for interpre­
tation o f geom orphic processes and landforms re­
sulting therefrom wherein he gave more emphasis to
‘q u antity’ in place of ‘q u ality’ and applied the laws
of thermodynamics to the analysis o f geological
processes. According to first law o f thermodynam­
ics in any system o f constant mass, energy is neither
created nor destroyed but total energy remains con­
stant and it can be transferred from one type to
another type (the law is known as conservation of
energy) while the second law o f thermodynamics
states that ‘as time passes and the energy within the
system becom es m ore equally distributed the en­
tropy (measure o f order or disorder) increases until,
at the state o f m axim um entropy, all parts o f the
closed system have the sam e energy level’ (R.J.
Chorley et. at, 1985). In other w ords, with the
passage o f time a system tends to achieve m inimum
energy and m axim um entropy (m ax im um disorder).
Gilbert took n ature in the p resen t ten se i.e.
he was more interested in the present forms and
processes and their future trends (prediction) rather
than in the reconstruction o f past events and forms
(retrodiction). His concept o f nature was based on
two fundamental concepts o f natural philosophy i.e.
(i concept o f rhythm ic tim e, and (ii) co n cep t o f
equilibrium .
G ilb e rt's understanding o f ‘tim e ’ was quite
different from geologists’ concept o f time. A ccord­
ing to him geologic time is rhythm ic. ‘A ny event (of
the earth) represents a plexus o f particular rhythm.
The motion o f the earth is the basic rh y th m ’ which
affects climate which in turn affects and controls
processes which create different suites o f landforms.
It may be mentioned that motion o f the earth, which
is responsible for the genesis o f seasons and cli­
mates, includes rotation and revolution o f the earth.
Gilbert attempted to differentiate the traditional con­
cept of evolution (involving continual grow th or
decay on the basis o f basic tenet o f progressive
evolutionary change o f landform s) from non-evolutionary concept involving equilibrium model. He
ciiticised and rejected the evolutionary concept of
geologists involving continuous progressive change
in landforms through time and advocated the con­
cept o f time-independent model o f landform devel­
opment involving dynam ic equilibrium and steady
state.
His concept o f eq u ilib riu m envisages that in
the final form of any functional system ‘the sum of
the forces acting on the final form equalled zero.*
This is also known as the prin ciple o f least force*
The forces in question are o f two types, i.e. driving
force and resisting force. He explained his model o f
equilibrium with specific examples which were based
on his own field studies. First, he applied the concept
o f equilibrium for the explanation o f the formation
o f loccoliths resulting from vulcanicity. The forma­
tion and rise o f laccolith depends on the competence
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tain, California, Sierra Mt. etc.) but did not prostulate
any com m on theory regarding the evolution and
development o f landforms, rather he postulated a set
o f principles regarding different geomorphic fea­
tures viz. ‘law o f uniform slo p e’, ‘law o f stru c­
tu re’, ‘law o f d ivid e’ (law o f increasing acclivity),
‘law o f tendency to eq u ality’, ‘dynam ic equilib­
riu m ’, ‘law o f interdependence o f p arts’ etc. In
fact, Gilbert was ahead o f his time as he propounded
such advanced concepts as ‘steady states’ ‘graded
curve and profile of equilibrium,’ ‘dynamic equilib­
riu m ’ etc. in the beginning of the 20th century which
became the base of the ruling theory o f landform
development (e.g. dynamic equilibrium theory in­
volving time-independent development o f landforms)
and became the pivot of drastic methodological shift
in the post-second world war geomorphology.
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THEORIES OF LANDFORM DEVELOPMENT
of driving force (rise o f m agm a) and resisting force
(overlying pressure o f superincum bent load). The
formation and growth of loccoliths continue so long
as the driving force o f rising m agm a is not countered
by resisting force (acting downward) of equal m ag­
nitude. In other words, so long as driving force
exceeds the resisting force, m agm a continues to rise
upward and loccoliths register continuous growth
but when the driving force is balanced by resisting
force, the state of equilibrium sets in and the growth
of laccoliths becomes static. Thus, the principle of
least work becomes operative wherein the sum of
driving and resisting forces becomes zero.
Gilbert also applied this principle of least
force leading to establishment of equilibrium condi­
tion in the case o f river to elucidate profile of
equilibrium. The downstream flow of river water
(river discharge) is guided by the force of gravity
wherein the potential energy is converted into ki­
netic energy. The driving force in the case of a river
(say energy o f the river system) is provided by its
flow velocity while the resistance is offered by the
bed-load and lithology of river valley. More pre­
cisely, the friction to flow velocity is offered by the
materials of the valley. So long as the system energy
say driving force (flow velocity) equals the resisting
force say frictional force, the state of equilibrium is
established and this condition prevails till the equilib­
rium condition is maintained and thus the principle
of least force works. The long profile of a river which
has attained the equilibrium state is called profile of
equilibrium (i.e. equilibrium of actions) and such
river (in the state o f equilibrium) is called graded
river. It may be mentioned that Gilbert applied the
concept of ‘grad e’ to all of the landforms and
processes which he studied in the field e.g. ‘graded
h each ’ in the case of Bonneville Lake, ‘graded
hillslope’ in the case of Sierra mountain etc.
Thus, Gilbert propounded that ‘the landscape
is the result o f two competing tendencies i.e. ten­
dency towards variability ( when driving force ex­
ceeds resisting fo rce) and tendency towards uni­
fo rm ity (when driving fo rce equals resistingforce).'
2. Geomorphic Theory of Davis
The general theory of landform development
of Davis is not the ‘geographical cycle’ as many of
the geomorphologists believe. His theory m a y b e
expressed as follows—
"There are sequential changes in landforms
through time (passing through youth, m ature and
old stages) and these sequential changes are d i­
rected towards well defined end product-developm ent o f peneplain. ”
The basic goal of Davisian model of geo­
graphical cycle and general theory of landform de­
velopment was to provide basis for a systematic
description and genetic classification of landforms.
The reference system of Davisian general theory of
landform development is ‘that landform s change in
an orderly manner as processes operate through
time such that under uniform external environm en­
tal conditions an orderly sequence o f landform d e ­
velops” (R.C. Palmquist, 1975). Various models
were developed on the basis of this reference system
e.g. normal cycle of erosion, arid cycle of erosion,
glacial cycle of erosion, marine cycle of erosion etc.
Thus, ‘geographical cycle’ is one o f the several
possible models based on Davis’ reference system
of landform development.
Davis postulated his concept o f ‘geographi­
cal cycle’ popularly known as ‘cycle o f erosion9 in
1899 to present a genetic classification and system­
atic description of landforms. His ‘geographical
cycle' has been defined in the following manner.
‘Geographical cycle is a period o f time during
which an uplifted landmass undergoes its transfor­
mation by the process of landsculpture ending into
low featureless plain or peneplain (Davis called
peneplane).’
According to Davis three factors viz. struc­
ture, process and time play important roles in the
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W illia m M o rris D av is, an A m eric an
geomorphologist, was the first geomorphologist to
present a general theory of landform development.
Infact, his theory is the outcome of a set ol theories
and models presented by him from time to time e.g.
(i) ‘com plete cycle o f river I if e \ propounded in his
essay on “The Rivers and Valleys of Pennsylvania’
in 1889, (ii) ‘geographical cycle’ in 1899, (iii)
‘slope evolution’ etc. He postulated the cyclic con­
cept of progressive development o f erosional stream
valleys under the concept o f ‘complete cycle of
river-life’, while through ‘geographical cycle’ he
described the sequential development of landforms
through time.
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GliQMOK PI IOLQG Y
66
(5) Erosion docs not start until the upliftment
is complete. In other words, upliftment
and erosion do not go hand in hand, Thi*
assumption of Davis bccarnc the focal
point o f severe attacks by the critic* of the
cyclic concept.
Davis has described his model o f geographi­
cal cycle through a graph (fig. 3 . 1).
origin and development of landfonns of a particular
place. These three factors are called as ‘Trio o f
D avis’ and his concept is expressed as follows—
‘Landscape is a function of structure, process
and tim e’ (also called as stages by the followers of
Davis).
Structure means lithological (rock types)
and structural characteristics (folding, faulting, joints
etc.) of rocks. Tim e was not only used in temporal
context by Davis but it was also used as a process
itself leading to an irreversible progression of change
of landforms. Process means the agents of denuda­
tion including both, weathering and erosion (run­
ning water in the case of geographical cycle).
The c y c le of erosion begins with the upliftment
o f landmass. There is a rapid rate o f short-period
upliftment of landmass o f hom ogeneous structure.
This phase o f upliftment is not included in the cyclic
time as this phase is, in fact, the preparatory stage of
the cycle of erosion. Fig. 3 . 1 represents the model of
geographical cycle wherein UC (upper curve) and
LC (lower curve) denote the hill-tops or crests of
water divides (absolute relief from mean sea-level)
and valley floors (lowest reliefs from mean sealevel) respectively. The horizontal line denotes time
whereas vertical axis depicts altitude from sea-level,
AC represents maximum absolute relief whereas BC
denotes initial average relief. Initial relief is defined
as difference between upper curve (summits o f wa­
ter divides) and lower curve ( valley floors) o f a
landmass. In other words, relief is defined as the
difference between the highest and the lowest points
of a landmass. A DG line denotes ba.se level of
erosion which represents sea-level. No river can
erode its valley beyond base level (below sca-lcvcl
Thus, base level represents the limit o f maximum
vertical erosion (valley deepening) by the rivers.
The upliftment of the landmass stops after p o im C
(fig. 3.1) as the phase o f upliftment is complete. Now
erosion starts and the whole cycle passes through the
following three stages—
The basic prem ises of Davisian model of
‘geographical cycle’ included the following assump­
tions made by Davis.
(1) Landforms are the evolved products of
the in te r a c tio n s o f e n d o g e n e tic
(diastrophic) forces originating from within
the earth and the external or exogenetic
forces originating from the atmosphere
(denudational processes, agents of weath­
e rin g and e r o s io n - r iv e rs , w ind,
groundwater, sea waves, glaciers and
periglacial processes).
(2) The Evolution of landform takes place in
an orderly manner in such a way that a
systematic sequence of landforms is de­
veloped through time in response to an
environmental change.
(3) Streams erode their valleys rapidly down­
ward until the graded condition is achieved.
(4) There is a short-period rapid rate of up­
liftment in land mass. It may be pointed
out that Davis also described slower rates
o f upliftment if so desired.
(1)
Y outhful S ta g e — Erosion starts after the
completion of the upliftment o f the landmass.
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Fig. 1 1 : Graphical presentation of geographical cycle presented by W.M Davis.
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THEORIES OF LANDFORM DEVELOPMENT
67
T h e top-surfaces or the summits of the water
divides are not affected by erosion because the rivers
are small and widely spaced. Small rivers and short
tributaries are engaged in headward erosion due to
which they extend their length. The process is called
stream len gth en in g (increase in the lengths of the
rivers). B ecause o f steep slope and steep channel
gradient rivers actively deepen their valleys through
vertical erosion aided by po th ole drilling and thus
„ there is gradual increase in the depth of river valleys.
This process is called valley deepening. I he valleys
become deep and narrow characterized by steep
valley side slopes of convex plan. The youthful stage
is characterized by rapid rate of vertical erosion and
valley deepening because fi) the channel gradient is
very steep, (ii) steep channel gradient increases the
velocity and kinetic energy of the river flow, (iii)
increased channel gradient and flow velocity in­
creases the transporting capacity of the rivers, (iv)
increased transporting capacity of the rivers allow
them to carry big boulders of high calibre (more
angular boulders) which help in valley incision (val­
ley deepening through vertical erosion) through
pothole drilling. The lower curve (LC, valley floor)
falls rapidly because of valley deepening but the
upper curve (UC. summits of water divides or
interstrcam areas) remain almost parallel to the hori­
zontal axis (AD, in fig. 3.1) because the summits or
upper parts of the landmass are not affected by
erosion. Thus, relative relief continues to increase
till the end o f youthful stage when ultim ate m axi­
m u m relief (EF, in fig. 3.1) is attained. In nutshell,
the youthful stage is characterized by the following
characteristic features.
(i) Absolute height remains constant (CF is
parallel to the horizontal axis) because of
insignificant lateral erosion.
(ii) Upper curve (UC) representing summits
of water divides is not affected by ero­
sion.
(iii) Lower curve (LC) falls rapidly because of
rapid rate of valley deepening through
vertical erosion.
(iv) Relief (relative) continues to increase.
(v)' Valleys are of V shape characterized by
convex valley side slopes.
(vi) Overall valley form is gorge or canyon.
ally diminish with march of time and
these practically disappear by the end of
late youth. The main river is graded.
(2) M a tu r e Stage— The early mature stage is
heralded by marked lateral erosion and well inte­
grated drainage network. The graded conditions
spread over larger area and most of the tributaries are
graded to base level of erosion. Vertical erosion or
valley deepening is remarkably reduced. The sum ­
mits of water divides arc also eroded and hence there
is marked fall in upper curve (UC) i.e. there is
marked lowering of absolute relief. Thus, absolute
relief and relative relief, both decrease. The lateral
erosion leads to valley widening which transforms
the V shaped valleys o f the youthful stage into wide
valleys with uniform or rectilinear valley sides. The
marked reduction in valley deepening (vertical ero­
sion or valley incision) is because o f substantial
decrease in channel gradients, flow velocity and
transporting capacity of the rivers.
(3) Old Stage— Old stage is characterized by
almost total absence of valley incision but lateral
erosion and valley widening is still active process.
Water divides arc more rapidly eroded. In fact, water
div id es are reduced in d im e n s io n by both,
downwasting and backwasting. Thus, upper curve
falls more rapidly, meaning thereby there is rapid
rate of decrease in absolute height. Relative or avail­
able relief also decreases sharply because of active
lateral erosion but no vertical erosion. Near absence
of valley deepening is due to extremely low channel
gradient and remarkably reduced kinetic energy and
maximum entropy. The valleys become almost flat
with concave valley side slopes. The entire land­
scape is dominated by graded valley-sides and di­
vide crests, broad, open and gently sloping valleys
having extensive flood plains, well developed me­
anders, residual convexo-concave m onadno ck san d
extensive undulating plain of extremelyMow relief.
Thus, the entire landscape is transformed into
peneplain. As revealed by Fig. 3.1 the duration of
old stage is many times as long as youth and maturity
combined together.
Evaluation of the Davisian Model of Landform
Development
Davisian model of landform development
involving progressive changes in landforms through
time and his concept o f ‘geographical cycle’ re­
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(vii) Long profiles of the rivers are character­
ized by rapids and water falls which gradu­
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GEOMORPHOLOGY
68
(6) His model is capable o f both predictions and
historical interpretation o f landform evolution
(retrodictions).
ceived world wide recognition and the geomorphologists readily applied his model in their geomorphological investigations. The academic intoxica­
tion of Davis’ model o f cycle of erosion continued
from its inception in 1899 to 1950 when the model
had to face serious challenges though hi';, model was
being criticised from the very beginning of its pos­
tulation. S. Judson (1975) while commenting on
Davis' geographical cycle remarked, “His grasp of
time, space and change; his com m and of detail; and
his ability to order his information and frame his
arguments remind us again that we arc in the pres­
ence o f a giant” . C. G. H iggins (1975) admitted that
“ Davis system came to dominate both teaching and
research in the descriptive and genetic-historical
aspects o f geomorphology. Its continued validity is
attested in part by continuing objections to it by
recent critics such as R.C. Flemal (1971) and C.R.
Twidale (1975), that such an obviously flawed doc­
trine could have enjoyed such prolonged popularity
among large segment of the geomorphic community
suggests that there must be compelling reasons for
its appeal” (Charles G. Higgins. 1975).
NEGATIVE ASPECTS OF DAVIS' MODEL
(1) Davis' concept o f upliftment is not acceptable.
He has described rapid rate o f upliftment of
short duration but as evidenced by plate tec­
tonics upliftment is exceedingly a show and
long continued process.
(2) D av is' c o n c e p t o f r e la tio n s h ip b etw ee n
upliftment and erosion is erroneous. A ccord­
ing to him no erosion can start unless upliftment
is complete. Can erosion wait for the com ple­
tion o f upliftment ? It is a natural process that
as the land rises, erosion begins. Davis has
answered this question. He adm itted that he
deliberately excluded erosion from the phase
of upliftment because o f tw o reasons- (i) to
make the model sim p le,a n d (ii) erosion is
insignificant during the phase o f upliftment.
(3) The Davisian model requires a long period of
crustal stability for the com pletion o f cycle of
erosion but such eventless long period is
tectonically not possible as is evidenced by
plate tectonics according to w hich plates are
always in motion and the crust is very often
affected by tectonic events. Davis has also
offered explanation to this objection. Accord­
ing to him, if crustal stability for desired period
is not possible, the cycle o f erosion is inter­
rupted and fresh cycle o f erosion may start.
POSITIVE A SP EC T S O F DAVIS' MODEL
(1) Davis' model of geographical cycle is highly
simple and applicable.
(2) He presented his model in a very lucid, com ­
pelling and disarming style using very simple
but expressive language. Commenting on the
language of Davis used in his model Bryan
remarked, “Davis' rhetorical style is just ad­
mired and several generations of readers be­
came slightly bemused by long, though mild
intoxication of the limpid prose of Davis' re­
markable essay.”
(4) Walther Penck objected to over em phasis of
time in Davis' model. In fact, Davisian model
envisages ‘tim e-d ep en d en tseries’ o f landform
development whereas Penck pleaded for ‘timeindependent serie s’ o f landforms. According
to Penck landiorm s do not experience pro­
gressive and sequential changes through time.
He, thus, pleaded for deletion o f ‘tim e’ (stage)
from Davis' ‘trio ’ of ‘stru ctu re, process and
tim e’. According to Penck “geom orphic forms
are expressions o f the phase and rate of
upliftment in relation to the rate o f degrada­
tion” (Von Engcln, 1942).
(3) Davis based his model on detailed and careful
field observations.
(4) Davis' model came as a general theory of
landform development after a long gap after
Hutton's ‘cyclic nature of the earth history.’
(5) This model synthesized the current geological
thoughts. In other words, Davis incorporated
the concept of ‘base level’ and genetic classi­
fication o f river valleys, the concept of ‘graded
stream s’ of G.K. Gilbert and French engi­
neers’ conccpt o f ‘profile of equilibrium’ in
his model.
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(5) A.N. Strahler, J.T. H ack and R.J. Chorley and
several others have rejected the Davisian con­
ccpt o f ‘historical ev o lu tio n ’ o f landforms.
They have forwarded the dyn am ic equilib­
rium theory for the explanation o f landform
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THEORIES OF LANDFORM DEVELOPMENT
(6) Though Davis has attempted to include struc­
ture, process and time in his model but he
overemphasized time. His interpretation of
geomorphic processes was entirely based on
empirical observation rather than on field in­
strumentation and measurement. Though Davis
decribed the structural control on landforms
but he failed to build any model of lithological
adjustment of landforms.
(7) Davis attempted to explain the concept of
grade in terms of ability to work (erosion and
deposition) and the work that needs to be done.
It is evident from the essays of W.M. Davis
that in the initial stage o f landform develop­
ment (in terms of cycle o f erosion) the avail­
able energy is more than needed to transport
the eroded sediment. Thus, the river spends
additional available energy to erode its valley.
As the river valley is deepened the sediment
supply (the work needed to be done increases)
for transportation increases but available en­
ergy decreases. Ultimately, required energy
and available energy become equal and a con­
dition ofequilibrium isattained. Butthe critics
maintain that the concept o f balance between
available energy and the work to be done has
not been properly explained by Davis. It is
apparent from the writings of Davis that the
work to be d o n e’ refers to transportation of
debris by the rivers and energy is spent in two
ways e.g. in transportation o f debris and in
valley deepening. Such division of expendi­
ture of energy is not justified. Thus, there are
two shortcomings in this concept viz. (i) ero­
sion in itself depends on the mobility of
sediments and erosion is never effective in the
abscnce o f sediments, (ii) such condition when
the whole energy is spent in transporting the
sediments and erosion becomes totally absent
is practically not possible.
It may be concluded in the words of C.G.
Higgins (1975) that ‘if the desire for a cyclic, time-
dependent model stems from an unacknowledged
fundamental postulate that the history of the earth is
itself cyclic, then no non-cyclic theory o f landscape
development can win general acceptance until this
postulate is unearthed, examined and possibly re­
jected*.
3. Geomorphic Model of Penck
W. Penck is perhaps the most misunderstood
geomorphologist of the world. It is not yet sure
whether he used the word ‘cycle’ or not in his model
of landform development. Penck's views could not
be known in true sense and could not be interpreted
in right perspective because of (i) his incomplete
work due to his untimely death, (ii) his obscure
composition in difficult German language, (iii) illdefined terminology, (i v) misleading review by W.M.
Davis and (v) some contradictory ideas. His work
was posthumously published in the form o f ‘Die
morphologische Analyse’ in 1924.
It may be pointed out that German scientist
Walther Penck pleaded for the rejection o f Davisian
model of geographical cycle based on time-depend­
ent series of landform development and presented
his own model o f ‘m orphological sy stem ’ or ‘m o r ­
phological analysis’ for the explanation o f land­
scape development. The m ain goal of Penck's model
of morphological system was to find out the mode o f
development and causes o f crustal movement on the
basis of exogenetic processes and m orphological
characteristics. The reference system o f Penck's
model is that the characteristics of landforms o f a
given region are related to the tectonic activity of
that region. The landforms, thus, reflect the ratio
between the intensity of endogenetic processes (i.e.
rate o f upliftment) and the magnitude o f displace­
ment of materials by exogenetic processes (the rate
of erosion and removal of materials).
According to Penck landform development
should be interpreted by means of ratios between
diastrophic processes (endogenetic, or rate of uplift)
and erosional processes (exogenetic, or rate of ver­
tical incision).
Following arc the basic premises of Penckian
model of landscape development—
(1)
The morphological characteristics of any
region of the earth's surface is the result of competi­
tion between crustal movement and denudational
processes.
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development. It may be pointed out that noncyclic concept of ‘dynamic equilibrium* as
valid substitute o f Davis' cyclic concept of
landform development and other so-called
‘open sy stem ’ and non-cyclic models of
landform development could not arouse any
e n th u sia s m
am ong
the
m o d e rn
geomorphologists.
69
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GEOMORPHOLOGY
70
(2) Landscape development is time-independ­
ent.
(3) Tectonic m ovem ents can be explained
and their causal factors may be ascertained on the
basis of morphological characteristics.
(4) The shape of the hillslope depends on
relative rates of valley incision by rivers and re­
moval o f debris from hillslope.
(5) There are three crustal states e.g. (i) state
o f crustal stability when there is no active displace­
ment, (ii) state o f initial domed uplift in a limited
area followed by widespread uplift and (iii) state of
extensive crustal upliftment.
(6) There are three states of adjustment be­
tween crustal m ovem ent and valley deepening viz.
(i) if crustal upliftment remains constant for longer
period of time, the vertical erosion by the river is
such that there is balance between the rate of upliftment
and erosion, (ii) if the rate of uplift exceeds the rate
of valley deepening, then the channel gradient con­
tinues to increase till the rate of valley deepening
matches with the rate of upliftment and the state of
equilibrium is attained when both become equal, and
(iii) if the rate of valley deepening exceeds the rate
of crustal upliftment, then the channel gradient is
lowered to such an extent that the rates of upliftment
and erosion become equal and the state o f equilib­
rium is attained.
(7) Upliftment and erosion are always co­
existent. Penck is supposed to have deliberately
avoided the use of stage concept in his model of
landscape development either to undermine the cy­
clic concept of W.M. Davis or to present a new
model. According to O.D. Von Engeln (1960) “Penck
found escape from the concept of cyclic change
marked by the stages youth, maturity and old age’1.
In the place of ‘stage’ he used the term entwickelung
meaning thereby ‘development’. Thus, in the place
o f youth, mature and old stages he used the terms
aufsteigende entwickelung (waxing or accelerated
rate o f development), gleichformige entwickelung
(uniform rate of development) and absteigende
entwickelung (waning or decelerating rate of devel­
opment).
Contrary to the concept o f W .M . Davis, ‘that
landscape is a function o f structure, process and time
(stage)’, Walther Penck postulated that, ‘geomorphic
forms are an expression o f the phase and rate of
uplift in relation to the rate o f degradation. It is
assumed that interaction between the two factors,
uplift and degradation, is continuous. T he landforms
observed at any given site give expression to the
relation between the two factors (uplift and degrada­
tion) that has been or is in effect, and not to a stage
in a progressive sequence” (O.D. Von Engeln, 1960,
pp. 261-62).
The landscape developm ent (we may say the
cycle of erosion) begins with the upliftm ent of
primarumpf (initial landscape with low height and
relief) representing an initial featureless broad land
surface. In other w ords, p rim a r u m p f is initial
geomorphic unit for the beginning o f the develop­
ment of all sorts o f landforms. Penck is supposed to
have assu m ed v ary in g rates o f u p liftm e n t of
prim arum pf for the developm ent o f landforms. In
the beginning the uplift is characterized by exceed­
ingly slow upheaval of long duration and thereafter
the rate o f uplift is accelerated and ultimately it stops
after passing through the intermediate phases of
uniform and declerating rates o f upheaval. In fact,
‘the most tectonic m ovem ents began and ended
slowly, and that the com m on pattern o f such move­
ments involved a slow initial uplift, an accelerated
uplift, a deceleration in uplift and, finally, quies­
cence’ (R.J. Chorley, et al., 1985, p. 28). The initial
uplift begins with regional updoming and the landform
development passes through the following three
phases.
(1) Aufsteigende Entwickelung means the
phase o f waxing (accelerating) rate o f landform
development. Initially, the land surface rises slowly
but after some time the rate o f upliftment is acceler­
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Penck used the term prim aru m p f to repre­
sent the characteristic lanscape before upliftment.
Primarumpf is, in fact, initial surface or primary
peneplain representing either new ly em erged sur­
face from below sea level or a ‘fastenbene’ or
‘pen ep lain ’ type of land surface converted into fea­
tureless landmass by uplift. A ccording to Von Engeln
(1942) the “prim aru m p f is a prim ary peneplain, one
which could, in either case, exhibit truncated beds
and structures, and yet need n ever have had a greater
altitude or a higher re lie f’. In other words, primarumpf
is the initial landscape with ev iden ces o f erosion but
with low altitude.
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THEORIES OF LANDFORM DEVELOPMENT
71
ated. Because o f upliftment and consequent increase
in channel gradient, flow velocity and kinetic energy
and of course increase in discharge (not due to uplift)
the rivers continue to degrade their valleys with
accelerated rate of downcutting (valley deepening or
incision) but the rate o f upliftment far exceeds the
rate of valley deepening (say degradation of uplifted
landmass). Continuous active downcutting and val­
ley deepening results in the formation of deep and
narrow V -shaped valleys. As the rate of uplift
(aufsteigende entwickelung) continues to increase
the V-shaped valleys are further deepened and sharp­
ened. Since valley deepening does not keep pace
with the upliftment of landmass, the absolute height
continues to increase. In other words, the altitudes of
divide summits as well as the altitudes of valley
bottoms continue to increase as the rate of upliftment
far exceeds the rate o f vertical erosion (fig. 3.2 ) but
the relative or available reliefs continue to increase
due to everincreasing rate o f vertical erosion or
valley deepening. Thus, both maximum altitude
(absolute height from sea level) and maximum relief
o f U p lift
(relative) increase (1 in fig. 3.2). The slopes o f valley
sides are convex in plan.
The valley side slopes are continuously steep­
ened due to continued valley deepening. The radius
of convexity o f .slopes is reduced with passage o f
time due to parallel retreat o f the steeper slope
segments. With the passage of time and more accel­
erated uplift and degradation the primary peneplain
or say primarumpf is surrounded by a series of
benches called as piedm ont treppen. Each o f such
benches develops as a piedmont flat, called in G er­
man as piedm ontflache on the slowly rising m ar­
gins o f the dome.
(2)
G leichform ige E n tw ick elu n g means
uniform development of landforms. This phase may
be divided into 3 subphases on the basis o f rate o f
uplift and degradation (2 in fig. 3.2). P hase (a) is
characterized by still accelerated rate o f uplift. A b ­
solute height still increases because the rate o f ero­
sion is still less than the rate o f upliftment. Altitudes
of both summits of water divides and valley floors
Curve
No F u rth er Uplift
i
Roschung o r G ravity Slope
Ilaldcnhag o r W ash Slope
Case Level
A ltitude
Insell>erg
Fig. 3.2 : Graphic presentation o f Penck's model o f landform development.
due to matching o f upliftment by the lowering of
divide Summit due to denudation. It means that
upliftment still continues. Relative relief also re­
mains constant because the rate o f erosion o f divide
summits matches with the rate o f valley deepening
while both are uplifted uniformly. The slopes of
valley sides are still straight as in phase 2 a because
of parallel retreat. This phase is, thus, characterized
by constant absolute and relative reliefs and thus
uniform developm ent o f landform s. P h a s e (c)Upliftment of the land stops com pletely. A bsolute
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continue to increase but at relatively lower rate than
in the phase o f aufsteigende entwickelung. M axi­
mum altitude (absolute relief) is attained but relative
relief remains constant because the rate of valley
deepening equals the rate of lowering of divide
summits. The valley sides are characterized by straight
slopes (2a in fig. 3.2). This phase is called the phase
o f uniform development probably because of uni­
form rate of valley deepening and lowering of divide
summits. P h a s e (b)-Altitude (absolute relief) nei­
ther increases nor decreases i.e. remains constant
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GEOMORPHOLOGY
72
EVALUATION OF PENCK'S MODEL
reliefs or altitudes o f summit divides start decreas­
ing because of absence o f upliftment but continued
erosion of summits o f divides. Relative reliefs also
remain constant bccausc the rate o f the lowering of
divide summits equals the rate o f valley deepening.
Thus, this subphasc is also characterized by uniform
development o f landscape.
The Penck’s model o f landscape develop­
ment. as pointed out in the beginning, could not be
correctly interpreted because o f its publication in
obscure G erm an language and wrong interpretation
of his ideas by English translators. P enck’s morpho­
logical system was severely criticised in the USA in
the same way as the ‘geographical c y c le ’ was criti­
(3)
A b steig en d e E n tw ic k e lu n g means w an­
cised in G erm any. P en ck ’s concepts o f parallel re­
ing development o f landscape during which the
treat o f slope and continued crustal movements
landscape is progressively dominated by the proc­
became the most sensitive points o f attacks by Ameri­
ess of lateral erosion and consequent valley w iden­
can geologists. It m ay be pointed out that earlier
ing and marked decrease in the rate o f valley deep­
translation o f Penck s w ork in E nglish revealed that
ening through vertical downcutting. This phase is
Penck believed in parallel retreat o f slopes but sub­
marked by progressive decline o f landforms. A bso­
sequent English translations sho w ed that Penck be­
lute relief (altitude from sea level) decreases re­
lieved in slope replacem ent w herein each upper
markably because o f total absence of upliftment but
slope unit o f hillslope and valley sides w as consid­
continued downwasting o f divide summits. Relative
ered to he replaced by low er slope unit o f gentler
relief also decreases because the divide summits are
slope. It may be, thus, forw arded that m ost o f the
continuously eroded down and lowered in height
criticisms o f Penck's m orphological system came
while downcutting of valley floor decreases remark­
out o f the faulty interpretations o f his views. Some
ably due to decrease in channel gradient and kinetic
o f the American critics stooped do w n to such an
energy. Parallel retreat o f valley side slopes still
extent that they rem arked that ‘his p eculiar notions
continues. Nov/ the valley side slope consists o f two
owed to his incomplete recovery from a head wound
segments. The uppermost segment maintains its
suffered in World W a r I ’ (quoted by C.G. Higgins,
steep angle inspite of continuous lowering of ridge
1975). His concept o f long con tin ued upliftm ent and
crests. T his slope is called g r a v it y slo p e or
tectonic speculations could not find any support but
b o sch un gen . The lower segment o f the valley sides
his concepts of slope d ev elo p m en t and weathering
is called wash slope or h a ld e n h a n g . Haldenhang,
processes are definitely o f m uch geom orphological
composed o f talus materials o f lower inclination, is
significance.
formed at the base o f the valley sides due to rapid
4. Geomorphic Model of L.C. King
parallel retreat o f gravity slope or boschungen and
The geom orphic theory or very com m only
consequent elimination of much of the convex wax­
known
as geom orphic system o f L.C. King co m ­
ing slopes. Divide summits are continuously low­
prises a set of cyclic m odels such as the lan d scap e
ered by the intersection o f the retreating boschungen
cycle, the epigene cycle, the p ed ip la n a tio n cycle,
o f adjoining valleys. Thus, the intersection of
hillslope cycle etc. essentially based on the land­
boschungen and haldenhang produces sharp knick
scape characteristics o f arid, sem i-arid and savanna
(break in slope). Haldenhang or wash slope contin­
regions of South Africa as studied by him.
ues to expand at the cost of upper gravity slopes. In
the advanced stage o f the phase o f absteigende
entwickelung the gravity slopes or boschungen are
reduced to steep-sided conical residuals called
inselbergs (fig. 3.2). Eventually, inselbergs arc also
consum ed and the whole landscape is dominated by
a series o f concavc wash slopes or haldenhang. Such
extensive surface produced at the end o f absteigende
entwickelung is called ‘endrumpf% which may be
considered equivalent to D avis’ peneplain.
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The reference system o f K ing's m odel is that
'there is uniform d evelo p m en t o f la n d fo rm s in varying environm ental co n d itio n s a n d th ere is insignifi­
cant influence o f clim a tic ch a n g es in the develop­
m ent o fflu v ia lly o rig in a ted la n d fo rm s. M a jo r land­
scapes in a ll the co n tinents have been evo lved by
rhythm ic g lo b a l tectonic events. There is continuous
m igration (retreat) o f h illslope a n d such retreat is
alw ays in the fo r m o f p a ra lle l retreat. ‘
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THEORIES OF LANDFORM DEVELOPMENT
73
Each cycle begins with rapid rate o f upliftment
followed by long period o f crustal (tectonic) stabil­
ity. Thus, King's concept o f upliftment and crustal
stability is similar to the concept o f Davis. It may be
pointed out that cycle of pediplanation begins with
the upliftment o f previously form ed pediplains and
not of any structural unit. T he pediplanation cycle
passes through the stages o f youth, mature and old as
in the Davisian cycle o f erosion.
As stated above King formulated his model
(theory) on the basis o f information of landform
characteristics derived through his personal studies
of landscape scenery of South Africa having arid,
semi-arid and savanna environment and then as­
serted that his model may be practicable in other
parts of the globe. According to L.C. King an ideal
hillslope profile consists of all the four elements of
slope viz. summit, scarp, debris slope and pediments
and such hillslopes develop in all regions and in all
climates where there is sufficient relief and fluvial
process is dominant denudational agent.
The stage o f youth is characterized by initia­
tion of rapid rate o f active dow n cutting of valleys by
the rivers consequent upon upliftment. Thus, the
long profile of the rivers is punctuated by a series o f
nick points which move upstream. The valleys are so
deepened that they assume the form o f gorges and
canyons. With the march o f time active dow n cutting
of valleys is slowed down and as a consequence o f
which the valley side slopes are characterized by
constant slope angles. The form o f valley side slope
is controlled by physical processes operating on the
slopes and lithologica! characteristics. ‘Eventually,
downcutting will become less active, and small
pediments will begin to appear in the valley bottoms.
These will become more extended as interfluve and
upland areas are consumed by scarp retreat’ (R.J.
Small, 1970). By the late youth most o f the interfluves
are narrowed down due to scarp retreat and are
converted to steep sided hills which are called as
inselbergs. The rounded inselbergs are called as
bornhardts and castle koppies.
King, through his extensive field observa­
tion, identified ‘remarkable surfaces of planation,
surmounted by isolated hills (inselbergs) and piles
o f rock boulders (castle koppies), that are such an
obvious feature o f the landscape in arid, semi-arid
and savanna parts o f A frica’ (R.J. Small, 1970).
Thus, King propounded an entirely new ‘cyclic
m odel o f p ed ip lan ation ’ (known as pediplanation
cycle) in 1948 to account for the unique landscapes
as referred to above as he was convinced that Davisian
model o f arid cycle o f erosion was not competent to
explain these landscapes. It may be mentioned that
King claimed to have propounded his geomorphic
system as entirely different from Davisian cyclic
model and based on some assumptions of Penckian
model but in fact King's model is nearer to Davisian
model than the Penckian model.
After extensive study of South African land­
scape scenery King was convinced that the African
landscape consisted of three basic elements e.g. (i)
rock p e d im e n ts flanking river valleys and having
concave slope varying in angle from 1.5° to 7° cut
into solid rocks, and (ii) scarps having steep slopes
bounding the uplands and varying in angle from 15°
to 30° and experiencing parallel retreat due to
backwasting by weathering and rainwash. (iii) The
third element com prises steep sided residual hills
known as inselbergs (bornhardts) which vary in size
and shape. The size o f inselbergs is determined by
the magnitude of erosion, less eroded inselbergs are
large in size (e.g. mesa) while intensely eroded ones
are small in size (e.g. buttes). The shape of these
inselbergs depends on the nature of underlying struc­
The beginning of m ature stage is heralded by
the absence of active valley deepening and initiation
of lateral erosion. There is backw ard retreat o f valley
side slope because of valley widening and hence
valley sides are distanced from the channel but there
is no significant change in the angle o f valley side
slope. Extensive pediments varying in slope angles
from 5° to 10° are formed at the base o f valley side
slope. The pediments are o f concave slope plan.
Continuous erosion and w eathering results in pro­
gressive decrease in the num ber 9f inselbergs. M any
o f the inselbergs are so greatly w eathered that they
are converted to castle koppies. G radually, m any o f
the inselbergs and castle koppies finally disappear
while there is continuous extension o f pedim ents
consequent upon gradual parallel retreat o f scarps
(upper segm ent o f valley side slope). Eventually,
many pediments coalesce to form extensive flat
ture.
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The cycle o f pediplanation is performed by
twin processes viz. scarp retreat and pedimentation.
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GEOMORPHOLOGY M
74
(pediplanation cy cle) both the m odels are compatible to som e extent as boch envisage cyclic develop­
ment o f landscape w herein cy cle o f erosion begun
with rapid rate o f upliftment o f short period followed
by long period o f crustal stability (tectonic stability
or tectonic inactivity). Eventually, the landmass is
eroded dow n to peneplain (D a v is) and pediplain
(King). Both the landscapes (peneplain and pediplain>
have com m on sim ilarity in that both have antique
characteristics, extensive areas and subdued reliefs.
Both the models are based on the assumption of
completion o f all the three stages (youth, mature and
old) of the cycle. Besides these sim ilarities, both the
models also differ from each o th e r viz. Davis
peneplain is formed due to do w n w astin g w hile King's
pediplain is formed due to c o alescen ce and integra­
tion o f several pedim ents w hich are form ed due to
parallel scarp retreat. D av is’peneplain, once formed,
does not experience further d e v elo p m en t (growth)
until it is,reuplifted. W hen uplifted, new’ cycle of
erosion is initiated and the rivers are rejuvenated. On
the other hand. K ing’s pediplain. once formed, fur­
ther grows headward. New scarp is initiated at the far
end o f the previously fo rm ed p ed ip lain which is
progressively consum ed by the retreat o f new scarp
and thus second pediplain is form ed w h ile the former
pediplain experiences decrease in its e x t e n t The
process co n tin u es and a series o f intersecting
pediplains are formed which extend headw ard. Thus.
King's pediplains, so form ed, are an alo g o u s to W.
Penck’s p ied m on t trep p en .
surface termed by King as pediplain which is char­
acterized by uneven surface with low reliefs and
subdued intersecting concave surfaces. The pediplain
surface is still characterized by the presence of a few
remnants o f inselbergs and mounds.
By old stage m o st o f the residual hills
(inselbergs) disappear. ‘The whole landscape will
now be dom inated by low-angled pediments; the
multi-concave surface is the ultimate form (pediplain)
o f the cycle, the pediplain its e lf (R .J. Small, 1970).
King has also postulated the concept of an­
tique pediplanation. According to King the rem ­
nants o f original pediplains developed during each
cycle are preserved and exist on all summits. ‘Par­
ticularly where formed in resistant rocks, pediplains
and pediplain remnants are believed to achieve great
antiquity, so much so that the highest pediplain
remnants are believed by King to have formed be­
fore the break-up o f the southern hemisphere conti­
nental plates in the Jurassic’ (R.J. Chorley et. al,
1985). King has identified a few antique pediplanation
surfaces in Africa, S. A merica and Australia viz. (i)
African G ondwana pediplain (formed in Jurassic
period) of 1300 m height having its counterpan at the
elevation of 7 0 0 -1000m in Brazil ; (ii) African
pediplain (formed in Creataceous period) at two
elevations i.e. 600-800m (in the coastal areas o f
Africa) and 1000-1600m (in the interior of South
Africa) which is comparable to Australian pediplain
at the elevation o f 400-500m.
Regarding the development of hillslope King
has opined that the form of migrating or retreating
(parallel retreat) slope is controlled by the processes
operating on them. The summit o f hillslope is con­
vex and summital convexity results from the process
o f soil creep. Scarp slope (free face element) is
carved out o f rock outcrops and is characterized by
parallel retreat due to backwasting under the influ­
ence o f rock fall, landslides and gullying. Scarp is the
most active element o f hillslope. Debris slope is
formed by the debris com ing from upslope and the
gradient is determined by the angle o f repose of
debris while the pediment, forming the lowermost
segment o f the hillslope, is formed due to erosion of
solid rocks by turbulent sheet flood.
A few' o f the assu m ptions o f K ing's model are
controvercial e.g. (i) K ing’s m odel is based on Afri­
can experience but ‘it is not su rp risin g to find that
King has gone to apply his con cept not only to the
African landscape, but also to the regions which
today experience clim atic con dition s quite different
from those o f Africa w hich exhibit ‘p en ep lain s', not
readily accounted tor by the D avisian theory* (RJ.
Small, 1970). (ii) K in g s assertion that there is uni­
form developm ent of landscapes in different envi­
ronmental conditions is doubtful, (iii) ‘Despite the
existense of these extensive surfaces (pediplain sur­
faces) ol low relief separated by cliff-lik e escarp­
m en ts in the trop ics, the co n cep t o f antique
pediplanation must rem ain questionable, if Q*dy
because ot vast periods o f time involved and our lack
ot knowledge regarding the nature and rapidity of
Evaluation
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If wc com pare the geom orphic models o f
W.M. Davis (geographical cycle) and L.C. King
J
i
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THEORIES OF LANDFORM DEVELOPMENT
75
erosional processes in subhumid environment’ (R.J.
Chorley, et. al, 1985).
differences in the rocks and the processes acting on
them’.
It may be pointed out that King's geomorphic
theory could not receive as much support and recog­
nition as it deserved because of the fact that his
‘canons of landscape developm ent’ came at the time
(1953) when most of the geomorphologists were
least interested in geomorphic theories as they were
busy in quantifying the landforms and processes on
the basis o f information and data obtained through
field instrumentation and laboratory experimenta­
tion at much shorter temporal and smaller spatial
scales.
The goal o f the theory of Hack is to explain
the landscapes of any region of the earth s surface on
the basis o f present denudational processes operat­
ing therein and to demonstrate lithological adjust­
ment to landforms (for which he presented examples
from the Shenandoah valley of the Applachians,
USA).
The reference system o f Hackian model is
that ‘geomorphic system is an open system which
always tends towards steady state while his m odel
may be stated as 'the shape o f the landform s reflects
the balance between the resistance o f the underlying
m aterials to erosion and the erosive energy o f the
active processes. ’
5. Geomorphic Model of J.T. Hack
J. T. Hack, an American geomorphologist, is
a supporter and advocate of dynam ic equilibrium
theory of landscape development, which implies a
delicate condition o f energy balance and envisages
that ‘so long as the factors controlling landscape
development and denudational processes and en­
ergy in the open geomorphic system remain con­
stant, there is no appreciable change (evolution) in
landforms through tim e’. In fact, Hack's geomorphic
model is a serious attempt to fill the conceptual
vacuum created by the criticism and rejection of
Davisian evolutionary model of geographical cycle
and Penck's ‘m orphological system ’. According to
Hack multilevel landscape (polycyclic relief) can­
not be explained on the basis of multiple erosion
cycles as m aintained by W.M. Davis and his follow­
ers, albit these landscapes can be explained on the
basis of dynam ic equilibrium theory. He further
admitted that ‘eq u ilib riu m co n cep t’ is not in itself
a m o d el’ rather it is a reality in nature. Hack’s
geomorphic model is exclusively based on the con­
cept o f open system but minute analysis of Hackian
model also reveals clear glimpse of evolutionary
model in it. The basic tenet o f Hack's model is that
(as referred to above) geom orphic system is an open
system and so long as energy remains constant in the
geomorphic system, landscapes remain in the condi­
tion o f steady slate though there is lowering in the
landscape by denudational processes. It is, thus,
o b v io u s that Hack's model envisages time inde­
pendent or timeless developm ent of landscapes.
Besides, Hack also invoked a model of lithological
ad justm ent to lan d form s as he stated that topo­
graphic forms and processes are closely related to
The basic p rem ise o f Hackian model o f land­
scape development is that ‘the landscape a nd the
processes that fo rm it are p a rt o f an open system
which is in steady steady o f b a la n c e ' (Hack^ i960).
Hack further conceived the following reference sys­
tems on the basis of his basic assumptions—
(i) T h e r e is balance between denudational
processes and rock resistance’.
(ii) ‘There is uniform rate o f dow nwasting in
all components o f landscapes.’
(iii) ‘Differences and characteristics o f form
are explicable in terms of spatial relations in which
geologic patterns are primary consideration’ (Hack,
1960).
(iv) The processes (denudational) which o p ­
erate today have carved out the landscapes of the
earth's surface.
(v) ‘T h e re is lith o lo g i c a d j u s t m e n t to
landforms’.
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Though J.T. Hack did not construct evolu­
tionary model of landscape developm ent directly
but he did opi ne *that evolution is also a tact o f nature
and that the inheritance o f form is always a possibil­
ity’ (Hack, 1960). Though he did not build a model
of progressive changes in landform s through time
with changing environmental conditions but he opined
that ‘landforms do experience changes w ith chang­
ing equilibrium conditions but these changes are not
like Davisian evolutionary changes.
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GEOMORPHOLOGY
76
remains stable for long geological period (stable
base level) then the landmass is eroded down and
lowered to base level o f erosion and thus the changes
in landform s from initial stage to the final stage
occur in evolutionary sequ en ce (like D avisian model
o f cycle o f erosion). A cco rd in g to H ack in the case
o f stable base level ‘an orderly netw ork o f ridges and
ravines’ is produced in the final ph ase o f landscape
development. Thus, there is gradual and sequential
lowering in reliefs w hen base level o f erosion is
Hack postulated the concept o f variations in
landscapes in relation to varying conditions of bal­
ance between rates o f upliftment and erosion viz.—
(i) The rate o f upliftment is balanced with the
rate o f erosion. If there is rapid rate o f upliftment and
erosion, there is produced high reliefs. This condi­
tion is m aintained so long as the higher rate of
upliftm ent and erosion remains constant.
(ii) So long as the rate o f upliftment increases,
the relief also increases so that rate of erosion matches
the increasing rate of upliftment.
stable.
(ii) If the base level o f erosion rises because o f
positive change in sea-level then the lo w e r segment
o f the rivers is subm erged due to transg ressio n o f sea
water on coastal land but there is n o appreciable
effect o f base level chang e (positive) on the up­
stream segm ent o f the streams. T h e p ositiv e change
in base level also leads to low ering o f relief. Hack
maintains that the long profiles o f rivers and their
normal work w hich controls the d ev elop m en t o f
valley side slopes are influenced and controlled by
upstream conditions o f the drainage basin and not by
the dow nstream conditions. Thus, H ack on the basis
of this concept justified the validity o f R.E. Horton's
scheme o f ordering o f stream s and stream segments.
It may be mentioned that H orton (1942 and 1945)
attempted to determ ine the hierarchy o f stream seg­
ments in the fluvially originated d rain ag e basins
from upstream section (source tributary streams).
(iii) When the rate o f upliftment becomes
zero i.e. when upliftment stops, then relief also
declines, though ridge and ravine topography is still
maintained.
H ack has opined that if the diastrophic move­
ment is gradual and if it is balanced by the denudational
processes (i.e. rates o f upliftment and erosion are
equal) then landscape, while changing from one
form to the other, remains in equilibrium condition.
O n the other hand, if there is rapid rate o f diastrophic
movement, then relict landforms are preserved until
new equilibrium condition is not attained.
R.C. Palmquist has rightly revealed inherent
glimpse o f evolutionary model o f Davis in Hack's
model— ‘H ack (1965) paraphrases Davis' ideal geo­
graphical cycle in terms o f the equilibrium concept
and develops a similar evolutionary scheme. An
initial disequilibrium stage (youth) of rapid stream
incision is followed by an equilibrium stage (ma­
ture) wherein the rounded interfluves are lowered as
potential energy decreases though they do not change
in fo rm ’ (Palmquist, 1975).
H a c k a lso d e v e l o p e d a ‘c o n tin u o u s
dow n w astin g m o d e l’ which though envisages ten­
dency for dynam ic equilibrium but it is not neces­
sary that the dynamic equilibrium is in steady state.
He him self admitted that ‘though there is possibility
for steady state but it is not possible in reality.’
He further opined, ‘that evolutionary models
can be conceived on the basis o f base level of
erosion. In this context he considered three condi­
tions o f base level viz. (i) stable base level, (ii)
positive (rise) change in base level and (iii) negative
(fall) change in base level.
(iii) If there is low ering o f base level o f
erosion (negative change) then there is rapid rate o f
erosion in the dow nstream section (m ainly near the
new base level i.e. m outh o f the river) o f the stream
which influences larger areas o f the d rain age basin.
New adjustm ent betw een erosion (rapid rate) and
rock resistance is attained.
H a c k a lso p r o p o u n d e d th e c o n c e p t o f
lithological ad ju stm en t to landform s* ‘F o r exam ­
ple, it has been s u g g e s te d that, in the folded
Applachians, the local relief and slo pe angles have
been so adjusted that each m ajor geological outcrop
yields an equal sedim ent load p er unit area (i.e. hard
rocks-high, rugged and steep ; soft r o c k s — low,
gently rolling and w ith low slo pes— Hack, I960)*
(quoted by R.J. C horley et. al, 1985). R.J. Chorley c t
al have rem arked that ‘although this is an attractive
alternative explanation for geological limited 4cy*
clic surfaces, but it is difficult to su ppo rt’ (RJ«
Chorley et. al, 1985).
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In the case o f stable base level o f erosion he
maintains that if any landmass is uplifted and then
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THEORIES of lan dform developm ent
77
T h e advocates o f dynam ic equilibrium theory
including J.T. H ack m aintain that the so-called
peneplain and planation surfaces at different eleva­
tion levels (the outcom e o f rejuvenation and succes­
sive cycles of erosion as envisaged by Davis and his
followers) are not the result o f completion o f succes­
sive cycles o f erosion bul they have been formed
differently. T h e y argue, in any area of rocks which
are reasonably uniform in terms of resistance, when
the stream spacing (drainage density) is uniform,
and where the slopes are at the same maximum
angle, it is to be expected that the summits and the
divide crests will all reach the same height and so
give the impression of a former level surface which
has, subsequent to its formation, been dissected by
valleys. Hack has even gone so far as to propose that
such a landscape, which he refers to as ‘ridge-andravine topography', is the normal expression of a
condition ofdynamic equilibrium’ (R.J. Small, 1970).
6. Tectono-Geomorphic Model of M. Morisawa
The appearance o f plate tectonic theory since
1960 has provided impetus to geomorphological
i n v e s t ig a t io n s in n ew d i r e c t i o n s as som e
geomorphologists have attempted to explain land­
scape development on the basis o f gradual and
continuous tectonic movements as evidenced by
plate movements and sea-floor spreading. American
geom orphologist Marie M orisaw a's geomorphic
model of landscape development (1975) is based on
such premise. The following are basic premises o f
Morisawa's ‘tectono-geom orphic model’—
(1) Landforms are the result o f inequality o f
force or inequality of resistance or o f both.
(2) The variations in landforms are due to
inequality of rates of operation o f exogenetic proc­
esses acting on different geomaterials o f the earth’s
surface and inequality o f the rates o f endogenetic
processes.
Evaluation
(3) Nature tends to attain balance/equilibrium
between force (of processes) and resistance (o f
geomaterials) but this situation (of balance) is not
always possible because the earth is unstable and
dynamic. Thus, the earth's surface is characterized
by frequent changes and hence in stead o f static
equilibrium there is tendency to equilibrium. D y­
namic earth system is characterized by isostatic
feedback which affects upliftment and erosion, and
deposition and subsidence i.e. upliftment is fol­
lowed by erosion and erosion is followed by deposi­
tion which is followed by subsidence which again
leads to upliftment and thus the process continues.
The isostatic feedback also affects the rates o f
upliftment and erosion, and deposition and subsid­
ence.
The Hack's concept that ‘most of the land­
scapes are in uneasy dynamic equilibrium between
available energy for work (erosion and transporta­
tion) and the work being done’ cannot be validated
because if there is gradual and continuous lowering
in regional elevation (and hence decline in energy
available for denudational work) then no landscape
of open system may remain in steady state. Simi­
larly, the concept o f Hack that landscapes are adapted/
adjusted to changing environmental conditions is
doubtful because there are very little landscapes
which have instantaneously adjusted/adapted to new
environmental conditions. R.J. Rice ( 1977) has aptly
remarked, ‘to an extent all landforms are prisoners
of their own evolutionary history. A few of the
assumptions or precepts of dynamic equilibrium
theory are merely deductions which do not have
ground support. For example, the fact that ‘there is
perfect relationship between present-day processes
and landforms' is not always true. A.L. Bloom (1978)
has evaluated the Hackian model in right perspec­
tive— ‘If, however, tectonics and climatic changes
invalidate the assumption ol initial upliit or other
constructional processes followed by still stand and
landscape evolution, then the dynamic equilibrium
model, changing only from disequilibrium to equi­
librium, is most suitable as a basis for interpreting
the present landscape1 (A.L. Bloom, 1978).
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(4) The present landforms are the result o f
difference of ratios of the actions o f endogenetic and
exogenetic processes. It may be mentioned that W.
Penck also postulated identical concept (landforms
reflect the ratio between the intensity o f endogenetic
processes i.e. rate o f upliftment and the m agnitude o f
displacement of materials by exogenetic processes
i.e. rate of erosion and removal o f eroded materials).
The ratio ot rates o f action by endogenetic and
exogenetic processes varies temporally and spa­
tially. This aspect is responsible for temporal and
spatial variations in landform characteristics. Thus,
the landforms o f the earth's surface becom e com plex
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G V / M W n S fA / fff
78
(1968). It m ay be pointed out that the ta id rate
erosion is for those rivers w hich corns out (rtntt the
and hence it becom es difficult to understand the
mode o f their genesis and development.
Himalayas.
Based on aforesaid in f o r m a l/f t MorHawa
hypothesised that ‘there h d ire c t p<*tt'tvc rc te irm ship between rate of upliftm ent and rate o f
,
It may be mentioned that Mori>av/a * modeJ f* \/4Xtd
on empirical studies and not on m erely deductsf/M ,
(5) Some morphological features can be ex­
plained on the basis o f plate tectonics.
(6) Any landmass when uplifted or newly
created landmass undergoes rapid transformation of
its form through exogenetic (denudational) proc­
esses. The rate o f change (transformation) of form
depends on the nature o f force and resistance.
The major prem ise o f M o m a v / a * r o o d d h
that variations in landscape-; and theirdcveJop/rrjeat
arc due to inequality of force or rc.u?>tancc, S he haa
attempted to explain this concept with the help o f a
diagram (fig. 3,3). The potential energy of %ttc&rn\
with varying heights differs considerably. In fig, 3.3
Marie M orisawa first collected information
about the results o f geomorphological studies per­
taining to erosion and reliefs conducted by different
geomorphologists in different parts o f the world and
then formulated the hypothesis that there is high rate
o f erosion on uplifted landm ass because potential
energy required f o r erosion increases due to greater
height (and high potential energy results in high
kinetic energy due to increased channel flow veloc­
ity which ultimately accelerates erosion).
Based on the result o f the study of stream
erosion by F. Ahnert (1970) in middle latitudes
Morisawa concluded that the rate of denudation and
basin reliefs were highly positively correlated and
90 per cent of the total differences in erosion rates in
different drainage basins were due to average reliefs
o f the basins. She also cited examples of the work of
B.P. Ruxton and I. McDougall (1967) in Papua
regarding the erosion o f volcanic mountains. The
rates of erosion on different volcanic mountains
(e.g. 75 cm /1000 year over 760 m high mountain and
8 cm/10 0 0 year over 60 m high mountain) again
revealed positive correlation between height of land­
mass and rate o f erosion. Similarly, T. Yoshikawa's
(1974) studies also revealed positive correlation
between the rates o f upliftment and denudation.
According to him the rate o f denudation substan­
tially increased because o f Quaternary upliftment in
Japan but the rate o f upliftment in drainage basins
exceeded the rate o f denudation. He further reported
higher rate of denudation on highest mountainous
areas than the rate o f tectonic upliftment. According
to Yoshikawa the present rate o f denudation of
0.84rn/1000 year is more or less equal to the present
rate o f upliftment (0.863 m/1000 year). B. Isacksct.
al (1973) estimated the average rate o f upliftment of
the Himalayas as 0.3 m m /1000 year which matches
with the rate of erosion (0.3 mm/1000 year) by the
rivers in South Asia as estimated by J.N. Holeman
Fig. 3.3 : Graphic presentation o f potential energy o f
two streams o f two different height: but with
same base level (after M.Morisawa).
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the base level for tv/o stream s (S ( and S j is the same
but they emerge from different height’s (h. and h j
with the result the potential energy o f S. is more fdue
to higher height, hj) than S ] and hence the available
energy (for denudational w ork) o f S, w ould be more
than S r It is, thus, inferred that there is difference in
available energy o f stream s for denudational work if
their base level is the same but source heights arc
different. She also considered such situation where
base level and source height o f three stream s 'S ?, S,,
S3) are same but channel gradient is different (fig 3.4). The water discharge is also same for these th r e e
streams. In such situation potential energy and iLs
transformation into kinetic energy for all the three
streams is same but available energy for work to be
done (erosion and transportion) would be different
for three streams because available energy for work
depends upon the travel distance (channel length)
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t h e o r ie s o f l a n d f o r m d e v e l o p m e n t
79
covered by the stream s during transformation of
potential energy into kinetic energy. The travel dis­
tance of a stream (S 3) with gentle channel gradient is
longer (fig. 3.3, A D distance for S3 stream) whereas
it is much shorter for a stream with steep channel
gradient (A B distance for stream S p fig. 3 .3). The
longer the travel distance, the lesser the available
(kinetic) energy for erosion and transportation be­
cause there is greater loss of energy due to friction of
longer distance. On the other hand, if the travel
distance is shorter, the available kinetic energy would
be more because there would be comparatively less
loss of energy due to friction by the surface (valley
floor).
Thus, there is variation in the rate of denuda­
tion because of unequal resistance to resultant un­
equal or equal force (available energy). In other
words, if there is uniform height, base level and
water discharge for different streams but there is
difference in slope gradient, then the streams having
gentle channel gradient would have to cover longer
distance (channel length) and hence there would be
more friction and hence more loss of energy and less
available energy for work. On the other hand, the
stream having steep channel gradient would have to
covercomparatively shorter travel distance and hence
there would be comparatively less loss of energy due
to leaser friction but more available energy for work.
Thus, the stream with steep channel gradient and
consequent higher resultant available energy would
erode the valley at faster rate than the stream with
gentle channel gradient and lower amount o f result­
ant available energy. Thus, the deduced geomorphic
model of Morisawa may be stated as follows—
‘That unequal fo r c e s o r unequal resistance to
the sam e fo rc e will result in differen t rates o f d e n u ­
dation. Unequal fo rc e s at work, o r u nequal resist­
ance to sam e fo rc e results in individuality a n d va ri­
ety o f landforms*.
(M. Morisawa, 1975)
BASE
Morisawa has attempted to establish relation­
ship between tectonic force and denudational force.
When tectonic force and denudational force are
equal, then there is equilibrium condition but there
would be disequilibrium when tectonic force is ei­
ther higher or less than the denudational force. She
further maintains that the state of disequilibrium is
temporary because two opposing forces (tectonic
and denudational) tend towards equilibrium state.
Relief increases at faster rate if upliftment occurs at
faster rate but the rate of erosion lacks far behind the
rate of upliftment. Consequently, the rate of denuda­
tion would go on increasing with growing reliefs
until denudational force (rate of erosion) matches
w'ith tectonic force (rate of upliftment). Conversely,
if denudational force exceeds tectonic force, then the
decay of landscape is slowed down because of de­
crease in reliefs and available energy and eventually
equilibrium state between denudational and tectonic
forces is attained.
LEVEL
Fig. 3.4 : Graphic presentation o f difference in kinetic
energy when the base level and height fo r
different streams is same but there is differ­
ence in channel gradient, (after M. Morisawa,
1975).
It is, thus, evident that stream S, has highest
available kinetic energy for erosional and depositional
work while stream S 3 has lowest available kinetic
energy. It is further apparent that there is inequality
in force (available en erg y ) o f three streams inspite of
same height, same base level and same water dis­
charge. Similarly, if height and channel gradient are
same but discharge varies from one stream to the
other, then available kinetic energy would again be
different for different streams because kinetic en­
ergy = 1/2 M V 2 (M = mass, here discharge i.e. water
mass, V = velocity, while potential energy = M x G
X H where M = mass, G = gravity and H = height).
Even if height, slope and relief are same for different
streams, the force (available energy) would be un­
equal for different stream s because o f unequal force
Morisawa has further clarified that the afore­
said equilibrium state is possible only when either
there is decrease in tectonic force and increase in
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GEOMORPHOLOGY
plate margins arc characterized by block faulting
and lava flow. The rivers draining across the upthrown
fault block resorts to active dow ncutting and deepen
their valleys and form d eep gorges and canyons. As
the erosion proceeds, several gcom orphic events
like reversal o f drainage pattern, river capture, for­
mation of w ater gaps etc. form typical landforms.
Mountain ranges are formed becau se o f subduction
of one plate margin below com p aratively lighter
plate margin along destructive plate margin (con­
vergent plate m argins;. Stream s e ro d e these uplifted
and folded m ountain ranges with accelerated rate
because o f increase in available kinetic energy due
to greater height, steep gradient and less frictional
(resistance) force and form d eep and narrow gorges,
canyons, high altitude terraces etc. R iv e r terraces are
deformed and long profiles o f the riv ers are punctu­
ated by nick points due to co n tin u o u s intermittent
upliftment. C ontinued upliftm ent results in the for­
mation of stepped features (like terraces and benches)
and chain of nick points. T h us, acco rd in g to M.
M orisaw a some o f the geo m o rp h ic features of the
earth's surface m ay be ex plained on the basis of
widespread neo-tectonic events.
degradation o f landscape by denudational force or
there is increase in tectonic force and decrease in
degradation. It is evident that equilibrium state may
not be stable (static).
The upliftment is followed by lowering of
landmass by denudation and eroded materials arc
deposited in low lying areas. This leads to positive
feedback mechanism i.e. there is isostatic adjust­
ment following degradation o f landmass by erosion
and aggradation by deposition o f sediments. C onse­
quently, the landmass degraded by erosion (lower­
in g o f h e ig h t) ris e s w h e r e a s a g g ra d e d a r e a
(depositional area) is subjected to subsidence under
the mechanism o f isostatic readjustment. Such isostatic
readjustment may be accomplished instantaneously
or m ay be delayed. If there is time-lag in isostatic
readjustm ent i.e. if the isostatic readjustment is
delayed, then erosion is renewed. With the result
there is intermittent upward movement in the land­
mass and consequently different erosion levels are
formed at different altitudes. It may be mentioned
that this concept validates Davisian mode of evolu­
tionary change and polycyclic reliefs or multi-level
erosion surfaces. On the other hand, instantaneous
or continuous isostatic feedback supports Penck's
model of geomorphic system (continuous change in
the rate o f upliftment and erosion). M orisawa has
claimed that both the models may be applicable in
the geomorphic personality o f any region.
Evaluation
T h e t e c t o n o - g e o m o r p h i c m o d e l o f M.
M orisaw a is technically m ore sound and is easily
applicable in the explanation o f g en esis and devel­
opment o f some, if not all, sim ple m orphological
features because it is based on em pirical studies of
different geologists and g eo m o rp h o lo g ists in differ­
ent parts o f the globe. H er m odel is m ore flexible
because it acco m m o dates both the m o dels o f evolu­
tionary change in landform s and d y n a m ic equilib­
rium concept. Besides, it is based on the evidences of
plate tectonics about w hich co n v in cin g evidences
have been provided by n um ero us stud ies conducted
by a host o f scientists.
Based on above mentioned premises Morisawa
postulated that ‘when denudational processes (forces)
operate on rocks o f varying resistance then there is
temporary disequilibrium state between work (ero­
sion) and form (landscape) but there is a tendency
tow ards the attainment o f equilibrium o f form in
relation to force and resistance. In other words, any
stream tries to attain such slope gradient that re­
quired energy to transport the eroded sediment
becom es available i.e. when the geomaterials are
resistant, there is temporary increase in energy which
increases the force so that it equals the increased
high resistance and equilibrium is attained. C o n ­
versely, when geom aterials are less resistant, there is
decrease in energy so that it matches with the resist­
ance and equilibrium is attained.
7. Ep iso dic Ero sio n Model of S.A . Schum m
I he episodic erosion model o f S.A. Schumm
is, in Iact, the m odified version o f g eo m orphic cycle
and is related to ev o lu tio n a ry co n cep ts involving
two basic concepts viz. c on cep t o f geom orphic
threshold and concept ot co m p lex response- He
constructed his model on the argu m ents that most of
the geom orphic m odels are oversim plified and lack
in a ccom m od atin g m inor chang es in landforms duf-
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M orisaw a has attem pted to explain the g en ­
esis and developm ent o f landforms o f the earth's
surface on the basis o f plate tectonics. Constructive
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81
THEORIES o f l a n d f o r m d e v e l o p m e n t
ing short periods. According to him there is no
progressive change in the level of valley floor and
channel gradient through geological (long) time.
The reference system of Schumm's model is
that there is no progressive lowering or reduction of
stream gradient and altitude of valley floor because
there are frequent obstructions in such progressive
changes due to functioning of fluvial system. The
minor details in the landforms cannot be explained
on the basis of Davisian model of cycle of erosion.
The main goal of Schumm's model is to ex­
plain minor details of landforms (stepped valley
floor) in the channel gradient and valley floor during
the functioning of fluvial system on the basis of the
concepts of geom orphic thresholds and complex
response involving dynam ic equilibrium model.
His m odel/theory states that denudation is
not gradual and continuous rather it is episodic. The
geomorphic history of landscape development in­
cludes numerous periods of rapid erosion (period of
instability) and deposition. Period of rapid erosion is
followed by long period of deposition (example of
geomorphic response). There is repetition of periods
of erosion and deposition and thus there is complex­
ity in the evolution and development of landforms
(example of complex response).
model of cycle o f erosion the concept o f progressive
loweirng of channel gradient appears longical but
graded state is not attained in youth and mature
stages. Graded stage is attained in the penultimate
(old) stage o f cyclic model. On the other hand, if the
graded state is attained then progressive reduction o f
channel gradient and valley floor cannot be possible.
Schumm has suggested that one o f the concepts o f
progressive erosion and progressive reduction in
channel gradient and valley floor should be dropped
in order to solve the above geo m orph ic riddle. S o,
Schumm has suggested for the construction o f alter­
native model which instead o f envisaging progres­
sive reduction of channel gradient and valley floor
includes rapid changes o f short periods w hich sepa­
rate graded periods of long duration. In other words,
there is a period of rapid change (by episodic ero­
sion) of short duration between two graded periods
of long duration.
According to Schumm the com plexity o f land ­
scape may be explained on the basis o f two geom orphic
concepts viz. (i) the con cep t o f g eo m o rp h ic th r e sh ­
olds, and (ii) the con cep t o f co m p lex resp on se.
According to the concept of geom orphic thresholds *
changes may occur in the fluvial system but these
changes are not occasioned by external factors (e.g.
isostatic upliftment) but are effected by inherent
geomorphic controls o f eroding fluvial system (say
drainage basin). For exam ple, if there is deposition
of eroded sediments in a fluvial system , these d e p o s ­
ited sediments become unstable at a critical thresh­
old slope i.e. channel slope gradient increases due to
sedimentation and a limit (threshold) is attained
when no further sediments may be accom m odated.
Consequently, the channel gradient b eco m es such
(due to deposition) that erosion o f deposited sediments
begins due to increased channel flow velocity. It is
evident that such changes (deposition and erosion )
have not been effected by external variables o f the
fluvial system but have been caused by the internal
geomorphic controls.
R.W. Lichty and S.A Schumm (1965) first
attempted to dispel controvercies regarding the models
of landscape development propounded by W.M.
Davis, W. Penck and J.T. Hack on the basis of
different time spans o f landscape development e.g.
cyclic tim e, graded tim e and steady state tim e (see
chapter 2, time scales, pp. 45-48). Cyclic time in­
volves long geological period (hundreds of millions
of years) characterized by exponential decrease in
channel gradient (fig. 2.13 A). There are several
periods o f graded time and steady state time. C han­
nel gradient (average) almost remains constant but
there may be fluctuations (rise and fall) with time in
average channel gradient. Steady state has a period
of very short duration during which there is no
According to the co ncept o f co m p lex re ­
sponse when a fluvial system is re ju v e n ated (say
drainage basin) then the respon se o f the fluvial
system to rejuvenation is not only re n e w e d acc e le r­
ated rate o f valley deepen in g but the resp o n se is in
the form o f attainm ent o f new equilibrium (it m a y b e
stated that the equilibrium is distu rb e d d u e to
change (fig. 2.13 B and C).
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The basic prem ise o f Schum m 's model is that
the model of geom orphic cycle (as propounded by
W.M. Davis) cannot accom m odate both the aspects
°f progressive low ering (reduction) in channel gra­
dient and valley floor. For exam ple, in Davisian
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82
GEOMORPHOLOGY
effect o f w hich is reflected in the form o f accelerated
erosion at the m outh o f the river w hich represents the
response (of rejuvenation) o f the system (in the form
o f accelerated erosion) at a particular place (river
mouth) and particular time. T he effect o f such change
by accelerated erosion due to rejuvenation is not
immediately extended in upstream segment o f the
river and by the tim e the effect o f change is extended
upstream, the fluvial system responds in the form of
deposition.
rejuvenation) through downcutting, aggradation and
renewed erosion. If the effects o f external variables
o f the fluvial system (isostatic upliftment) is co m ­
bined with geomorphic thresholds and complex re­
sponse then at least during the initial stage (youth) o f
geom orphic cycle erosion cannot be progressive
rather there would be complex response o f events o f
relative periods o f stability separated by periods o f
episodic erosion. In other words, there is repetition
o f periods o f erosion and erosionless periods (peri­
ods o f stability), the response (result) o f which is
that, the fluvial system and the resultant landscape
become very complex. The main reason o f the re­
sultant com plexity o f landscape is the fact that if any
event occurs in any segment of a river, there is no
instantaneous impact o f such event on the entire
channel length. For example, if the river is rejuve­
nated due to negative fall in sea-level, the immediate
j
YOUTH
^ w i I inr\
BASE LAVE L
MATURI TY
Schum m has attem pted to explain his model
o f episodic erosion with the help o f graphs (fig. 3 .7).
First, he suggested modification in the Davisian
model o f cycle o f erosion (Fig. 3.5 and 3.6). Figs. 3.5
(presentation o f Davis' geographical cycle by oth­
ers) and 3.6 (presentation by D avis him self) repre­
sent geographical cycle o f D avis while fig. 3.7
represents the geom orphic model by Schum m .
f m o i n vo
Fig. 3.5 : Graphic presentation o f Davis' geographical cycle (by others).
Fig. 3 .6 :
Graphic presentation o f geographical cycle by W.M. Davis. BFHK = upper curve-swnm it o f water divide,
CEGJ = valley flo o r = lower curve ; CEG = deposition shown by dotted line.
In all the three diagrams (figs. 3.5,3.6 and 3.7) upper
line (upper curve) denotes sum m it levels or altitude
o f w ater divides from sea-level while lower line
(low er curve) denotes altitude o f valley floor from
sea-level. Part A o f fig. 3.7 represents youth and
early m ature stages o f Davisian model (fig. 3.6) but
erosion is not progressive but these (youth and early
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mature stages) are frequented by disturbances caused
by isostatic adjustment. D otted line (CEG ) in Davis’
graph (fig. 3.6) represents deposition in the valley
floor. It may be noted that in Davis' graph (fig. 3.6)
upper curve (sum m its o f w ater divides, BFH K) and
lower curve (valley floor, C E G ) are sm ooth curves
whereas in Schum m 's graph (fig. 3.7A) both the
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THEORIES OF LANDFORM DEVELOPMENT
83
curves (upper and low er) arc stepped ones which (upliftment). T he dotted line in fig. 3.7 denotes
represent obstructions causcd by isotatic adjustment
progressive lowering o f altitude.
B
O
o
VF 2
^*PPed
^H !J_orvo//eyw
>•
o
>
Instabi/rty
Episodic Erosion
c
o
“D
D
T I M E
Fig. 3 7 : Modified concept o f geomorphic cycle o f erosion. A - dotted line denotes progressive, lowering o f altitude as
envisaged in Davis'm odel while solid lines indicate stepped features as suggested by Schumm. B - portion o f
v a l le y floor C - Portion o f valley floor V F2 (as shown in B) which indicates dynamic equilibrium period
between two periods o f instability o f shorter duration. After S. A. Schumm, 1975.
portion indicated by VF1 in fig. 3.7 A represents
normal pattern o f valley floor o f river channel but
when observed minutely at smaller spatial scale then
it looks stepped as is evident from fig. 3.7B where
real form of VF1 in fig. 3.7 A has been extended and
projected. Normally, such stepped form o f valley
floor is explained in terms o f influences o f external
variables like upliftment, subsidence, climatic changes
etc. but according to Schum m such stepped valley
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Schumm maintains that divide summits un­
d e r g o m o d e ra te c h a n g e s b e c a u se o f lim ited
downwasting caused by surface runoff resulting
from rainfall but downwasting is more or less uni­
form on all summits. The form of valley floor be­
comes stepped because o f reduction in valley floor
but for shorter duration. It may be mentioned that the
stepped form o f valley floor is because of sediment
storage (deposition) and sediment flushing. The
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GEOMORPHOLOGY
84
ates period o f valley deepening and the process is
repeated over and again. It is, thus, evident that if the
episodes o f erosion (period o f instability, of short
duration) and deposition (period o f stability, of long
duration) are repeated then there is no need of
external variables to explain minor details o f land­
scapes like small terraces, alluvial fills, riffles and
pools etc. because these features are the result of
internal variables of the fluvial system. R.J. Chorley
et. al (1985) have aptly remarked that ‘this dynamic
metastable equilibrium model of episodic erosion
shows, in addition, that many o f the details of the
landscape (e.g. small terraces and recent alluvial
fills) do not need to be explained by the influence of
external variables because they develop as an inte­
gral part of system evolution’.
floor is not of external variables rather it is because
of control of internal variables of the fluvial system.
Such model is in fact representative of dyn am ic
m etastable e q u ilib riu m model. It may be men­
tioned that in steady state equilibrium model (here is
fluctuation around a stable average value whereas
dynamic metastable equilibrium envisages ‘a condi­
tion o f oscillation about a mean value of form which
trending through time and, at the same time, is
subjected to step-like discontinuties as a threshold
effect’ (R.J. Chorley et. al, 1985). According to
Shumm there is possibility of influences of external
variables on system equilibrium but in terms of
denudation of landmass dynamic mestastable equi­
librium reflects reponses of inherent geomorphic
thresholds of the fluvial system i.e. internal vari­
ables of the fluvial system influence and control
dynamic metastable equilibrium. Forexample, depo­
sition of sediments in the valley floor upsets the said
equilibrium state and introduces changes in the sys­
tem (e.g. increase in channel gradient due to sedi­
mentation) and when these changes exceed the criti­
cal geomorphic threshold, the eroding fluvial sys­
tem i.e. fluvially originated drainage basin is rejuve­
nated leading to accelerated rate of erosion (valley
downcutting). Such situation of accelerated erosion
is called p erio d o f episodic erosion. The period of
episodic erosion, when it exceeds the geomorphic
threshold, is succeeded by period of deposition.
Thus, the bedrock valley floor of the river becomes
step-like which denotes the period of instability
(period of episodic erosion) and period of stability
(period of dynamic metastable equilibrium). It may
be pointed out that the period of instability/erosion
is of short duration while the period of stability
(dynamic metastable equilibrium or graded period)
is o f longer duration. It may be clarified that the
periods o f instability and stability are, in fact, peri­
ods of erosion and deposition respectively (fig. 3.7 C).
Schumm has also postulated the concept of
several subcycles within a larger fluvial cycle. Ac­
cording to him the major cycle begins with denuda­
tion of uplifted landmass. In the initial stage maxi­
mum sediments are produced because of active
vertical erosion (valley deepening) and the quantity
and size of sediments decreases with time because of
decrease in the rate and magnitude o f erosion due to
lessening of channel gradient. Within major cycle
second order cycles are initiated due to isostatic
adjustment (upliftment) in the 1 st cycle and climatic
changes. Within the second order cycles third order
cycles are initiated when geomorphic thresholds in
the fluvial systems are exceeded. The fourth order
cycles are initiated due to complex geomorphic
responses which are the result of changes in any one
of the variables of the fluvial system e.g. tectonic
events, isostatic adjustment (upliftment or subsid­
ence), climatic changes or geomorphic thresholds.
The fouth order cycles o f smaller magnitude are
initiated as a result of adjustment to changes in the
1st, 2nd and 3rd order cycles. The final or 5th order
cycles are initiated due to seasonality o f hydrologic
events or large floods.
Schumm has further stated that during peri­
ods o f stability there may be changes in the channel
pattern because of changes in the nature of sediments
passing through the channel, i.e. straight channel
courses may be transformed to sinuous and mean­
dering courses. Again, the sinuous or meandering
course of the river may be straightened during exten­
sive floods. The straightened and thus shortened
river course again stimulates erosion and thus initi­
Evaluation
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The Schumm's model o f landform develop­
ment is, in fact, modified form o f Davisian model of
geographical cycle which envisaged progressive
changes in landforms through time. Schumm has
successfully attempted to remove the major draw­
backs of Davis’ decay model and has tried to blend
the cyclic model with equilibrium model. His model
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A
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THEORIES OF LANDFORM DEVELOPMENT
is nearer to m ore reality than Davisian model. He has
also attempted to explain minor landscape details
mainly in the valley floors which were obscured in
Davis' m odel. But the concept of numerous subscycles
within a m ajor or say super cycle in a fluvial system
is difficult to digest but the effort of S.A. Schumm
is commendable. There is a need o f blending o f
decay and equilibrium models to build a more
flexible model as R.J. Chorley et. al (1985) have also
opined, ‘more than this, modern studies of thresh­
olds and complex response have suggested how the
Davisian cyclic decay model and the steady state
model o f Gilbert may be effectively, combined into
a united vision of landform evolution.’
8. Geomorphic Theories : In Indian Context
Now, the author presents geomorphic prob­
lems of a typical nature from the sub-humid tropical
environm ent of India for critical evaluation of the
landscape developm ent of the region which may
85
lead us to corroborate the concept o f composite
theory o f landscape development.
Bhander plateau (24° 3’ 29" N— 24° 39' 1” N
lat. and 80° 16’ 30" E— 80° 53’ 15” E long.), located
between Panna plateau in the northwest and Rewa
plateau in the east, is characterized by Vindhyan
sandstones, shales and limestones generally lying in
a horizontal manner with alternating bands o f hard
and soft rocks. It registers an ascent o f about 350m
above the general surrounding surface o f lower
uplands and is drained by the feeders o f the Tons, the
Satna and the Ken rivers. Mean annual rainfall is
1137mm and mean monthly m axim um temperatures
of January and June are 30.5°C and 45.3°C respec­
tively whereas mean monthly temperatures o f corre­
sponding months are 20.4°C and 23.1°C respec­
tively. Hilly tract of the plateau has mixed vegeta­
tion of open and dense forests whereas low er up ­
lands have scattered bushes.
Fig. 3 .8 : Bhander Plateau, M.P., India (after Savindra Singh, 1974).
lower and rolling upland developed over V indhyan
basement which has been m oderately incised by
shallow valleys, the depth o f which m atches with the
thickness of alluvia (4m to 18m). This low er upland
is dotted with flat-topped hills, the exam ples o f
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A well-marked zonation of three distinct topo­
graphic features (fig. 3.8) from the higher plateau to
the outer margins upto the river valleys is identified
on three sides (fig. 3.8) viz. north, west and north­
east— (i) at the outer margins, there is significantly
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86
OliOMOimiOLOOY
m e sa s and buttes (K u sh la hill, Siiuliirin paluir*
Shankargarh hill, Lai pahar, Pithaurahad liill, Nurduliit
pahar, D hark ana pahar, Patna hill, liandhnurn hill,
Satani hill, M utw ari hill etc.). These accordant re­
sidual hills having sandstone capping above and the
alternate hands ol sandstones and shales helow are
flat-topped m esas and huttes having vertical sleep
scarps o f free-face element and rectilineal flanks of
30° to 40° slope helow and join the billowing surface
form ed at their base which seldom exceeds 3 to 4
degrees in slope; (ii)T h e second ring o f topographic
features incorporates numerous cinhaymcnts and
indentations which girdle the plateau proper from
three sides and indicate massive breaching o f the
plateau rims. The most outstanding feature o f this
zone isa c re n u la te d line o f imposing and precipitous
scarps ; (iii) The third zone includes the top-surface
of the central plateau and lacks in pronounced reliefs
cxccpt som e convexo-concave low hills having lim ­
ited flat tops but in majority o f the cases they have
round tops, some long and narrow ridges, knolls and
irregular and asymm etrical valleys. The major river
courses have graded profiles over the higher plateau
and lower uplands but arc punctuated by sudden
falls when they descend through the precipitous
scarps. The existence o f numerous waterfalls along
the rims o f the escarpm ent ranging between 10m and
60m makes the riddle o f the geom orphic history o f
the region moc complex.
The region appears to be in equilibrium stage
as there is gradual parallel retreat o f scarps and thus
there is no significant chang e in landscape. The
hack wasting is the most dom inant process. Various
detached hills projecting above the general rolling
surface of lower uplands are the left-over remnants
of the recession o f the escarpm ents and thus the
surrounding lower flat and rolling uplands arc not
the outcome o f lateral planation by the rivers rather
they are the results o f parallel retreat o f the scarps.
This explanation, no doubt, goes in favour o f equi­
librium model but the existence of Sharda Pole hill
(488m), only a km away from the precipitous Naktara
escarpm ent, exhibits an exam ple o f dow nwasting
and reduction o f relief because the recession of
scarps (of sandstone capping) is com plete, the sand­
stone capping has been stripped o ff and the weaker
shales have been exposed. T hus, the absence o f hard
and resistant lithologic elem ent (sandstones) has
effected d ow nw asting in D avisian style o f lowering
of reliefs. During the sam e erosional history o f the
region, Sharda Pole hill has un dergo ne the reduction
of relief of at least 72m w h ereas the tops o f central
plateau and flat-topped m esas (ranging in height
between 500m and 58 0 m ) are least affected by
dow nw asting though they have undergone parallel
retreat and the process is still continuing. Such
conditions again support D avisian model of land­
scape evolution and taboo H ack's equilibrium model
and co rrob orates the slope rep lacem en t model of
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The entire Bhandcr plateau is a maturely
dissected plateau but the existence of waterfalls
cannot be accom m odated in Davisian model of g eo ­
graphical cycle. The heights o f the scattered hills
standing over the lower uplands (fig. 3.8 : block
diagram ) equal the central plateau surface in height
(accordant level) and the exposed rock beds over the
escarpm ents and these hills show perfect parallel­
ism. Such conditions do not support any upliftment,
a necessary requirement for rejuvenation and nick
points as required in Davisian model o f landscape
development. Further, dow nw asting seem s to be
ineffective in this region. This problem can be, for
the time being, solved if we look at the locations and
nature o f these waterfalls. There arc two distinct
locations o f waterfalls viz., (i) the steepest and
highest waterfalls (upto 60m ) are located along the
rim s o f the plateau generally at the heads o f the
cmbayrncnts and small tributaries ; (ii) the second
line ol waterlalhi in located further inland over the
higher plateau and in n ge in height from lOrn to30m
and are characterized by deep, long and narrow
gorges helow their banc*, ft may be pointed out that
the find category ol falls is, in fact, head* of
em baym enls or scarp heads where water falls down
the vertical walls only when there is rain, otherwise
they remain dry during rnoM period o f the year.
Thus, these waterfalls are not true falls signifying
heads or rejuvenation rather they arc structural in
character, liven this is accepted, the coexistence of
the drainage net with graded profile of equilibrium
over the higher plateau and lower uplands, signifi­
cant breaks in slope in their middle courses and
above all steep slopes having frcc-facc elem ent of
scarp faces in no case can be explained on the basis
of Davisian model and thus his model miserably
fails in the present case.
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87
t h e o r ie s o f l a n d f o r m d e v e l o p m e n t
Penck due to backw asting and hillslope cycle model
o f L.C. King.
Thus, both the exam ples o f steady state and o f
no substantial changc in the landscape on the one
hand and effective low ering o f relief and progres­
sive change (from free face, rcctilincar hillslope to
convexo-concave slope and reduction of height from
560m to 488m ) on the other hand within a distance
of one km, over a region o f uniform structure and
same geological history, having no trace o f any
fossil landform apparently different from the present
ones, no subaerial processes in the past history of the
Fig. 3.9:
geomorphological evolution o f the region at least
since Cretaceous period etc. nullify the need and
desirability and even the authenticity o f a single
theory o f landscape developm ent all over the globe.
I f wc p r o c e e d f u r t h e r e a s t w a r d a n d
northeastward from Bhander Plateau (say towards
R cw a p la te a u ) ‘t e c to n o -g e o m o r p h ic m o d e l ’
(Morisawa, 1974 and 1975) becom es valid in ex­
plaining the landscape characteristics. The northern
rim o f Rewa Plateau (fig. 3.9) overlooking transYamuna plain ascends slowly from 160m to 200m
and then is characterized by an abrupt, vertical and
Part o f Rewa scarps with indentation, valley embayments, nicks and waterfalls (after Savindra Singh, 1974)
em bayments similar to Bhander escarpm ents but o f
lower heights. The T on s river, the upper course o f
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precipitous escarpment from 200m to 260 or 280m
and is highly c ren u laied and indented having
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GEOMORPHOLOGY
88
(very slow) due to plate tectonics is equalled by
degradation and the scarps are experiencing parallel
retreat maintaining their original character (free face
above and middle rectilinear segm ent together with
lower segments of concave elem ent below) but it
should be remembered that equilibrium stage is not
static as the earth is so dynamic.
which is graded over the eastern lower upland of
Bhander plateau, abruptly descends through a steep
vertical waterfall of 70m height carvcd out in hori­
zontal but massive V indhyan sandstones (24°47' N
and 81°r56" E) and after draining for a distance of
about 6 km downstream in a narrow, deep and
vertical gorge (valley walls rise upto 60m from the
river bed) receives the Bihar river which makes the
m ost outstanding Chachai Falls o f 127m hardly 1.5
km upstream from its confluence with the Tons river
and the gorge (1.5 km long) is very massive and has
been carved out of horizontal massive beds of Vindhyan
sandstones. Further eastward, the Mahanadi, a tribu­
tary of the Tons, makes a 98m falls at Kevati (only
9km cast of Sirmaur market) and drains through a
straight but narrow and deep gorge having a vertical
valley-side wall o f 80m for a distance of 4 km and
thence the gorge widens out further downstream.
Further in the east and northeast there is a line of
waterfalls ranging between 20m and 145m in height.
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The above discussion and observation of
Palmquist (that ‘only two premises are necessary to
produce a reference system which allow s both for
landform evolution and dynam ic equilibrium, (i)
geomorphic systems arc multivariate open systems
which tend towards a steady state equilibrium and
(ii) the mass of rock existing above base level con­
stitutes an external variable to which the system is in
constant disequilibrium’, Palmquist, 1975, p. 159)
warrant the necessity of multiple theories. Thus, it
facilitates us to conclude that the landscapes are
complex rather than simple and these should be
studied with no bias of a particular theory or model
but should be viewed with open mind taking into
Such conditions (nick points in the long pro­
account the consideration of adjustment of landforms
files of major rivers of 5th to 8th order) indicate
to lithology, geologic history of the region, tectonic
rejuvenation of northern rim of the Deccan Fore­
activity and magnitude of denudational processes
land. The subduction of Indian plate beneath Asiatic
plate culminated in the Himalayan orogency and
and above al I minute observation of landforms in the
jerks caused by the Himalayan upliftment intro­
field and laboratory. Thus, the composite approach
duced rebound impact on northern rims of the Deccan
envisages detailed objective description of landforms
Foreland which was responsible for relative uplift of
through field observation and morphometric details,
the latter in relation to the trans-Yamuna plain. This
their classification into gcnetic/non-gcnetic catego­
activity accelerated the rate of denudational proc­
ries and their explanation highlighting their devel­
esses and caused disequilibrium of action. It is to be
opment whether they may be the result of progres­
noted that landscape is the outcome of the relation­
sive change through time (as is the case of Sharda
ship between the rates of intensity of tectonic force
Pole hill, referred to above), or they may be the
and denudational processes and between the force of .
outcome of the balance between continuing uplift
resistance of materials and energy. Whenever there
and erosion (as is the case of the northern rim of
is difference between these tw-o, disequlibrium re­
Deccan Foreland) as a case of open-system steadysults and when these two equal, equilibrium condi­
state model of landform development or they may be
tion is maintained. If M orisawa’s statement is fol­
the product of interaction between diastrophic ac­
lowed, ‘denudational and tectonic forces in Japan
tivity and climate or they may be due to parallel
and in the Himalayas have reached an equilibrium of
action at present’ (Morisawa, 1975, p. 211), equilibretreat etc. A combination of m ore than one possi­
•ium model w'orks in this case as the rate of uplift
bilities may be possible in a single region.
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CLIMATIC GEOMORPHOLOGY AND MORPHOGENETIC
REGIONS
D iagnostic landforms ; geomorphological processes and climatic con­
trol ; direct control of clim ate; indirect climatic control; climatic changes
and landforms ; morphogenetic regions.
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CHAPTER 4
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89-104
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4
CLIMATIC GEOMORPHOLOGY AMD
MORPHOGENETIC REGIONS
The concept o f climatic geomorphology envis­
ages that each clim atic type produces its own
characteristic assemblages of landforms and set of
geomorphic processes which shape them. Though
the concept o f climatic geomorphology found grcund
in Germany and France by the end of the 19th
century based on the works o f scientific explorers
like Yon Richtchofen in China, Passarge, lessen,
Walther, and Thorebecke in Africa, and Sapper in
central A merica and M alanesia but certain funda­
mental problems regarding this concept could not be
solved as yet. Even W.M. Davis recognized humid
temperate region as ‘n o r m a l ’ for landscape develop­
ment but ‘he treated the landforms of non-temperate
climatic regions as deviants from the ‘normal’ scheme’
(D.R. Stoddart, 1969). The German scientists, who
were convinced about the imposing influences of
climate on geomorphic processes and landforms
resulting therefrom, propounded that in Germany
each climatic region was characterized by distinc­
tiv e a s s e m b la g e o f la n d fo rm s w hile French
geoscientists identified climate as a major control­
ling factor o f landscape development.
D.R. Stoddart (1969), L. Wilson (1969, 1973), J.
Tricart and A. Cailleux (1972) etc. The advocates of
climatic geomorphology argue that the rate of oper­
ation of weathering and erosional processes, vegeta­
tion type, surface runoff, nature and rate of erosion
and mechanisms of landform genesis and develop­
ment differ considerably from one climatic region to
the other but it may be pointed out that they could not
be able to present convincing evidences in support of
their arguments as yet.
The concept of climatic geomorphology is
based on the following three major themes (D.R.
Stoddart, 1969)—
(1) Landforms differ significantly in different
climatic regions.
(2) Spatial variations of landforms in differ­
ent climatic regions are because of spatial variations
in climatic parameters (e.g. temperature, humidity,
precipitation etc.) and their influences on weather­
ing, erosion and runoff.
(3) Quaternary climatic changes could not
obscure relationships between landforms and cli­
mates. In other words, there are certain diagnostic
landform s which clearly dem onstrate climatelandforms relationships.
The concepts of climatic geomorphology and
morpho-climatic / morphogenetic landscapes and
regions were further enriched by the classical work
of J. Budel (1948, 1982), L.C. Peltier (1950), C.
Troll (1958), W.F. Tanner (1961), P. Birot (1968),
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(4) Besides, ‘not only do different levels of
magnitude and frequency of processes have differ­
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GEOMORPHOLOGY
90
than present climatic con ditio ns9 (D .R Stoddart,
ent geomorphic effects in different environments,
but within a single environment different attributes
of morphometry (e.g. hydraulic geomorphometry,
slope forms and divide configuration) may them ­
selves be formed by processes of different magni­
tude and frequency’ (R. J. Chorley, ct. al, 1985). The
above mentioned themes of climatic geomorphology
need explanation separately.
1969).
In se lb e rg s representing steep sided residual
hills are considered to be the representative landforms of hot and arid and semi-arid clim ates and the
end product o f arid cycle o f erosion but insclberg*
have been found in different parts o f the world
having different ‘climatic conditions, from hurnid
subtropical in Georgia, N orth A m erica to humid
tropical in the Guinea coastlands, south India, Bra­
zil, and to desert areas in western North America,
M auretania, and south-w est A fric a ’ (D.R. Stoddart,
1969). It is argued that inselbergs are structurally
controlled rather than clim atically controlled and
most o f the present inselbergs w ere form ed before
Quaternary epoch, ‘hence present clim ates are not
necessarily those in which the inselbergs were formed’
(D.R. Stoddart). It may be possible that inselbergs
might have been formed when the clim ate was arid
or semi-arid which m ight have changed after their
formation.
4.1 DIAGNOSTIC LANDFORMS
The advocates of climatic geomorphology
have attempted to collect information about the
characteristics of such landforms w'hich may be
regarded as diagnostic landforms to determine climate-landforms relationships. Such typical diag­
nostic landforms are regarded as representatives of
a particular climate. Climatogenetic or climatically
controlled landforms are identified and differenti­
ated in two ways e.g. (i) general observation and
acquaintance of whole landscape of each climatic
region, and (ii) identification o f typical or distinctive
landforms which represent the control of a particular
climate. The typical landforms are, in fact, main
tools o f climatic geomorphologists which help them
in determining climate - landforms relationships in
different climatic regions. Such distinctive landforms
are designated as diagnostic landform s. The diag­
nostic landforms, identified by the climatic geomor­
phologists so far include inselbergs, duricrusts, ped­
iments, tors etc.
P ed im en ts, characterized by low-anglerockcut surfaces surrounding m ountains, are considered
to be the representative landform s o f arid (desert)
and semi-arid climates. P edim ents are also found in
a variety of climatic conditions e.g. tropical wet and
dry climate, subtropical and tem perate climate. A
few geomorphologists (e.g. W. Penck) argue that
pediments are structurally and tectonically rather
than climatically controlled. L.C. King has opined
that the process of pediplanation and pedimentation
is universal and it occurs in all environm ental condi­
tions. In fact, ‘many arid zone pedim ents are clearly
polycyclic, developed during the com plex sequence
of Pleistocene pluvials (period o f prolonged rain­
fall) and interpluvials : many appear to be being
distroyed under present climatic conditions, rather
than being form ed’ (D.R. Stoddart, 1969).
D uricrusts are indurated hardened surfaces
of different kinds such as laterites, silcretes, cal­
cretes, alcretes, ferricretes etc. depending on domi­
nance of constituent minerals. Normally, lateritic
crusts are supposed to have been formed in hot and
humid climate of tropical and subtropical areas and
therefore these are indicative of hot and humid
climates. Lateritic crusts are predominantly found in
Chotanagpur highlands (Patlands of Ranchi and
Palamau plateaus) of Bihar (India) and over many
areas o f Decean plateau (e.g. Mahabaleshwar and
Panchgani plateaus of Maharashtra). The presence
o f lateritic crusts in certain parts of Europe (e.g.
U.K., Germany etc.) clearly demonstrates the fact
that these are not the result o f the present climate.
‘Such crusts are often interpreted as o f Tertiary age,
or as having been under continuous formation since
the end o f the Mesozoic. Exposers o f silcretes and
calcretes similarly are often related to past rather
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T o rs, ‘one o f the m ost controvercial land­
forms, are piles o f broken and exposed masses of
hard rocks particularly granites having a crown of
rock-blocks ol different sizes on the tops and clitters
(trains of blocks) on the sid e s ’ (Savindra Singh,
1977). Tors have been considered to be o f periglacial
origin by J. Palmer and R.A. Neilson (1962), of
fluvial origin (humid climate, deep chem ical weath­
ering and exhum ation ol rock debris by running
water) by D.L. Linton (1955), w hereas L.C. King
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CLIMATIC g e o m o r p h o l o g y a n d m o r p h o g e n e t i c r e g io n s
has opined that tors are the result o f universal proc­
ess of pediplanation in different climatic conditions
It may be pointed out that ‘various theories of torformation have been put forth but there is no una­
nimity among the exponents and it must not be as
tors, as mentioned earlier, are not confined to a
particular rock type and climate but a variety of
rocks and climates claim their existence’ (Savindra
Singh, 1977). In fact, the presence of tors right from
Dartmoor of England through Nicaragua to India has
complicated the problem o f origin of tors rather than
solving it.
91
physical weathering are considerably slowed down.
Dense vegetation covering the valley sides and even
reaching the valley floors discourages lateral ero­
sion by streams and thus the processes of valley
widening becomes sluggish. Dense vegetation o f
humid tropics also reduces surface runoff because a
sizeable portion of rainfall is intercepted by forest
conopy and thus rainwater reaches the ground sur­
face in the form of aerial stream lets through the
leaves, twigs, branches and stems o f trees and thus
allows more infiltration.
It may be concluded that the aforesaid repre­
sen tative/diagn ostic la n d fo rm s are older than
Pleistocene climatic changes, so they are definitely
not related to present climates where they are found.
It may be pointed out that climatic relation of landforms at least in glacial, periglacial and desert cli­
mates are undoubtedly confirmed but more mor­
phometric evidences are needed to establish close
relationship between climate and landforms in other
climatic regions. ‘This is not to deny that climati­
cally conrolled landform differences exist, though
morphometric confirmation o f this is scanty; but it is
to assert that the climatic inputs and geomorphic
outputs in denudation system are so litle known that
one cannot be inferred from the other’ (D.R. Stoddart,
1969).
Annual R a in fa ll(in c h e s )
70 60 50 U0 30 20 10
Chemical
4.2 GEOMORPHIC PROCESSES AND CLIMATIC
CONTROLS
It is an established fact that different pro­
cesses work in different climatic regions and with
climatic variations there is also variability in the
nature and mode of influences of climatic parame­
ters which affect denudational (weathering and ero­
sion) processes. Tem perature and humidity have
emerged as the most significant climatic parameters
of the control of geomorphological processes in
different climatic regions. High mean annual tempera­
ture and rainfall (and hence perennial humid condi­
tion with high temperature throughout the year)
favour deep chemical weathering in humid tropics,
but the presence of gullies on steep slopes and
canyons in the same humid tropics presents a geo­
morphic riddle. Besides, vegetation also plays im­
portant role in controlling geomorphic processes in
tropical humid areas, because the combination of
high mean annual temperature and rainfall favour
dense vegetation even on steeper slopes with the
result the processes o f soil erosion, sheetwash and
weathering
80 70 60 50 <*0 30 20 10
Physical weathering
Chemical weathering, B : physical weather­
ing in relation to mean annual temperature
and rainfall (After LC. Peltier, 1950J.
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Fig. 4.1 A
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92
GEOMORPHOLOGY
The areas, clearcd o f natural vegetation through
h u m a n activities in the h um id tropics, are subjected
to activc vertical erosion. S apper (1935), Friece
(1 9 3 5 ) and W e n tw o rth (1928) have identified active
d eep ch em ical w eathering and vertical erosion in
h o t an d h u m id clim ates due to high mean annual
tem perature and rainfall. E xcessive humidity accel­
erates the process o f landslides, soil creep and slum p­
ing. D ifferent com binatio ns o f temperature and pre­
cipitation generate different types o f weathering
m e c h a n is m s (figs. 4.1), w eathering regions (fig. 4.2)
and effectiv en ess o f m assm ovem ent, wind action
and pluvial erosion (fig. 4.3) in different climatic
regions.
T.C. C ham berlin and R.T. Cham berlin (1910)
differentiated landform s o f humid tropics from those
Mean
Annual
o f the m id-latitude tem perate landform s. Different
rock types respond differently to the combinations
o f w ater and tem perature in different climates. For
exam ple, limestones becom e chem ically weak to
w eathering and erosion in hot and h um id climate
because chemical w eathering becom es m ore active
but these becom e resistant to chem ical weathering in
hot and arid clim ate because o f scarcity o f water and
humidity. Soil creep is also m ore or less absent in
arid regions because o f scarcity o f w ater (and hence
undersaturation o f soils).
Even there is such spatial variation in the
climatic param eters within a single climatic region
that geom orphic processes are also influenced spa­
tially by such variation. Altitude, slope aspect, di­
rection, insolation, and precipitation are significant
Rainfall
Cinches)
<JL-------- i f - r f
M od erate
M echanical^
Moderate Chemical
weathering Frist action
very
Moderate
C hemicol
strong
weathering
Weathering
Chtmica I
Weathering
Peliierr^I950)Si° nS
re^ ° n l° vary‘n8 co,nbinations o f mean annual rainfall and temperature,
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Fig. 4.2:
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c l im a t ic g e o m o r p h o l o g y a n d m o r p h o g e n e t i c r e g i o n s
Mean A n n u a l R a i n f a l l ^ i nc he s )
80 70 60 50 U P 30 20 in
variables which influence processes and landform s
resulting therefrom. For exam ple, southw ard facing
slopes o f east-west trending valleys are steeper than
northward facing slopes because the latter receive
comparately less am ount o f insolation and thus are
covered with snow for longer duration, freeze-thaw
is less effective and hence w eathering and erosional
processes are also less active while southw ard slopes
are more affected by w eathering and erosion due to
greater amount o f insolation.
Several examples may be cited which dem on­
strate strong influence of climatic param eters on
geomorphological processes and landform s result­
ing therefrom. This aspect will be detailed out in the
succeeding sections. Climate controls morphogenetic
processes and landforms both directly and indi­
rectly.
Mass mo ve ment
r
i
i
i i
10
20
-
/ 1 i
i
/ Mi nimum
It may be pointed out that different m orpho­
genetic processes operate in different climatic re­
gions and with climatic variation the m ode and rate
of operation of geomorphic processes also differ
from one climatic region to the other. Besides w eath ­
ering, climate also influences the m echanism s o f
transportation and deposition. A few g e o m o r­
phologists have studied in detail the m orphoclim atic
mechanisms in some climatic regions.
y
/
o U0
/
>
Direct Controls of Climate
-
/^^Mod e r a t
30
t
Maximum
/-
X
70
80
Temperature is a very significant climatic
parameter which not only influences but also co n ­
trols the mechanisms of different m orphogenetic
processes. It is known to all that tem perature varies
considerably in different climatic regions. I f te m ­
perature (mean) o f a region is below freezing point
(less than 1°C), then there is frequent and w ide­
spread frosting. If there is such fluctuation in daily
temperature that it goes dow n below freezing point
during night but rises above freezing point during
day time, then there occurs diurnal freeze (during
night) and thaw (during day) cycle which leads to
alternate processes o f contration (due to freezing
during night) and expansion (due to thaw during day
time). The repetition o f this m echanism causes frost
weathering in periglacial climate (congelifraction)
during transitional periods o f sum m er and winter
seasons in temperate climate. Jointed rocks are shat­
tered under the impact o f frost weathering w hich is
responsible for the origin o f distinctive landforms
/
/Mini;
11 J l . i
i \i '\ \ i
E
r
o
s
i
o
n
Pluvial
1 -11I 1---- 1 1 " 1
/M inim um \
93
/
(
80 70 60 50 to 30 20 10
Wind Action
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Fig. 4.3 :A : Nature ofmassmovement, B : pluvial erosion
and C : wind action in different climatic
conditions (after L C . Peltier, 1950).
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GEOMORPHOLOGY
94
(m o rph og enetic) pro cesses in a variety o f ways.
T here is large daily ran g e o f te m p e ra tu re (u pto 33°C)
in h ot desert clim ate w ith the resu lt the ex pansion
and contraction co efficien t reg isters in c re ase w hich
ham pers d ev e lo p m e n t o f jo in ts in rocks. R o c k s are
shattered due to altern ate e x p a n s io n (d ue to very
high tem perature du ring d ay ) a n d co n tra c tio n (due
to considerable fall o f te m p e ra tu re d u r in g n ig ht) into
granular disintegration. H ig h diu rn al ra n g e o f te m ­
perature leading to rep etition o f e x p a n s io n a n d c o n ­
traction for longer duration c a u se s flaking in the
rocks w herein thin sheets o f rocks are p e e le d o ff
layer after layer, the process is called exfoliation or
on io n w e a th e r in g . This process is not o n ly co n fin e d
to hot desert areas but is also o p e ra tiv e in m o n so o n
climates. F or exam ple, the case o f flak in g and e x fo ­
liation w eathering can well be seen o v e r ex p o sed
granito-gneissic dom es o f C h o ta n a g p u r in general
and Ranchi plateau in particular.
like tors w here the rocks are w idely jointed. The
im p act of freeze-thaw m echanism on unconsolidated
geo -m a terials beco m es very interesting in active
layers of periglacial areas w here if this m echanism
b e c o m e s a c tiv e in clay m aterials, solifluction
(congelifluction) becom es operative as clay de­
posits resting on slopes are softened and loosened
d u e to frost action and slum p dow nslope when
lubricated by m eltw ater (when frost thaws due to
rise in tem perature).
Fro st action also influences surface runoff
and undergroun d drainage. For example, there is
m o re or less regularity in stream discharge in hot
and h um id climate but there is much variation and
fluctuation in the climates having frost actions as
discharge becom es m inimum during winter due to
freezing o f a sizeable portion o f water but there is
m axim um discharge o f water during sum m er due to
thaw o f frozen water. The whole o f active la y er
lying above p e r m a f r o s t in periglacial climate freezes
but the upper part o f it thaws during sum m er but the
thawed water does not reach greater depth in active
layer and hence water flows rapidly as active surface
runoff (though for very short duration) and the
streams become able to transport loads of large size
even on gentle slopes but the streams soon become
overloaded and are called stone stream s.
The regions having m a rk e d d iffe re n c e in h u ­
mid and dry conditions (i.e. clim ates h av in g sea­
sonal variations in wet and dry c o n d itio n s) g en era te
different types o f conditions for w e a th e rin g and
erosion. The amount, intensity and p erio d ic ity o f
rainfall are significant aspects w hich co ntro l and
condition denudational processes in clim ates c h a r ­
acterized by seasonality. The area having clays gives
birth to polygons w hen dehydrated due to h ig h
temperature during dry condition. M o n tm o rillo n ite
is subjected to largest im pact o f variation in h u m id ­
ity. Desiccation o f m ontm orillonites d u e to long
spell of dry condition results in the d e v e lo p m e n t o f
numerous polygons o f varying sizes and d im ension.
Rainwater reaches the depth o f 2-3 m th ro ugh the
cracks o f such polygons and collects at the base
where the geomaterial is m ore w et and relatively
impermeable. Thus, the w ater at the base o f p o ly ­
gons becomes sliding plane and stim ulates earthflow wherein polygons ju st above the sliding plane
move downslope. Such geo m orphic activities are
operative in the areas o f frequent alluviation during
floods in the alluvial flood plains o f rivers in m on ­
soon climate (e.g. India). M editerranean climatic
regions, characterized by m arked contrast in wet and
dry seasons, present ideal conditions for such geo ­
morphic mechanism i.e. slum ping and earthflow.
Conversely, clay rocks having kaolinite as major
constituent mineral has lowest contraction coeffi-
Aeolian process is influenced by frost action
in a variety o f ways. Generally, frost discourages
transportation o f materials in cold climates as the
loose fine materials are consolidated due to frosting
but some times strong winds like blizzards remove
these consolidated materials but the mechanism of
abrasion is not effective and hence topographic
features produced by deflation, abrasion, sandblast­
ing, and pitting in hot desert areas are not found in
frost susceptible climates. The resultant deposits are
called niveo-aeolian deposits in cold climates.
Coastal processes are also affected by frost
action. The coastal rocks are hardened due to frost
action during winter in cold climates, with the result
they protect the coasts from active erosion by sea
waves but the sea cliffs suffer from rock disintegra­
tion due to frost weathering caused by freeze-thaw
action.
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The changes in thermal conditions above freez­
ing point influence and control mechanisms of weath­
ering and erosion by different geomorphological
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q j MATIC g e o m o r p h o l o g y a n d m o r p h o g e n e t i c r e g io n s
cientdue to desiccation. Clay particles, in such case
are coalesced and consolidated due to raindrops and
are hardened w hen desiccated. Consequently the
impermeability o f clay rocks increases, which’dis­
courages infiltration o f rainw ater and encourages
surface runoff.
from one vegetation type to the other type. There is
maximum interception of rainwater in equatorial
rainforests o f humid tropics wherein ‘aerial streams’
become most effective. Some geomorphologists (e.g.
Rougerie, 1960) have also emphasized the geomorphic
importance of surface runoff. The rainfall - intercep­
tion depends upon seasonal conditions in tropical
and subtropical deciduous forests. Rainfall intercep­
tion is minimum during peak summer season with
high temperature and complete dry condition be­
cause most of the trees and bushes become leafless.
In such condition, if there is occasional rainfall,
splash erosion becomes very effective. On the other
hand, splash erosion is lessened during rainy season
when there is maximum interception because the
vegetation becomes lush green. The Indian monsoon
lands come under the influence of seasonal varia­
tions in vegetation control on rainfall interception
and hence resultant seasonality in the effectiveness
of fluvial and weathering processes. There is least
interception of rainwater and hence maxim um splash
erosion (though occasionally) because of near ab­
sence of vegetation in tropical and subtropical hot
desert climates. Surface runoff and consequent over­
land How is lessened in temperate steppe climates
because forests and grass cover protect the ground
surface from direct impact o f falling raindrops and
thus allow more infiltration of rainwater. It may be
mentioned that most of the grasslands o f temperate
climates in different continents (e.g. Steppe in E ura­
sia, Prairies in N. America, Pam pas in S. America,
Velds in South Arica nad Downs in Australia) have
now been converted into agricultural farmlands which
have now become famous 'granaries o f the w o rld ’
and thus these converted farm lands (from original
grasslands) are subjected to m a x im u m splash and
sheet erosion resulting into im m ense loss o f rich
soils.
Indirect Climatic Controls
(Climate -» Vegetation —» Morphogenetic Proc­
esses)
Climate influences and controls morphogenetic
processes (geom orphological processes) indirectly
through (i) vegetation and (ii) soils. The world
distribution o f vegetation is azonal which is closely
related to climatic zones. In fact, climate and vegeta­
tion and climate and soils are so intimately interre­
lated that these influence each other. For example,
vegetation determ ines pedogenesis (soil formation)
while soils determ ine vegetation types which again
depend on climate. V egetation, in turn, also influ­
ences floral characteristics. These interactions be­
tween climate, soils and vegetation, in turn, influ­
ence and control nature, type and mode of operation
o f different denudational processes.
The kinetic energy o f rainfall (say raindrops)
and its geo m orph ic significance is greatly con­
trolled by interception capacity of vegetation. It
may be pointed out that the areas devoid of vegeta­
tion (open areas) are directly pelted by falling rain­
drops with m a x im u m kinetic energy and causes
sp lash ero sio n w herein loose particles are resettled
on the ground surface and form a strong cuirasse
which d iscou rages infiltration o f rainwater and fa­
vours increased surface runoff. On the other hand,
densely vegetated areas m ainly o f forests are charac­
terized" by least splash erosion because o f maximum
interception o f rainw ater. In fact, the kinetic energy
o f falling raindrops is consideraly reduced due to
interception o f raindrops by forest canopy and con­
sequently the ground surface is protected from direct
pelting by raindrops as rainw ater reaches the ground
surface very slow ly through leaves, branches and
stems o f trees in the form o f ‘aerial strea m lets’
which incourage m ax im u m infiltration o f rainwater
if the surficial materials or regoliths are permeable.
If the ground surface is im perm eable, then the rain­
water becomes surface runoff.
Like rainfall, vegetation cover also affects
snowfall through its interception capacity. There is
maxim um interception o f snowfall by forest cover
in temperate and taiga climates.
V egetation greatly influences the soil tem per­
ature which in turn influences m icn> g eom o rp hological processes. The variation in soil tem perature
during sum m er and winter, and during day and night
is minim ised because o f forest cover. T he ground
surface under thick forest cover receives relatively
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It may be m entioned that interception o f rain­
water by vegetation varies from season to season and
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GEOMORPHOLOGY
%
o p ined that ‘like c lim a te v e g e ta tio n typ e a ls o p r o ­
d u ces its d istin c tiv e a sse m b la g e o f la n d fo rm s. ’
>;
96
less am o un t o f insolation because about one third o f
insolation is used by plants in the process of p h o to ­
synthesis and evapo-transpiration and hence ground
tem perature decreases resulting in low soil te m p era­
ture. N ight tem p eratu re o f forested areas becom es
higher than open areas even in the sam e clim atic
zone because ground radiation is retarded in forested
areas. Thus, daily range o f tem perature becom es
high in open areas in com parison to forest-covered
areas. This aspect o f diurnal range o f tem perature
also increases variation in soil tem perature m ore in
open areas than in the forested areas. Thus, the
variation in soil tem perature in open and forest
covered areas even within a single climatic zone
causes spatial variations in the nature, pattern and
intensity o f weathering.
Climate->Vegetation->Soils and Morphogenetic
Processes
It m ay be m e n tio n e d th a t c lim a te influen ces
vegetation and in turn v eg etatio n in flu e n c e s soils
and thus there is in terractio n b e tw e e n soils and
m o rp h o g en e tic p rocesses. It m a y b e f u rth e r pointed
out that it is not nece ssary th a t c lim a te alw ays
influences m o rp h o g e n e tic p ro c e s s e s v ia veg etatio n
and soils. T h us, the f o llo w in g in te rre la tio n s h ip s
betw een these variab les m a y b e id e n tifie d viz. (i)
clim ate —> v eg etation —> m o r p h o g e n e tic p ro c e s s e s
—» landform s, (ii) clim a te —>soils —» m o rp h o g e n e tic
p ro c e sse s—^landform s, an d (iii) c l im a te - m o r p h o genetic p ro c e s s e s —^landform s.
Vegetation cover m inim ises variation in soilm oisture because the process of desiccation o f soil is
slowed down as forest-covered areas receive rela­
tively less am ount o f insolation and are p rotectedby
the shades provided by the vegetation cover. The
desiccation o f soil, in turn, influences soil cracks
(mud eracks), surface ru noff and groundwater.
T h ere is very clo se re la tio n s h ip b e tw e e n p e ­
do genesis and c h em ica l e ro sio n w h ic h is a c c e le ra te d
by infiltration o f w ater and d e c o m p o s itio n o f h u m u s .
F o r exam ple, infiltrating w a te r i.e. d o w n w a r d m o v e ­
m ent of w ater rem o v e s m a te ria ls fro m ‘A ’ h o riz o n
or eluviated zone o f soil p rofile th r o u g h th e m e c h a ­
nism o f eluviation (lea ch in g ) a n d tra n s p o r ts th e m to
illuviation zone (B horizon). C h e m ic a l e l e m e n t s are
further transported d o w n w a rd to C h o riz o n . T h u s ,
part o f soil profile abo ve C h o riz o n is s u b je c te d to
chemical erosion. T he m e ch an ism o f elu v atio n (le a c h ­
ing) is co n tro lled by te m p e ra tu re a n d in filtra te d
water. L e a ch in g or e lu v iatio n b e c o m e s m i n i m u m in
tem p erate clim ates b e c a u se o f d e c r e a s e in m e a n
te m p eratu re and b io log ical a c tiv itie s d u r in g w in te r
season w h ereas it is m a x im u m in h o t a n d h u m id
clim ates b ecause o f h igh te m p e r a tu r e , h ig h rain fall
am o u n t and a b u n d a n t veg etal c o v e r th r o u g h o u t the
year. L ea ch in g d e c re a se s in m o n s o o n c lim a te c h a r ­
acterized by w e t an d dry s e a s o n s (b u t it b e c o m e s
active d u rin g w et m o n s o o n m o n t h s e.g. Ju n e to
S ep te m b er) w h e re a s le a c h in g b e c o m e s practically
ab sent in arid c lim ates. C h e m ic a l e r o s io n a n d w e a th ­
ering in soil h o riz o n s leads to m e c h a n ic a l c h a n g e s o f
various sorts in the reg o lith s. F o r e x a m p le , the solid
rock b e c o m e s triab le du e to su c h c h a n g e s . F riab le
horizo n is in d u rated an d h a r d e n e d to fo rm cuirarsses.
C o llo id al h u m u s re stin g o v e r clay la y e r co n so lid ates
and a g g re g a te s clay p article s w ith the re s u lt clay
b e c o m e s co h esiv e . L im e c o n te n t in th e soil protects
h u m u s from d e c o m p o s itio n a n d p r o v id e s stab le co ­
hesion. T h u s , c a lc a re o u s so ils m ix e d w ith hu m us
It may be opined that vegetation cover m in ­
imises the influences o f atm ospheric processes and
thus the m orphogenetic processes become sluggish.
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Climate, through vegetation, also influences
transportation m echanism by different geom orphic
processes. As mentioned earlier, dense vegetation
cover m inim ises overland flow and m axim ises in­
filtration o f rainwater. C onversely, open areas (u n ­
covered) generate m axim um overland flow because
in case o f strong rainstorm s with high rainfall inten­
sity rainfall am ou nt exceeds w ater absorption cap a c­
ity o f ground surface and hence instantaneous o v er­
land flow is generated and subsoil rem ains dry. It
may be m entioned that vegetation cover, on one
hand, influences and controls the volum e, nature and
intensity o f surface runoff, it also influences the
geom orphic effects o f ru n o ff on the other hand.
Dense grass co ver lessens surface ru n o ff and tran s­
portation and erosion by it m ore than forest cover. In
fact, grass cover reduces soil erosion considerably
and protects the g rou nd surface from sheet erosion.
Trees obstruct w inds and hence reduce wind v elo c­
ity and hence aeolian erosion and transportation are
rem arkably reduced. C o n sid erin g the g eo m o rp h ic
significance o f vegetation T ricart and C ailleaux have
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CLIMATIC g e o m o r p h o l o g y a n d m o r p h o g e n e t i c r e g i o n s
become resistant to m echanical erosion (i.e. abra­
sion). The permeability and porosity o f geomaterials
allows more infiltration o f rainw ater and thus re­
duces surface ru n o ff and overland flow considerably
whereas soil erodibility is rem arkably reduced due
to increase in resistance in soil particles consequent
upon aggregation and cohesion o f particles.
97
1969). It may be m entioned that large-scale clim atic
changes throughout geological history pose serious
problem regarding identification and substantiation
of climate-landforms relationship as m o st o f the
evidences might have been either destroyed or o b ­
scured by subsequent changes. Thus, it becom es
very difficult to ascertain as to w hether the landforms
found in the present climate are the result o f the sam e
climate or of the result o f palaeo-clim ates. C onse­
quently, it is very difficult to find out and identify
real links between climate and landforms.
The illuviation horizon (zone of deposition of
materials) w hen com pacted and hardened loses its
erodibility but obstructs further dow nw ard m ove­
ment (infiltration) o f water. This mechanism causes
over saturation o f upper soil horizon (eluviation
zone) with the result surface runoff and overland
flow increases w hich causes m ore surface erosion. It
may be m entioned that cuirasses formed dur to
illuviation o f insoluble (o f different forms depend­
ing on the nature o f materials e.g. alcrete, silcrete,
ferricrete, calcrete etc.) are resistant to mechanical
erosion and hence are responsible for the develop­
ment of bold reliefs and protect reliefs and erosion
surfaces in tropical and subtropical humid climates.
Soils play m ajor role in the attainment and
maintenance of equilibrium in relief, climate and
vegetation. P edogenesis also helps in the recon­
struction of palaeo-processes and changes in topo­
graphic features.
Following Tricart and Cailleux it may be
stated that landscape form ing processes are con­
trolled by tectonic forces, climate and biological
factors ; climatically controlled vegetation cover
produces topographic variations through the mecha­
nism of pedogenesis. Thus, vegetation cover, on one
hand, controls chem ical erosion, on the other dis­
courages mechanical erosion.
4.3 CLIMATIC CHANGES AND LANDFORMS
It may be pointed out that m any o f the as­
sumptions and premises o f climatic geom orph olo gy
could not be substantiated. D oughlas has opined that
climate plays insignificant role in the developm ent
of landforms. D.R. Stoddart (1969) has rem arked
that, ‘taken together, the evidence suggests that
climatic changes have been so continuous in the last
50 million years, and so rapid in the last 2 million
years, that equilibrium landform s can rarely have
been developed’. It may be concluded that unless
sufficient morphometric data o f landform s are m ade
available from different climatic regions and landform
variations from one climatic region to other climatic
region are not ascertained and substantiated on the
basis of these morphom etric data, concepts o f cli­
matic geom orphology cannot be validated but can be
retained as a working hypothesis.
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Sufficient evidences have been collected to
substantiate m ajor climatic changes at global level
throughout geological history of the earth. The cli­
matic geom orphologists are convinced that inspite
°f largescale climatic changes during Quaternary
epoch the evidences o f clim ate-landform s rela­
tionships could not be obscured but the climatic
changes pose a critical problem in climatic geo­
morphology, for it cannot be assum ed that the land­
forms found in any given climate have developed in
response to it. How, then, can the links between
climate and landform be identified ? ’ (D.R. Stoddart,
Regarding the present landscapes and cli­
matic changes during Quaternary the scientists are
of opinion that though Q uaternary climatic changes
were rapid and of great intensity but these could not
be of much geomorphological significance because
these were of shorter duration in com parison to
earlier climatic changes. A few geom orphologists
have expressed skepticism regarding influences o f
earlier climatic changes on landform s e.g., ‘ex cep t
in case of ice action, it is likely that m any clim atic
conditions existed for so short a time that they w ere
morphologically significant only in areas o f w eak
rocks and considerable r e li e f (D.R. Stoddart, 1969).
N.M. Starkhov (1967) aftercareful study o f T ertiary
and Quaternary climatic changes and their im pacts
on landforms has stated that, ‘in m o st areas, h o w ­
ever, the present landscapes are com plex m o saics
consisting of small areas inherited from Tertiary
conditions, and tracts of forms developed durin g the
Quaternary complex climatic c o n d itio n s’ (quoted
by D.R. Stoddart, 1969).
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GEOMORPHOLOGY
98
eters e.g. m ean annual tem perature and m ean annual
rainfall w hich determ ine m a jo r m orph ogenetic pro­
cesses. Thus, Peltier defined and classified m orpho­
genetic regions on the basis o f d o m in a n t processes
a n d n o t on th e b a s is o f l a n d f o r m g e o m e tr y
(m orphom etric data) into 9 types e.g. (i) glacial, (2)
periglacial, (3) boreal, (4) m a ritim e, (5) selva, (6)
m oderate, (7) savanna, (8) sem i-arid , an d (9) arid
m orphogenetic regions (fig. 4.4).
4.4 MORPHOGENETIC REGIONS
The concept o f m orphogenetic/m orpho-climatic regions is based on the basic concept o f cli­
matic geomorphology that ‘each geom orphic proc­
ess produces its own characteristic assem blage o f
landforms, and each geom orphic process is the re­
sult o f a particular clim ate’ and thus, ‘each climatic
type produces its own characteristic assem blage of
distinctive landform s’. According to R.J. Chorley
et. al (1985) ‘m o r p h o g e n e tic reg io n s are large areal
units w ithin w hich d is tin c tiv e asso ciatio n s o f
geom orphic processes (e.g. weathering, frost action,
mass movements, fluvial action and wind action) are
assum ed to operate, tending towards a state of
m o r p h o c l im a tic e q u i l i b r i u m w herein regional
landforms reflect regional clim ates’ (R.J. Chorley,
et. al, 1985).
J. Tricart and A. C aille u x , th o u g h strong
advocates o f clim atic g eo m o rp h o lo g y , a d m itted that
researches related to asso ciatio n b etw ee n clim ate
and landform s are not a d eq u a te to su b stan tiate die
concept o f climatic g e o m o rp h o lo g y b e y o n d criti­
cisms. They are of the firm view th a t sin ce clim ate
influences landform d e v e lo p m e n t both directly and
indirectly and hence m o rp h o c lim a tic classification
should not be based on clim atic d a ta alone. Thus,
they suggested follow ing criteria for the d e te rm in a ­
tion and definition o f m o rp h o g e n e tic re g io n s —
The concept of morphogenetic regions was
initiated by Sapper (1935) and Friese (1935) and was
developed by J. Budel (1948, 1982), L.C.Peltier
(1950), W.F. Tanner (1961), P. Birot (1968), D.R.
Stoddart (1969), L. Wilson (1969), J. Tricart and A.
Cailleux (1972).
(a) Identification and c la ssificatio n o f m ajor
m orphogenetic regions on the basis o f m a jo r cli­
matic and zoo geograp hical regions.
J. B u d el p r o p o u n d e d the c o n c e p t o f
fo r m k r e is e n (morphogenetic region) in 1944 and
1948 and further developed the concept in 1982.
L.C. Peltier divided the world into 9 morphogenetic
regions (1950) on the basis of two .climatic paramMe o n
Annuol
(b) Subdivision o f m a jo r m o rp h o g e n e tic re­
gions on the basis of present climatic, zoogeographical
and palaeoclim atic factors.
On the basis o f these tw o crite ria they divided
the globe into 4 m ajor m o rp h o g e n e tic re g io n s and 9
sub-regions (total, 13) as fo llo w s—
Roinfoll (finches )
1 C o ld Z o n e M o r p h o g e n e t i c R e g i o n s
Further divided into tw o s u b re g io n s on the
basis o f intensity and d o m in a n c e o f frost action.
(a) g lacial z o n e (c h a ra c te riz e d by r u n o f f in
solid form e.g. g la ciers)
( b ) p e r ig la c ia l z o n e (c h a ra c te riz e d by runoff
in liquid fo rm -w a te r d u rin g s u m m e r)
2 . F o r e s te d Z o n e M o r p h o g e n e t i c R e g i o n s
F urther divided into 3 s u b re g io n s on the basis
o f intensity o f w inter frost and e ffe c ts o f palaeoclimates.
(a) m a r i t i m e z o n e : n o rm al w in te r season, no
significant frost action, more
influence o f P leisto cene gla­
cial and p erig la c ia l relict
features.
(b) c o n t in e n ta l z o n e : w in te r sev erely cold, effects
o f P leisto cen e an d present
frost m o st do m inant.
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Fig. 4.4 : Morphogenetic regions according to L.C.
Peltier, J950.
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c l im a t ic g e o m o r p h o l o g y a n d m o r p h o g e n e t i c
99
REGIONS
(c) mediterranean zone : dry summer, wet winter,
m o rp h o g en e tic regio ns and (ii) se c o n d o rd er
morphogenetic regions and have identified alto­
gether 8 morphogenetic regions e.g. ( 1 ) glacial, (2 )
arid, (3) humid tropical, (4) tropical wet-dry, (5)
semi-arid, (6) dry continental, (7) humid mid-latitude and (8) periglacial morphogenetic regions.
insignificant effects of
Q u a te rn a ry periglacial
relict features.
3. Semi-arid and Arid Morphogenetic Regions
(of both low and m iddle latitudes)
Further divided on the basis of aridity and
winter temperature.
(A) On the basis o f aridity
(a) step p e region
(b) x ero p h y tic region
(c) d esert region
(B) On the basis o f w inter temperature
(a) m id d le la titu d e region
(b) su b tro p ica l region
(c) trop ical region
4. Hum id T rop ical M orp h ogen etic Regions
(a) savanna region : dry and wet seasons, seasonal
rainfall, m oderate vegetation
cover, enough overland flow,
active chem ical weathering
during wet season.
(b) forest region : hum id tropical region, rainfall
throughout the year, maximum
vegetation cover, chcmical and
bio lo g ic a l w eatherin g most
dominant.
R. J. Chorley, S.A. S chum m and D.E. Sugden
(1985) have presented classification of morphogenetic
regions on the basis o f temperature, precipitation
and seasonality and have attem pted to integrate all
the existing classificatory schem es o f morphogenetic
regions into a com m o n schem e. T hey have proposed
the classification at tw o levels i.e. (1) first order
(i) First-order m orphogenetic regions are
characterized by non-seasonal processes, low av­
erage erosion rates, highly infrequent and episodic
erosional activity such as glacial surges, desert rain­
storms, slope mass failures etc. They have identified
3 morphogenetic regions under this category i.e. (1)
glacial, (2) arid and (3) humid tropical morphogenetic
regions.
(ii) Second-order m orphogenetic regions
include 5 morphogenetic regions i.e. (1) tropical
wet-dry, (2) semi-arid, (3) dry continental, (4) hu­
mid mid-latitude and (5) periglacial morphogenetic
regions. These morphogenetic regions are charac­
te riz e d by s e a s o n a lity o f th e o p e r a t i o n o f
morphogenetic processes, occasional high rate o f
erosion in specific areas under extra-ordinary condi­
tions, some consistency in erosional activity inspite
of episodic nature, changes in response to climatic
changes. Following R.J. Chorley et. al (1985) sec­
ond-order morphogenetic regions can be divided
into two groups :
( 1 ) ‘warmer climates (tropical wet-dry and
semi-arid) where geomorphic processes differ m ost
significantly in terms of the length of the wet season ;
(2 ) cooler climates (dry continental, humid
mid-latitude, and periglacial) w hose geom orphic
processes differ mainly in respect o f sum m er tem ­
peratures, as well as some regard to precipitation
am ount’ (R.J. Chorley et. al, 1985).
Table 4.1 : Peltier’s Morphogenetic Regions_____________ ___________________________________________
M orphogenetic
Regions
1.
M ean A nnual
T em perature
(0°F)
Mean Annual
Morphological Characteristics
Rainfall
(inches)_______________________________ ________________
0-20
2. Periglacial
5-30
^-55
strong massmovement, moderate to strong
wind action, low fluvial action ;
15-38
10-60
moderate frost action, moderate to low wind
action, moderate fluvial action;
3. Boreal
glacial erosion, nivation, wind action;
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Glacial
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100
GEOMORPHOLOG’
4. M aritime
35-70
50-75
strong m a ss m o v e m e n t, m o d e ra te to strong
fluvial action;
5. S elva
60-85
35-90
strong m a ss m o v e m e n t, low slope wash, ab­
sence o f w ind action;
6. M o d erate
35-85
35-60
m a x im u m fluvial actio n , m o d e ra te m ass­
m o v e m en t, m o d e ra te frost action in colder
areas, insignificant w in d action
7. S a v a n n a
10-85
25-50
except coastal areas ; s tro n g to lo w fluvial
action, m o d e ra te w ind action ;
8. S em i-arid
38-85
10-25
strong w ind action, m o d e ra te to s tro n g fluvial
action ;
9. Arid
55-85
0-15
strong w ind action, low fluvial action.
Table 4.2 : Morphogenetic Regions of R.J. Chorley, S.A. Schumm and D.E. Sugden (1985)
M o rph og enetic
K oppen
R egions
R egion
O ther nam es
G eo m orph ic
M o rp h o lo g ic a l
Processes
F eatu re s
EF
subglacial
m ax im um frost weatheirng, mod. m echanical
w eathering, min. c h e m i­
cal w eathering,
m ass
w asting and fluvial p ro ­
cesses except for sea­
sonal m elt-w ater, max.
glacial sour and w ind ac­
tion.
alpi ne topography, abrasion
surfaces, kam es, till forms,
fluvio-glacial features.
2. A rid
BWh
desert, true des­
ert, tropical and
subtropical
desert
min. frost w eathering
ex cept at high altitudes,
max. m echanical w e a th ­
ering, m in im u m ch e m i­
cal w eathering, m assw asting and fluvial p ro ­
cesses, no glacialaction
and m ax. w ind action.
dunes, playas, deflation
basins, angular, debris
co vered slopes, fossil
fluvial form s (e.g. fans
and arroys).
3. H u m id T ropical
A f and
Am
selva, rainforest,
intertropical
zone
no frost w eathering, min.
m echanical w eathering,
max. chem ical w eath ­
ering, m assw asting, flu­
vial processes — m o d ­
erate to m in im u m slope
wash and rainbeat, m ini­
m u m stream erosion due
to lack o f coarse debris,
maximum transport o f che­
mical and suspended load,
no glaical and wind action.
low gradient rivers; wide,
flat, o r gently undulating
flood plain floors upt to
s ev eral kilom eters; steep
s lo p e s ; knife-edged ridges
m a in ta in e d by parallel
retreat o f slope
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CLIMATIC g e o m o r p h o l o g y a n d m o r p h o g e n e t i c r e g i o n s
5. Semi-arid
6. Dry continental
7. Humid m id ­
latitude
8. Periglacial
AW
savanna, tropi­
cal
sheetwash
zone, moist and
dry savanna, tro­
pical savanna
no frost weathering; min. steep irregular slopes o f
coarse debri s characterized
to moderate mechanical
by
parallel
retreat,
weathering;
seasonal
inselbergs,
pediments,
maximum deep chemical
weathering (in wet sea­
bajadas.
son), moderate to max.
masswasting, no glacial
scour, minimum to mod­
erate wind action,
BS,
peripheral
or
pediments, inselbergs,
minimum frost weather­
BW , Cs marginal
hot
arroys, badlands, alluvial
ing ; mini, to mod, me­
deserts,
thorn chanical weathering and
fans, local dunes.
savanna, semi- chemical weathering ;
arid steppe, med­ max fluvial processes (but
iterranean
or episodic in the form of
summer-dry sub­ sheetwash, gullying and
tropical zone.
ephemeral stream action ;
no glacial scour; mod. to
max. wind action,
BSk
steppe zone, midmin. to mod frost weath­
pediments flanked by steep
BWk
latitude grass­
scree-covered
ering but highly seasonal slopes,
lands semi-arid
min to mod. mechanical
slopes, badlands, alluvial
steppes, degraded and chemical weathering, fans, arroys.
mod. mass wasting, mod.
steppes.
to max. fluvial processes
no glacial scour, mod.
wind action.
smooth soil-coveredslopes,
Cf, Da, temperate marine min. to max. frost weath­
ering, min. to mod. mech­ ridges and valleys.
Db, Cs, and continental
anical weathering, mod.
Dc
zones,
humid
chemical
weathering,
tem perate m ed­
mod. to max. massiterranean zone.
wasting, no glacial scour,
min. wind action,
max. frost weathering,
scree slopes, solifluction
ET, D d, tundra, subpolar
max.
mechanical
w
eath­
slopes and cryoplanation
De
zone, high arctic
ering
(special
nivation),
surfaces, solifluction
barrens, humid
min. chemical weathering, lobes and terraces,
m icro-therm al,
max. masswasting, mod.
outwash plains, patterned
boreal.
fluvial processes (slope
ground, loess and dunes.
wash and valley cutting
concentrated in limited
thaw season), minimum
glacial scour, mod. to
max. wind action.
Based on R. J. Chorley, e t al, 1985
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4. Trophical
Wet-Dry
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GEOMORPHOLOGY
102
action, ob structio n o f vegetation an d an n u al am ount
o f precipitation periglacial m o rp h o g e n e tic region is
div ided into several s u b re g io n s e.g. (1) hyperperiglacial provinces; ( 2 ) meso-periglacial prov­
ince (w hich includes barren land s o f N. A m e ric a and
Eurasia, except E urasia, p e rm a fro st is w idespread;
vegetation c o v e r is n eg lig ib le; frost w eathering
(congclifraction), co n g eliflu c tio n (soliA uction) etc.
are im portant periglacial p ro cesses; clim a te is dry
continental having severe cold seaso n ; s u m m e r is
characterized by fog; w ind action is less significant;
m ain m o rp h o lo g ic a l featu res in c lu d e p a tte rn e d
ground, block fields, stone stream s, altiplanation
terraces etc.) ; (3) tundra region (vegetation ob­
structs runoff, d ev elo p m en t o f d e e p ‘active la y e r’,
solifluction and w ind action ; there are repeated
freeze-thaw m echanism s); (4) steppe periglacial
p ro v in c e (wind is m ost active, low frost action due
to aridity, A lberta o f C anada, M o ngo lia, north Ice­
land etc. are typical lo c a tio n s ) ; and (5) taiga prov­
ince (related to Pleistocene relic permafrost, geli Auction
becomes absent due to spring thaw, developed in
continuous and discontinuous permafrost areas).
Morphogenetic Reglone of Trlcart and Cailleux
(1972)
As slated earlier J. Tricart and A. Cailleux
(1972) identified four m ajor (say first-order) and
nine second-order m orphogenetic regions e.g. cold
zone (glacial and periglacial zones), forest-covered
zone (maritime, continental and mediterranean zones),
arid and semi-arid zone (steppe, xerophytic and
desert zones), and hum id tropical (savanna and for­
est regions) m orphogenetic regions. The following
brief descriptions o f characteristic features o f these
morphogenetic regions arc based exclusively on the
version of Tricart and Cailleux (1972)
1. Cold-zone Morphogenetic Regions
The boundary o f cold-zone morphogenetic
regions is dem arcated on the basis o f intensity of
frost action as frost is the major m orphogenetic/
geomorphological process which not only gives
birth to distinctive m orphogenetic processes and
their m echanism s but also influences work o f azonal
processes (e.g. waves, wind, streams etc.). It may be
mentioned that zonal processes are confined to a
particular climatic region whereas azonal processes
are active with varying intensities in many (almost
all) climatic regions (such as sea waves, wind, streams
etc.). Cold zone morphogenetic regions are divided
into (a) glacial zone and (b) periglacial zone.
2. Forested Mid-latitude Morphogenetic Region
This m orphoclim atic / m orpho genetic region
is located in the m id-latitude areas o f both the
hemispheres but it is m ore w idespread in the north­
ern hemisphere. This region extends from A tlantic
coast in Europe to Baikal lake in A sia in a long strip
and continues further eastw ard so as to include
A m ur basin, K orea and Japan. In N. A m eric a this
region extends from F lorida to Y uko n valley, from
Texas to Labrador and from N. C alifornia to Alaska.
Deep regolith has developed because o f w arm and
humid summer. The region is characterized by m in ­
imum intensity of m orphogenetic processes. G en ­
esis and developm ent o f m orphological features is a
slow process. The ground surface is covered with
thick litter because o f dense forest cov er and low
mineralization o f hum us. Litter cover discourages
surface runoff. M echanical, chem ical and biological
w eathering is m inim um with the result Pleistocene
surfaces have been well preserved. M ost o f the
landform s are relict features. There are spatial vari­
ations in the nature and intensity o f morphogenetic
processes due to local variations in climatic condi­
tions. This region is divided into (a) maritime, (b)
continental and (c) warm hum id tem perate zones.
(a) Glacial morphogenetic region is charac­
terized by low tem perature below freezing point
throughout the year with the result there is perm a­
nent snow cover on the ground surface and there is
no thaw ing of snow and hence the ru noff is always
in solid form (ice movement). The boundary o f this
zone coincides with glacier line. Glaciers are most
dom inant agents o f erosion (abrasion, attrition, pol­
ishing etc.) and transportation. The morphological
features include glacial valleys (U-shaped valleys
with han g in g valleys), cirques / corries, horn,
rochem outtonee, drumlins, moraininc ridges, eskers,
kam es etc.
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(b) Periglacial morphogenetic region is d e­
marcated on the basis o f tem perature which causes
and controls seasonal and diurnal freeze-thaw. Ground
has no perm anent ice cover i.e. ground surface is
covered with ice only during winter season. S um m er
season is characterized by surface runoff o f water
due to thaw-water. Based on periodicity o f frost
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CLIMATIC g e o m o r p h o l o g y a n d m o r p h o g e n e t i c r e g io n s
(a) M aritim e m orphogenetic region is es­
sentially humid zone and is characterized by low
variation in temperature range and humidity. This
region has most developed from Norway to Pyrenes
in Western Europe but it is also found in Poland.
Besides, this region has also developed in British
Columbia, Chile, T asm ania and New Zealand. Frost
action is moderate and o f short duration. Frost action
does not affect bedrocks. Soil desiccation does not
occur due to rainfall even during summer season.
Mechanical weathering is moderate but chemical
weathering is m axim um (strong). Granite rocks are
easily disintegrated due to abundance of humus
content in the soils.
(b) C ontin en tal m orphogenetic region has
developed in the eastern parts of Asia and N. America.
There is m axim um seasonal variability regarding
climatic param eters (viz. temperature, humidity,
precipitation etc.). W inters are severe. Precipitation
is characterized by high intensity, with the result
mechanical processes (erosion) are more active.
Frost becomes m ost active during winters. Sheet
erosion and gullying are activated during summers
because of strong overland flow resulting from spring
melt-water and ruinfall. Chem ical weathering and
erosion becom es m inim u m due to low infiltration of
water as a consequence of dom inance of frost action
during winters and m ax im um overland flow during
summers.
103
because infiltration is discouraged due to absence of
vegetation cover and thein soil cover. Pediments,
bajadas and playas are major landforms which are
associated with intermontane basins. Wind action is
most dominant and sand dunes most outstanding
depositional aeolian landforms. This region is di­
vided into (a) subhumid steppe region, (b) semi-arid
region and (c) arid region.
(a) Sub-hum id steppe region is located to
the north and south of Sahara, in eastern Africa, all
around Kalahari, Asia minor, middle Asia, A us­
tralia, Great Plains of USA, Prairies o f Canada,
Mexican plateau and Pampas o f Argentina. It may be
mentioned that previously (before the conversion of
temperate graslands into farmlands) mechanical ero­
sion was retarded due to dense grass cover but now
the vast areas are exposed to fluvial erosion because
of removal of grass cover for cultivation purposes in
all the temperate grassland areas of the world and
thus man has emerged as the m ost significant
geomorphic agent in this region. Deflation w ork was
previously confined to the dry beds o f rivers but now
cultivated farmlands are also affected by deflation.
Major aeolian depositional activity is the formation
of loess particularly in China. Loess is easily gullied
due to fluvial erosion caused by high intensity rain­
fall during occasional rainstorms. Leaching is not
effective due to relative aridity.
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(b)
S em i-a rid reg io n is also k n o w n as
m orphogenetic region and is character­
(c)
W a r m t e m p e r a t e / s u b tr o p ic axerophytic
l
ized
by
patchy
distribution o f steppe vegetation.
m orphogenetic region is m axim um developed in
Annual rainfall is low to m oderate but som e times
Mediterranean climate. Frost is practically absent.
there is occasional high intensity rainfall w hich
Landslides are com m on because of alternate dry
causes effective local overland flow. There is m ax­
(su m m e r) a n d w e t ( w i n t e r ) s e a s o n b e c a u s e
imum development o f inselbergs and pedim ents.
argillaceous rocks are subjected to contraction due
Ground surface is not protected from fluvial erosion
to dehydration during dry sum m er but to expansion
due to absence of vegetation cover. Fluvial process
due to hydration during w et winters. Fluvial erosion
is main geomorphic agent. W ind action is insignif­
is more active because o f high intensity rainfall
icant.
resulting in m axim um surface runoff and resultant
(c) A rid region / d esert reg io n is hot desert
overland flow and thus increased discharge of streams.
area characterized by lack o f rainfall and vegetation
cover. Surface runoff is practically absent. G round
3. Arid Morphogenetic Region
surface is sandy and rocky but is perm eable so that
Arid m orphogenetic region is located be­
rainwater, w henever received throug h very o cca­
tween mid-latitude forest-covered zone and humid
sional rainfall, quickly disappears through infiltra­
tropical zone. V egetation grades from steppe type to
tion. Sahara desert is typical exam ple o f this type o f
desert type. T his is characterized by extrem e aridity
region. W ind is most active geom orphic process but
and very variable rainfall. Surface runoll in case ot
is confined to deflation o f loose sands only. It may be
occasional rainstorm s generates rapid overland flow
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GEOMORPHOLOGY
104
is subjected to splash erosion and rillw ash when
there is occasional high intensity rainfall. Sheet
flood becomes m ore active in the areas o f dense
vegetation cover. The presence o f strong cuirases
protects the ground surface from fluvial erosion and
generate more surface ru noff and resultant overland
flow. Deep chemical w eathering is m ore active due
to high mean annual tem perature and rainfall result­
ing in the formation o f etchplains.
m entioned that w ater and w ind are essentially
transportational process in desert areas, thus their
morphogenetic importance is limited. Mechanical
disintegration is more active. The process of landform
development is exceedingly slow because of ab­
sence o f rainwater.
4. Humid tropical Morphogenetic Region
This region is divided into (a) savanna region
and (b) forest region on the basis o f humidity.
Savanna region is characterized by mean annual
rainfall o f 600 mm-800 mm and clearly defined dry
and wet seasons whereas hot-humid forest zone has
developed in the region having mean annual rainfall
of more than 1500 mm and short dry season. Both the
regions are characterized by high mean annual rain­
fall and total absence of frost and hence rock disin­
tegration is not very active. Chemical weathering is
most active due to high mean annual temperature
and rainfall.
.(b) Humid tropical forest morpho-genetic
region— Chemical weathering is m o st dominant
geomorphic process due to high tem perature and
rainfall throughout the year. Thus, active chemical
weatheing causes deep regoliths o f coarse materials.
Rivers are underloaded due to absence o f mechani­
cal weathering. Long profiles of the rivers are char­
acterized by breaks in slope (e.g. waterfalls and
rapids). J. Tricart and Cailleux (1972) have main­
tained that there is general absence o f bare rock
outcrops because of high rate o f infiltration due to
considerable vegetation cover even on steep slopes
and high ground surface.
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(a) Savanna m orphogenetic region is char­
acterized by dry and humid seasons which effectively
influence morphogenetic processes. Ground surface
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CONSTITUTION OF THE EARTH'S INTERIOR
S o u r c e s o f k n o w le d g e ; artificial sources, evidences from the theories o f
th e o rig in o f th e earth, and natural sources ; evidences of seism ology ;
c h e m ic a l co m p o sitio n and layering system of the earth ; thickness and
d e p th o f different layers of the e a r th ; recent views - crust, mantle and core.
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CHAPTER 5
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105-113
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5
CONSTITUTION OF THE EARTH'S INTERIOR
is commonly believed that the outer thinner part o f
the earth is composed of sedimentary rocks the
thickness of which ranges between half a mile to one
mile (0.8 km to 1.6 km). Just below this sedimentary
layer there is the second layer of crystalline rocks,
the density of which ranges between 3.0 and 3.5 at
different places. The average density of the whole
earth is about 5.5. Thus, it appears that the density o f
the core of the earth will be, without doubt, more
than 5.5. Generally, the density of the core of the
earth is around 11.0. Cavendish attempted to calcu­
late the average density of the earth in 1798 on the
basis of the Newton’s gravitational law. According
to him the average density of the earth is 5.48.
Poynting calculated the average density o f the earth
as 5.49 g cm-3 in the year 1878. Since 1950 several
attempts are being made to calculate the density of
the earth on the basis of satellites. The satellite
studies have revealed the following results about the
density of the various parts o f the earth-average
density of the earth = 5.517 g c n r \ average density
of the earth's surface = 2.6 to 3.3 g cm -3 and average
density of the core = 11 g cm'3
5.1SOURCE OF KNOWLEDGE
Though the study of constitution of the inte­
rior of the earth is out side the domain of geography
but its elementary knowledge is necessary for the
geographers because the nature and configura tion of
the reliefs of the earth's surface largely depend on the
nature, mechanism and magnitude of the endogenetic
forces which originate from within the earth. It is
decidedly true that it is very difficult task to have
accurate knowledge of the constitution of the earth's
interior because it is beyond the range of direct
observation by man but recently seismology has
helped to have some authenticated knowledge about
the mystery o f the earth's interior. The sources which
provide knowledge about the interior of the earth
may be classified into 3 groups.
1. A rtificial source
2. Evidences from the theories of the origin
o f the earth
3. Natural sources
e.g. volcanic eruption, earthquakes and seis­
mology
Thus, it is proved that (1) the density o f the
core o f the earth is highest o f all parts o f the earth.
(I) DENSITY
(II) PRESSURE
Numerous inferences can be drawn about the
constitution of the interior of the earth on the basis of
density of rocks, pressure of superincumbent load
(weight of overlying rocks) and increasing trend of
temperature with increasing depth inside the earth. It
Now question arises, what is the reason for
very high density of the core ? previously it was
believed that very high density o f the core was
because of heavy pressure of overlaying rocks. It is
common principle that pressure increases the den­
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1. Artificial Sources
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GEOMORPHOLOGY
106
w hich co m es in co n ta c t w ith th e asthen osphere re­
m ains 1200°C w hich is q u ite n eare r to the melting
point. I f we believe the rate o f general increase of
tem perature with in creasing d ep th the temperature
should be aroun d 25,000°C at the d e p th o f 2,900 km
but un der such c irc u m s ta n c e s m o s t p art o f the earth
w ould have m elted but this has not so happened. It is
evident from this d iscu ssio n that m o s t parts o f the
radioactive m inerals are c o n c e n tra te d in the upper­
m ost layer o f the earth. T h is fact e x p la in s the situa­
tion o f high te m perature in the co n tin en ta l crust as
described above b ecau se d is in te g ra tio n and decay of
radioactive m inerals gen era te m o re h e a t in the crustal
areas. It, thus, app ears that the rate o f increase of
tem perature d o w n w a rd s d e c re a se s w ith increasing
depth. The follow ing facts m a y be p resen te d about
the thermal condition o f the in terio r o f the earth.
sity o f rocks. Since the w eight and pressure o f rocks
increase with increasing depth and hence the density
o f rocks also increases with increasing depth. Thus,
it is proved that (2) very high density of the core of
the earth is due to very high pressure prevailing
there because of superincumbent load. This infer­
ence is proved w rong on the ground that there is a
critical limit in each rock beyond which the density
o f that rock canno t be increased inspite o f increasing
pressure therein. It may be, thus, forwarded that (3)
very high density of the core of the earth is not
because of very high pressure prevailing there. If
the high density o f the core o f the earth is not because
o f high pressure o f overlying rocks then (4) the core
must be composed of intrinsically heavy metallic
materials of high density. The experiments have
revealed that the core o f the earth is made o f the
m ixture o f iron and nickel. This inference is also
validated on the basis o f geocentric magnetic field.
T he metallic core is surrounded by a zone o f such
rock materials, the upper part o f which is com posed
o f crystalline rocks.
(i) T he a sth en o sp h ere is p artially m olten. The
tem perature is aro und 1 100°C at the d e p th o f 100 km
w hich is nearer to initial m e ltin g point.
(ii) The te m p eratu re at the d e p th s o f 400 km
and 700 km (from the e arth 's su rface) has been
estim ated to be I,500°C and 1,900°C respectively.
(Ill) TEMPERATURE
It is evident on the basis o f information avail­
able from the findings o f bore holes and deep mining
that temperature increases from the surface o f the
earth dow nw ard at the rate o f 2° to 3°C for 100
metres. It may be pointed out that it becomes very
difficult to find out the rate o f increase o f tem ­
perature beyond the depth o f 8 km. The rate o f
increase o f temperature in the continental crust has
been calculated based on geothermal graphs and the
follow ing generalization has been made. In the tectonically active areas (like the Basin and Range
P rovince o f the U SA ) tem perature rem ains 1000°C
at the depth o f 43 km from the surface o f the earth
w hile the tem perature remains only 500°C at the
depth o f 40 km from the surface in tectonically stable
areas. This inform ation provides significant k n ow l­
edge about the nature and behaviour o f the continen ­
tal crust. It is ev ident that high tem perature o f 1000°C
at the depth o f 43 km in the tectonically active areas
is nearer to the initial m elting point o f the rocks o f
low er crust and m antle mainly basalt and peridotite.
(iii) T he te m p eratu re at the ju n c tio n o f mantle
and outer moiten core s ta n d in g at the d ep th o f 2,900
km is about 3700°C.
(iv) The te m p eratu re at the ju n c tio n o f outer
m olten core and inn er solid c o re stan d in g at the
depth o f 5,100 km is 4,300°C .
Generation and Transfer o f heat inside the
Earth— It may be p o in ted o u t th a t th e heat in the
interior o f the earth is g en e ra te d th r o u g h the disinte­
gration o f radioactiv e m in e ra ls a n d co nv ersion of
gravity force into th erm al en erg y . It is believed that
about 4.7 billion y ears ag o the initial tem perature of
the earth generated by p la n e ta ry a c c re tio n and adi­
abatic co m pressio n w o u ld h a v e b e e n aro u n d lOOO^CL ater on the heat o f the in terio r o f the earth would
have gradually but s u b sta n tia lly in c re ased due to
heat supplied by the d is in te g ra tio n o f radioactive
minerals. A bou t 4 .0 to 4.5 billion y ears ag o the core
and m antle w ould h av e been se p a ra te d and their
boundary w ould h av e e v o lv e d w hen the temperature
w ould h av e increased to reach the m e ltin g point of
iron. T h us, due to fo u n d e rin g o f m olten iron into
core the gravity force e q u iv a le n t to 2 x 1037 erg (one
calorie = 4.9 x 107 erg) in the form o f heat energy
The tem perature o f the upper part o f the
m a g m a slab representing the upper portio n -o f the
oceanic crust has been estim ated to be 0°C w here as
the tem perature o f the low er part o f the m ag m a slab
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107
CONSTITUTION O F T H E EA R TH 'S INTERIOR
esis’ the earth was originated due to accretion and
a g g reg a tio n o f solid dust p a r tic le s know n as
‘planetesim als’. Based on this corollary the core o f
the earth should be in solid state. A c c o r d i n g to the
‘tidal h yp oth esis’ the core o f the earth should be in
liquid state because the earth has been taken to ave
been formed, according to this hypothesis, from the
tidal materials ejected from the p rim itive sun. A c ­
cording to the ‘n ab ular h y p o th e sis’ o f L ap lace the
core o f the earth should be in gaseous state. Z o e p p n tz
and Ritter have opined that the core o f the
is
made of gases but this co ncept m ay not be accepted
because if we assume the core o f the earth in gaseo u s
state many more problem s will em erg e. T h e re m a y
be only two possibilities viz. either the co re m a y be
in solid state or liquid state. T h is p ro b lem w o u ld be
dealt with while dealing with the ev id en ces o f se is­
might have been released. Large-scale melting and
rearrangement o f material inside the earth conse­
quent upon high thermal energy, as stated above,
probably became responsible for the formation o f
different zones o f the earth e.g. crust, m antle and
core.
On an average, there is gradual flow of heat
from the inner part o f the earth to its outer part. It may
be pointed out that the heat energy’ in the solids is in
the form o f vibrations o f atoms. It is to be rem em ­
bered that the rocks are poor conductor of heat. The
transfer o f heat from only 10 -m thick rock layer
takes 3 years. The 100-m thick lava flow takes 300
years to cool dow n and solidify. The transfer o f heat
from the low er part to the upper part o f a 400-km
thick layer o f rocks would take a long period of 5
billion years. If we take conduction as the only
mechanism o f the cooling o f the earth, the heat from
the depth o f 400 km would have not reached the
earth's surface till new.
mology.
3. Natural Sources
(I) VULCANICITY
The transfer o f heat from the interior o f the
earth towards its outer part may also not be effec­
tively performed by radiation because most o f the
minerals o f the interior o f the earth are opaque. Such
materials cannot effectively transfer or lose heat
through radiation. The third alternative possibility
for the transfer o f heat may be the process of convec­
tion but convective m echanism is more effective in
liquid materials.
Some scientists believe on the b asis o f upwelling and spread of hot and liquid lava on the
earth's surface during volcanic erup tion th at th e re is
at least such a layer below the earth's su rface w hich
is in liquid state. Such m olten layer has been te rm ed
as ‘m a g m a c h a m b e r ’ w hich supplies m a g m a and
lava during volcanic eruptions. It m ay be, thus,
surmised, on the basis o f ab ove co nno tation, that
some p a n o f the earth should be in liquid state bu t
this inference is refuted if one con siders the in c re a s ­
ing pressure with increasing depth inside the earth.
It is known to all that increasing pressure increases
the melting point o f the rocks. T h us, the inn er part
o f the earth m ay not be in m o lten state inspite o f very
high temperature prevailing therein because the e n o r­
mous weight and pressure o f the o v erly in g m aterials
(superincum bent load) increases the m eltin g point
o f the rocks. It, thus, appears that the core o f the earth
should be in solid state. N o w question arises, where
hot and liquid lavas co m e from during volcanic
eruption ? It may be pointed out that w hen the
pressure of su perincum bent load is released due to
fracturing and faulting in the crustal surface, the
melting point o f underlying rocks is red u ce d (lo w ­
ered) and thus the rocks are instantaneously m elted
because required degree o f high temperature is al­
ready present there It. thus, appears that no authen­
ticated knowledge about the com p osition o f the
The earth's surface receives heat from two
sources e.g. from the sun and from its interior part
itself. The heat received from these two sources is
ultimately sent into the space. Solar heat drives the
atmospheric and hydrological processes and gener­
ates denudational processes whereas the internal
heat o f the earth perform s constructive works e.g.
formation o f mountains, plateaux, faults e t c . vulcanicity, seismic events and other tectonic events.
‘In areal sense, the earth's internal heat engine builds
mountains and its external heat engine, the sun.
destroys them' (F. Press and R. Siever, 1974).
2. E vidences from the Theories of the Origin of the
Earth
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Various exponents o f different hypotheses
and theories o f the origin o f the earth have assumed
the original form o f the earth to be solid or liquid or
gaseous. According to the ‘p tM d w i n a l h y p o th ­
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108
GEOM ORPHOLOGY
trem o rs.’ After a brief interval the ‘secon d p r e lim } ,
nary trem o rs’ are recorded and finally the ‘main
trem o rs’ o f strong w aves are recorded (fig. 5.1).
earth's interior is obtained from the evidences o f
volcanic activities.
(II) EVIDENCES OF SEISMOLOGY
Seism ology is the science which studies var­
ious aspects o f seismic w aves generated during the
occurrence o f earthquakes. Seism ic waves are re­
corded w ith the help o f an instrum ent known as
seism ograp h . It may be pointed out that seismology
is the only source w hich provides us authenticated
inform ation about the com position o f the earth's
interior. T he place o f the occurrence o f an earth­
quake is called ‘fo c u s’ and the place which experi­
ences the seismic event first is called ‘ep icen tre’,
w hich is located on the earth's surface and is always
perpendicular to the ‘fo cu s’. On the other hand, the
focus or the place of the origin o f an earthquake is
always inside the earth. The deepest focus has been
measured at the depth o f 700 km from the earth's
surface. The different types o f tremors and waves
generated during the occurrence o f an earthquake
are called ‘seism ic w a v es’ which are generally di­
vided in 3 broad categories e.g. primary waves,
secondary waves and surface waves.
L
Fig. 5 .1 : Recorded seismic waves by a seismograph.
The nature and properties o f the composition
o f the interior o f the earth m ay be successfully
obtained on the basis o f the study o f various aspects
o f seismic w aves m ainly the velocity and travelpaths o f these w aves while passing through a ho­
m ogeneous solid body but these w aves are reflected
and refracted while passing through a body having
heterogenous composition and varying density zones.
If the earth would have been co m p o sed o f homog­
enous solid materials the seism ic w av es should have
reached the core o f the earth in a straight path but this
is not the case in reality. In fact, the recorded seismic
waves denote the fact that these w aves seldom fol­
low straight paths rather they adopt curved and
refracted paths. Thus, it becom es obvious that the
earth is not com posed o f h o m o g e n o u s materials
rather there are variations o f density inside the earth.
The seismic waves are refracted at the places of
density changes. A regular chang e o f density inside
the earth causes a curved path to be followed by the
seismic waves. Thus, the seism ic w aves become
concave tow ards the earth's surface (fig. 5 .2 ).
(i) P rim ary w aves— also called as longi­
tudinal or com pressional waves or simply ‘P ’ waves,
are analogous to sound waves wherein particles
m ove both to and fro from the 1i ne o f the propagation
o f the ray. P w aves travel with fastest speed through
solid materials. T hough these also pass through
liquid materials but their speed is slowed down.
(ii) S econ d ary w aves— are also called as
transverse or distortional or simply S waves. These
are analogous to w ater ripples or light waves wherein
the particles m ove at right angles to the rays. S
waves cannot pass through liquid materials.
As stated earlier S w aves cann ot pass through
liquid. A fter indepth study o f seism ic waves Oldhum
dem onstrated in the year 1909 that S w aves disap­
pear at the angular distance o f 120 ° from the epicen­
tre and P waves are w eakened. It is evident from fig5.2 that S waves are totally absent in the core of the
earth. It appears from this observation that there is a
core in liquid state w hich is located at the depth of
more than 2900 km from the earth's surface and
surrounds the nucleus o f the earth. Based on this
finding the scientists have estim ated that the iron and
nickel o f the core o f the earth m ay be in liquid state
(iii) S u rface w aves— are also called as long
period waves or simply L waves. These waves
generally affect only the surface o f the earth and die
out at sm aller depth. These waves covcr longest
distances o f all the seismic waves. Though their
speed is slow er than P and S waves but these are most
violent and destructive.
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When an earthequake occurs the seismic waves
are recorded at the epicentre with the help o f seism o­
graph. In the beginning a few small and weak swings
are recorded. Such tremors are called ‘p r e li m i n a r y
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CONSTITUTION O F TH E EA RTH S INTERIOR
109
and 3.3 km per second respectively in the upper part
of the earth. The density o f the rocks through w hich
these waves travel is about 2.7. It is proved on this
basis that the upper layer is com posed o f granitic
rocks.
(2) Interm ediate L a y e r — Conard identified
another set o f seismic waves term ed as P -S waves
on the basis of the study of Tauern earthquake o f
1923. The velocities of these waves are interm ediate
between P-S and Pg-Sg sets of w aves. P and S
waves travel at the rate o f 6-7 km and 3-4 km per
second respectively in the middle zone o f the earth.
It has been inferred on the basis o f interm ediate
velocity of these waves that there is an interm ediate
layer with average density o f 3 inside the earth.
There is difference of opinion about the nature and
type of the rocks o f this intermediate layer. A cco rd ­
ing to Daly and Jeffreys the intermediate layer c o n ­
sists of glassy basalt whereas W egener and H olm es
have identified amphibolite as constituent ro ck o f
this layer. But most of the scientists are o f the view
that the intermediate layer is com posed o f basalt.
Fig. 5.2 : Paths follow ed by seismic waves through the
earth's interior.
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Not only this, if we study the nature, charac­
(3) L o w er L a y e r — P and S waves penetrate
teristics and velocity of seismic waves, we may find
upto
greatest
depth inside the earth. T he velocity o f
the presence o f several density zones inside the
P and S waves is 7.8 km and 4.5 km per second
earth. Detailed studies of seismic waves of different
respectively. The highest velocity of seismic waves
epicentres all over the world have revealed the fact
in the innermost part of the earth indicates an inner
that there are extra sets of seismic waves which are
or lower layer of heavier materials, most probably
similar to P and S waves but with slower rate of
velocity. It is a known fact that the velocity of
peridotite or dunite. It is also possible that materials
seismic waves changes only when there are changes
may be in non-crystalline, glassy state. The depth of
in the density o f rocks. On the basis of velocity
this layer is estimated to be about 2900 km from the
seismic waves are divided in three sets of waves e.g.
earth's surface.
(i) first set of P-S waves o f maximum velocity, (ii)
5.2 CHEM ICAL COM POSITION AND LA Y E R IN G
second set of Pg-Sg waves of minimum velocity and
SYSTEM O F T H E EA R TH
(iii) third set o f P*-S* waves of medium velocity
According to S u e ss
falling between the first and the second sets of
waves. Thus, on the basis o f changes of velocity of
E.
Suess has thrown light on the chem ical
seismic waves it is proved that there are major
composition of the earth’s interior. T h e crust is
changes in the velocity o f waves at three places
covered by a thin layer o f sedim entary rocks o f very
inside the earth and hence it can be safely inferred
low density. This layer is com po sed o f crystalline
that there are three distinct zones or layers o f varying
rocks, mostly silicate matter. The do m in an t m inerals
densities inside the earth below the outer thin layer
are felspar and mica. The upper part o f this layer is
°f sedimentary rocks.
composed of light silicate m atter w hile heavy sili­
cate matter dominates in the low er part. S uess has
(1)
Upper L ayer— Jeffreys discovered a dif­
identified three zones o f different m atter below the
ferent set of seismic waves termed as Pg-Sg waves
outer
thin sedimentary cover.
on the basis of the record of the earthquake of the
Kulpa valley in Croatia in the year 1909. On an
(i)
Sial layer located just below the outer
average Pg and Sg waves travel at the rate of 5.4 km
sedimentary cover is com posed o f granites. This
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OEOMORPHOtOOY
110
(if) Intermediate layer is composed of the
mixture o f iron and silicate*. A verage density it
layer is dominated by silica and aluminium
(SIAL=SI+AL). The average density of this layer js
2.9 whereas its thickness ranges between 50 to 300
km. This layer is dominated by acid materials and
silicates of potassium, sodium and aluminium are
abundantly found. Continents have been formed by
sialic layer.
from 4.5 to 9 and the th ickness is 1,280 km.
(iii)
C e n t r a l z o n e is m ade o f iron and is i
solid state. A verage den sity and diam eter are 11.6
and 7,04 0 km resp ectively.
(2) ACCORDING TO HAROLD JE F F R E Y S
(ii)
S im a is located ju st below the sialic layer.
Jeffreys has id en tified , on the basis of the
This layer is com posed o f basalt and is the source o f
study o f seism ic w aves, four layers in the earth e.g.
m agm a and lava during volcanic eruptions. Silica
(i) outer layer o f sedim entary rocks, (ii; second layer
(S i-Silica+m a-m agnesium ) and m agnesium are the
o f granites, (iii) third layer o f thach ylyte or diorite
dom inant constituents. Average density ranges be­
and (iv ) fourth layer o f dunite, pcridotite or eclogiie.
tw een 2.9 to 4.7 w hereas the thickness varies from
1,000 km to 2,000 km. There is abundance o f basic
(3) ACCORDING TO HOMLE8
matter. The silicates o f m agnesium , calcium and
A rthur H o lm e s has r e c o g n i /^ d tw o major
iron are m ost abundantly found.
layers in the earth. T h e u p p e r la y e r is te rm e d as crust
w hich is co m p o sed o f w h o le o f S u e s s ’ sialic layer
and upper portion o f ‘s im a ’. T h e lo w e r layer has
been nam ed by H olm es as a s u b stra tu m which rep­
resents low er portion o f S u e s s ’ sima.
H om les has d eterm in e d the th ick n ess o f sial
below the continental su rface on the basis o f differ­
ent sources and ev id en ces as given below .
( i)O n the basis o f therm al co n d itio n s - 20 km
or less.
(ii) On the basis o f su rface seism ic waves (L
waves) - 15 km or more.
(iii) On the basis o f lo n g itu d in a l (P waves)
waves— 20-30 km.
(iv) On the basis o f s u b sid e n c e o f the deepest
geosynclines - 20 km o r m ore.
(4) ACCORDING TO VAN DER GRACHT
Van der G rach t has identified 4 - layersystem
o f the com position o f the interior o f the earth. He has
sum m arized the various p ro p erties o f the earth’s
(iii)
N ife is located just below ‘sim a’ layer.
interior in the fo llow ing m an ner.
This layer is com posed of nickel (NI) and ferrium
L ayer
T h ick n ess
D ensity
(Fe). It is, thus, apparent that this layer is made of
heavy metals which are responsible for very high
(i) O uter sialic
60 km
2.75 to 2.9
density (11) of this layer. The diameter of this zone
crust
(u n d er continents)
is 6880 km. The presence o f iron (ferrium) indicates
20 km
the magnetic property o f the earth's interior. This
(und er A tlantic
property also indicates the rigidity of the earth (fig. 5.3).
O cean)
A bsent
5.3 THiCKNESS AND DEPTH OF DIFFERENT
(u n d er Pacific
LAYERS OF THE EARTH
O cean)
(1) ACCORDING TO DALY
(li)Inner-silicate
6 0 -1 1 4 0 km
3. 1 t o 4.75
mantle
Daly has recognized three layers of different
(iii) Zone of mixed 1,140-2,900 km
4.75 to 5.0
density in the earth.
Fig. 5.3 : Layering system o f the earth according to E.
Suess. C - crust.
metals and
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(i)
O uter zone is com possed o f silicates.
silicates
Average density is 3.0 and the thickness is 1,600 km.
(iv)Metallie nucleus 2,900-6371 km
H-0
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CONSTITUTION O F T H E E A R T H S INTERIOR
111
It appears from the foregoing discussion that
there is difference o f opinions about ttic number,
thickness and various properties o f the layers o f the
earth. In order to avoid confusion the following
generalized pattern o f the layering system of the
earth's interior is com m only accepted by majority o f
the scientists.
obsolete. The scientific study and analysis o f various
aspects o f seismic waves (mainly velocity and travel
paths) o f natural and man-induced earthquakes have
enabled the scientists to unravel the m ystery o f the
earth's interior based on authentic information. Three
zones o f varying properties have been identified in
the earth on the basis o f changes in the velocity o f
seismic waves while passing through the earth (fig.
5.4) e.g. cru st, m antle and core. It m ay be pointed
out that there is still difference o f pinions about the
thickness of these zones, mainly abo ut the thickness
o f the crust. Various sources put the thickness o f the
crust between 30 km and 100 km. On the basis o f the
change in the velocity o f seism ic w aves crust is
further divided into (i) upper cru st and (ii) lo w er
crust because the velocity o f P w aves suud en ly
increases in the lower crust. For exam ple, the av er­
age velocity of P waves in the upper crust is 6 .1 km
per second while it becomes 6.9 km per second in the
lower crust. Fig. 5.4 depicts the different velocities
of Pand S waves in different parts o f the earth an d the
relationship between velocities o f seism ic w aves
and different zones o f the earth.
(i) L ith osp here with a thickness of about 1 00
km is mostly com posed o f granites. Silica and alu­
minium arc dom inant constituents. Average density
is 3.5.
(ii) P yrosp h ere stretches for a thickness of
2780 km having an average density of 5.6. The
dominant rock is basalt.
(iii) B arysphere is com posed o f iron and
nickel. Average density ranges between 8 and 11 and
this layer stretches from 2800 km upto the nucleus o f
the core.
5.4 R EC EN T V IEW S
The aforesaid views about the composition
and structure o f the earth's interior have now become
.
Fit . S.4
t
i
waves from the crust o f the earth to its interior and relationships between
" «,*d d f r r L t o . ' , o l ,H, ,a r,k M ' r K.E. B u ,M .
ve ocines
^ that in the beginning vast difference between the
structure and composition of the upper and low er
crust was reported by the scientists but now the
(1) CRUST
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^ ^ outer and lower
The
average
density
o
crust is 2.8 and 3.0 respectively. It may uc pointed
^
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GEOMORPHOLOGY
112
or simply ‘M oh o d isco n tin u ity ’. The mantle hav­
ing mean density o f 4 .6 g cm -3 extends for a depth of
2900 km inside the earth. It m ay be mentioned that
the thickness o f the mantle is less than half o f the
radius o f the earth (6371 km ) but it contains 83 per
cent of the total volum e and 68 per cent o f the total
mass of the earth. Previously the m antle was divided
into two zones on the basis o f changes in the veloci­
ties o f seismic waves and density e.g. (i) upper
m antle from M oho discontinuity to the depth of
1000 km and (ii) low er m an tle from 1000 km to
2900 km depth but now the mantle is divided on the
basis o f the information received from the discovery
of the International Union o f G eodesy and G eophys­
ics into 3 sub-zones e.g. (i) first zone extending from
Moho discontinuity to 200 km depth, (ii) second
zone extending from 200 km to 700 km depth and
(iii) third zone extending from 7 00 km to 2900 km
depth. The velocity o f seismic waves relatively
slows down in the upermost zone o f the upper mantle
for a depth of 100 to 200 km (7.8 km per second).
This zone is called the zone o f low velocity. Mantle
is believed to have been formed largely o f silicate
minerals rich in iron and magnesium.
evidences of seismology have revealed almost iden­
tical structure and composition o f these two sub­
zones of the crust. The difference o f density between
the upper (2.8) and lower crust (3.0) is because o f the
pressure of supperincumbent load. The formation of
the minerals of the upper crust was accomplished at
relatively lower pressure than the minerals o f the
lower crust.
D en sity
2-90
3-3
4-3
5-5
10-0
(3) CORE
The core, the deepest and most inaccessible
zone of the earth, extends from the lower boundary
of the mantle at the depth o f 2900 km to the centre o f
the earth (upto 6371 km). The mantle-core boundary
is determined by the ‘W eich ert-G u ten b erg D is­
continuity’ at the depth of 2900 km. It is significant
to note that there is pronounced change o f density
form 5.5 g cm"3 to 10.0 g cm"3 along the Gutenberg
Discontinuity. This sudden change in density is
indicated by sudden increase in the velocity o f P
waves (13.6 km per second) along the mantle-core
boundary or Gutenberg Discontinuity. The density
further increases from 12.3 to 13.3 and 13.6 with
increasing depth o f the core. It, thus, appears that the
density o f the core is more than twice the density of
the mantle but the volume and mass o f the core are
16 per cent and 32 per cent o f the total volume and
mass of the earth respectively.
12-3
13-3
13-6
Fig. 5.5 : Diagramatic presentation o f different zones
o f the earth, their densities and thicknesses on
the basis o f the information o f International
Union o f Geodesy and Geophysics.
(2) MANTLE
There is sudden increase in the velocity of
seismic waves at the base of lower crust as the
velocity of seismic waves is about 6.9 km per second
at the base of lower crust but it suddenly becomes 7.9
to 8.1 km per second. This trend of seismic waves
denotes discontinuity between the boundaries of
lower crust and upper mantle. This discontinuity
was discovered by A. Mohorovicic in the year 1909
and thus it is called as ‘Mohorovicic discontinuity’
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The core is further divided into two sub-zones
e.g. outer core and inner core, the dividing line
being at the depth o f 5150 km. S waves disappear in
this outer core. This means that the outer core should
be in molten state. The inner core extends from the
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113
CONSTITUTION O F T H E E A R T H S INTERIOR
depth o f 5150 km to the centre o f the earth (6371
km). This lowerm ost zone o f the interior o f the earth
is in solid state, the density o f which is 13.3 to 13.6.
p waves travel through this zone with the speed o f
1 1 .23 km per second. It is generally believed that the
core is com posed o f iron and nickel but according to
the second view point the core may be formed of
silicates. It is also believed that after disintegration
on high pressure the electronic structures have changed
into heavy metallic materials, thus the density of the
core has increased. A ccording to the third view point
initially the core was com posed of hydrogen but
later on hydrogen was transformed into metallic
materials due to excessive pressure (over 3 million
atmosphere). This possibility is questioned on the
ground that though the transformation of silicate or
hydrogen due to very high pressure in the core may
be believed tentatively but this process cannot in­
crease the density o f the core as high as it is at
present. For exam ple, the planet Mercury is smallest
of all the planets o f our solar system but its density
is highest o f all the planets. It may be argued that
least compression and pressure cannot generate highest
density in the core o f M ercury. Most of the presentday geophysicists and geochemists believe that the
core is made o f metallic materials mainly iron and
nickel.
asthen osphere, the lower part o f the lithosphere
(crust) is in partially molten condition wherein m ol­
ten (fluidj magma is in motion. The lithosphere
(crust; above hard mantle is characterized by a
network o f deformable m agm a ch a n n els which
have been termed as surge ch an n els. These surge
channels are. in fact, conduits through which fluid
magma moves upward from asthenosphere to upper
part of the lithosphere. W hen the asthenosphere
becomes too weak to support the lithosphere dy­
namically, the latter collapses into the former. The
surge channel system, fluid m agm a and the collapse
o f lith o s p h e r e in to d y n a m i c a l l y w e a k e n e d
asthenosphere, are parts o f ‘glob al g ia n t h yd rau lic
press system ’.
Strictly speaking, surge tectonics m eans up­
ward motion of fluid m agm a in surge channels
fmagma conduits), rise in tem perature o f regional
oceanic water and consequent decrease in pressure
and shift in regional gravity field o f oceanic crust.
The motions in the surge channels are caused by
earth s rotation. Magma, while rising through the
surge channels, undergoes its transform ation (defor­
mation; i.e. it becomes lighter (decrease in density)
and less compact and hence expands. This co n se­
quent expansion in magma reduces gravitational
attraction in the surge channels and w eakens the
regional gravity fields. The increase in seismic ac­
tivity along East Pacific Rise (ridge), increase in sea
level in the Pacific Ocean due to shift in regional
gravity field and increase in tem perature o f ocean
waters surrounding Indonesian archipilago etc. due
to surge tectonics have been associated w ith El N ino
phenomenon. Thus, it is concluded that El Nino
phenomenon is related to surge tectonics and in turn
the former affects weather and climate. This has
been termed as a Gravitationally Earth Teleconnected
Global Oscillation System (G E T G O S ) w hich con­
trols climatic fluctuation (D own to Earth, N ov 30
1999).
5.5 S U R G E T E C T O N IC S
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Surge tectonics refers to the genesis of global
surge waves caused by changes (weakenings in the
regional gravity fields due to upward movement of
deformable m agm a in the s u rg e ch an nels in the
lithosphere (crust, 100-200 km thick upper layer of
the earth) lying above mantle. Recently, surge tec­
tonics involving tectonic activity within the crust
has been related to climatic phenomena including El
Nino. Infact, there is paradigm shift from traditional
modelling o f climate change based on ocean-atmosphere interactions’ to ‘earth dynamics (surge
te c to n ic s)-o ce an -atm o sp h e re interactions . The
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CO N TIN EN TS AND OCEANS
I n tr o d u c tio n ; te tra h e d ra l hyp o th esis ; continental drift theory o f T ay lo r ;
c o n tin e n ta l d rift th eo ry o f W eg en er ; plate tectonic theory.
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CHAPTER 6
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6
CONTINENTS AND OCEAN BASINS
6.1 INTRODUCTION
Continents and ocean basins being fundamental
relief features o f the globe are considered as ‘relief
featu res o f the first o rd er’. It is, therefore, desir­
able to inquire into their m ode o f possible origin and
evolution. D ifferent view s, concepts, hypotheses
and theories regarding the origin of the continents
and ocean basins have been put forth by the scientists
from tim e to tim e. B efore exam ining these views
about their origin we should know the characteristic
features o f the distributional patterns and arrange­
m ent o f the continents and ocean basins as seen at
present. A bout 70.8 per cent o f the total surface area
o f the globe is represented by the oceans w hereas
rem aining 29.2 per cent is represented by the conti­
nents. Even the distribution o f different continents
and oceans in both the hem ispheres is not uniform .
T he follow ing characteristic features o f the distribu­
tional pattern o f the continents and the occean basins
m ay be highlighted-
w ould be ‘land h em isp h ere’ w h ile th e southern
hem isphere as ‘w ater h e m isp h ere’. T h u s, th e land
hem isphere w ould rep resen t 83 p er cen t o f th e total
land area o f the globe w hile the w ater h em isp h ere
w ould carry 90.6 per cent o f the total o cean ic areas
of the globe.
(2) C ontinents are arranged in ro u g h ly tria n ­
gular shape. M ost o f the co n tin en ts h av e th e ir bases
(of triangle) in the north w hile th eir ap ices are
pointed tow ards south. If w e take N o rth and S outh
A m ericas together, they rep resen t eq u ilateral tria n ­
gles, the base o f w hich w ould be alo n g the arctic sea
w hile the apex w ould be rep resen ted by C ap e H orn.
If we take these tw o co n tin en ts sep arately , again
they form tw o separate triangles. S im ilarly , E u rasia
also assum es the form o f a trian g le the base o f w hich
is along the arctic sea w hile its ap ex is n ear E ast
Indies. The base o f A frican trian g le is to w ard s north
w hile its apex is the C ape o f G ood H ope. A ustralia
and A ntarctica are the ex cep tio n s to this rule.
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(3) R oughly, the oceans are also trian g u lar in
(1)
T here is overhw elm ing dom inance o f land
shape. C ontrary to the co n tin en ts th e b ases o f ocareas in the northern hem isphere. M ore than 75 per
ceans are in the south w hile th e ir ap ices are in the
cen t o f the total land area o f the globe is situated to
north. T he base o f the A tlan tic O cean ex tends be­
the north o f the equato r (i.e. in the northern hem i­
tw een C ape H orn and C ape o f G o o d H o p e w hile its
sphere). C ontrary to this w ater bodies dom inate in
apex is located to the east o f G reen lan d . T he base o f
the southern hem isphere. If we divide the globe in
the Indian O cean is in the so u th bu t its tw o apices are
tw o such hem ispheres w here the north pole stands
located in the Bay o f B engal and the A rabian Sea.
located in the E nglish C hannel and the south pole
T he apex o f the P acific O cean is near A leutian
near N ew Z ealand, then the northern hem isphere
Islands w hile its base lies in the south.
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CONTINENTS AND OCEAN BASINS
is situated diam etrically opposite to water bodies.
There are only two cases o f exceptions to this gen­
eral rule e.g. (i) Patagonia is situated diametrically
opposite to a part o f north C hina and (ii) New
Zealand is situated qpposite to Portugal and Spain
(the Iberian Peninsula)
(6)
The great Pacific Ocean basin occupies
almost one-third of the entire surface area of the globe.
(4) The north pole is surrounded by oceanic
water while south pole is surrounded by land area (of
the Antarctic continent).
(5) There is antipodal arrangement (situation)
of the continents and oceans. Only 44.6 per cent
oceans are situated opposite to oceans and 1.4 per
cent of the total land area o f the globe is opposite to
land area. M ore than 95 per cent of the total land area
Fig. 6.1: Different geometrical shapes which were used to postulate the hypotheses o f the origin o f the continents and
ocean basins. The last one is a tetrahedron.
tions. In fact, all the previous hypotheses and theo­
ries dealing with the origin o f the continents and
ocean basins have faded away after the postulation
of plate tectonic theory. We will exam ine here only
the concepts o f Lowthian G reen, F.B. Taylor, A.G.
W egener and o f course plate tectonic theory.
6.2 TETR A H ED R A L H Y P O TH ESIS
A few scientists have attem pted to solve the
problems o f the origin o f the continents and ocean
basins on the basis o f fundam ental principles o f
geometry. The patagonal dod ecah ed ral hypoth­
esis (dodeca is a Greek word w hich means tw elve) o f
Elie dc Beaumont is considered to be the first at­
tempt in this field but the tetrahedral hypothesis o f
Lowthian Green is m ost significant o f all the hypoth­
eses based on geom etrical principles. ‘An attractive
hypothesis which has enjoyed a considerable vogue
was initiated by Lowthian Green in 1875’ (S.W .
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The validity and authenticity o f any hypothe­
sis or theory dealing with the origin and evolution of
the continents and the ocean basins would be de­
termined in the light o f aforesaid characteristics of
the distributional pattern o f the continents and ocean
basins. The presence o f the great Pacific Ocean basin
and island arcs and festoons o f the Pacific Ocean are
teething problem s before scientists who venture in
the precarious field o f the postulation o f the relevant
theory of the origin o f the continents and ocean
basins. Keeping the above facts in mind Lowthian
Green postulated his ‘tetrahedral hypothesis’ to
explain the intricate problem s o f the origin o f the
continents and oceans and characteristic features o f
their distributional pattern. Besides, Lord Kelvin,
Sollas, Love etc. have also attem pted to explain the
origin of the continents and ocean basins but their
views are not discussed here because they are based
on discarded and obsolete arguments and assump­
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GEOMORPHOLOGY
C onsequently, the upper part collapsed on the inner
part and ultim ately the earth began to assume the
shape o f a tetrahedron. L ow thian G reen has further
m aintained that th e earth has not been as yet changed
into a com plete tetrah ed ro n rath er as it is being
cooled, it is pro ceed in g tow ards attaining the true
shape o f a tetrahedron. H e has fu rth er opined that the
earth cannot be in the sh ap e o f a real tetrahedron
because o f its structural variatio n s and thus it is
natural that there m ay be som e d ev iatio n s from a true
tetrahedron.
W ooldridge and R.S. M organ, 1959). H is hypotehsis
is based on the characteristics o f a tetrahedron w hich
is a solid body having four equal plane surfaces, each
o f w hich is an equilateral triangle (fig. 6. 1 ).
Low thian G reen postulated his hypothesis
after considering the characteristics o f the distribu­
tional pattern o f land and w ater over the globe.
Barring a few draw backs and defects the tetrahedral
hypothesis successfully explains the follow ing char­
acteristics o f the continents and ocean basins.
(1) D om inance o f land areas in the northern
hem isphere and w ater areas in the southern hem i­
sphere ; (2 ) triangular shape o f the continents and
oceans ; (3) situation o f continuous ring o f land
around north polar sea and location o f south pole in
land area (A ntarctica) surrounded by water from all
sides; (4) antipodal arrangem ent o f the continents
and oceans ; (5) largest extent o f the Pacific Ocean
covering one third area of the globe and (6) location
of chain o f folded m ountains around the Pacific
Ocean.
In a tetrahedron a plane face rem ains always
opposite to an apex or coign. T he apex o r coign is
m ore sharpened in the case o f a real tetrahedron. In
the case o f the earth the oceans rep resent the plane
faces o f the tetrahedron and land m asses represent
the apices or coigns but in the case o f the earth the
coigns are not m uch sharpened, rath er they are flat
and convex. A ccording to L ow thian G reen oceans
were created on the plane faces o f the terrestrial
tetrahedron w hereas the coigns becam e continental
m asses (fig. 6.2 )
The hypothesis of Lowthian Green propounded
in the year 1875 is based on the com m on character­
istics o f a tetrahedron. He based his hypothesis on
the follow ing two basic principles of geom etry.
( 1 ) 4A sphere is that body which contains the
largest volum e with respect to its surface area’.
(2 ) ‘A tetrahedron is that body which contains
the least volum e with respect to its surface area’.
A fter many experim ents Low thian Green
opined that a sphere if subjected to uniform pressure
on all its sides would be transform ed into the shape
o f a tetrahedron. He applied this principle in the case
o f the earth. A ccording to him when the earth was
originated it was in the form o f a sphere. In the
beginning the earth was very hot but it gradually
began to cool dow n due to loss o f heat. First, the
outer part o f the earth cooled down and thus was
form ed the crust but inner part o f the earth continued
to cool dow n. C onsequently, the inner part o f the
earth was subjected to m ore contraction due to
continued cooling and thus there was marked reduc­
tion in the volum e o f the inner part o f the earth. Since
the upper part, the crust, was already cooled and
solidified and hence it could not be subjected to
further contraction. This resulted into possible gap
between the upper and inner parts o f the earth.
Fig. 6.2 : Distribution o f land and water on a tetrahe­
dron.
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Four oceans (e.g. the P acific O cean, the A t­
lantic O cean, the Indian O cean and the A rctic ocean)
were created on the four plane faces o f the terrestrial
tetrahedron. T hese plane faces could retain water
because o f the fact that these w ere low er than the
level o f the apices o r coigns o f the terrestrial tetrahe­
dron. C ontinents w ere form ed along the apices or
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CONTINENTS AND OCEAN BASINS
117
coigns o f the tetrahedron. T his fact may also be
proved on the basis o f an experim ent. If we sub­
merge a tetrahedron in a h em isphere o f water, the
flat surface o f the tetrah ed ro n w ould retain w ater
while the edges or apices or coigns will project
above the w ater. L o w th ian G reen claim ed to see a
tetrahedral arran g em en t in the distribution o f the
continents and o cean s in such a way that the earth
was linked to a tetrah ed ro n having to u r flat faces and
standing on one p o in t (fig. 6.2). T he upper flat face
represents the A rctic O cean w hile the rem aining
three faces represen t the Pacific O cean, the A tlantic
Ocean and the Indian O cean. S im ilarly, three verti­
cal m eridional edges represent N orth and South
Amrica, E urope and A frica and A sia w hile the lower
point is represented by A ntarctica. Thus, the pres­
ence o f w ater around north pole and the location of
south pole in land area (A ntarctic continent) are very
well explained on the basis o f tetrahedral hypoth­
esis. T hree coigns o ut o f four coings o f four equilat­
eral triangles are located in the northern hem isphere.
Only the fourth coign is located in the southern
hem isphere. T hese three coigns present the oldest
rigid m asses around w hich the present continents
have grow n. T hese three ancient shields are the
Laurentian or C anadian Shield, Baltic Shield and
Siberian Shield. T he fourth coign or the pivot o f the
tetrahedron represents the Antarctic shield. The present
continents have grow n out o f these four ancient
shields represented by four coigns o f the tetrahe­
dron. All the contin en ts developed along the edges
of the tetrahedron taper southw ard and thus triangu­
lar shape o f the continents is proved. The location of
the oceans along four plane faces and the continents
along the edges or coigns o f the plane faces o f the
tetrahedron proves antipodal position o f land and
water.
T hough G regory accepted the tetrahedral hy­
pothesis o f L ow thian G reen but he suggested certain
m odifications. A ccording to G regory due to shrink­
age o f the earth because o f contraction on cooling
‘the portion o f the vertical tetrahedral edges should
be fairly constant, but three edges around the polar
depression m ight develop som etim es in the northern
and at others in the southern h em isp h ere’.
characteristic features o f the distributional pattern of
the present-day continents and ocean basins but
because o f certain basic defccts and errors the hy­
pothesis is not acceptable to the m odern scientific
com m unity. It is argued that the balance o f the earth
in the form o f a tetrahedron w hile rotating on an apex
cannot be m aintained. S econdly, the earth is rotating
so rapidly on its axis that the spherical earth cannot
be converted into a tetrahedron w hile co n tractin g on
cooling. Thirdly, this hypothesis believes m ore or
less in the perm anency o f con tin en ts and ocean
basins w hile the plate tectonic theory has validated
the concept o f continental drift.
6.3 CONTINENTAL DRIFT THEORY OF TAYLOR
F. B. T aylor postulated his co n cep t o f ‘h o ri­
zontal displacem ent o f the co n tin e n ts’ in the y ear o f
1908 but it could be published only in the y ear 1910.
The main purpose o f his hypothesis w as to explain
the problem s o f the origin o f the folded m ou n tain s o f
Tertiary period. In fact, F.B. T ay lo r w an ted to solve
the peculiar problem o f the d istrib u tio n al pattern o f
Tertiary folded m ountains. T he north -so u th a rra n g e ­
ment o f the R ockies and the A ndes o f the w estern
m argins o f the N orth and S outh A m ericas and w esteast extent o f the A lpine m o untains (A lps, Cauca­
sus, H im alayas etc.) posed a serious problem before
Taylor which needed careful exp lan atio n . H e could
not find any help from the ‘co n tra ctio n th e o r y ’ to
explain the peculiar distribution o f T ertiary folded
m ountains and hence he p ropounded his ‘d rift or
displacem ent theory. The concept o f T aylor, thus,
is considered to be first, attem p t in the field o f
continental drift though A ntonio S n id er presented
his views about ‘d r ift’ in the year 1858 in France.
Main purpose behind the po stu latio n o f ‘d rift h y ­
poth esis’ o f S nider was to ex p lain the sim ilarity o f
the fossils o f the coal seam s o l C arb o n ifero u s period
in North A m erica and Europe.
T aylor started from C retaceo u s period. A c­
cording to him there w ere tw o land m asses during
Cretaceous period. L auratia and G ondw analand w ere
located near the north and south poles respectively.
He further assum ed that the con tin en ts w ere m ade o f
sial w hich was practically absent in the oceanic
crust. A ccording to T ay lo r co n tin en ts m oved to ­
w ards the equator. T he m ain d riv in g f o r c e 'o f the
continental drift w as tidal force. A cco rd ing to T ay lo r
continents w ere displaced in tw o w ays e.g. ( 1 ) eq u a­
Criticism
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Though the tetrahedral hypothesis throws light
on the problem s of the continents and ocean basins
and to m ajor extent it successfully explains the
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118
GEOMORPHOLOGY
force nor any external force can d rift the continents
apart and can help in the form ation o f mountains
T he responsible force m ust com e from within the
earth. T hough the co n cep t o f F.B. T aylor is not
acceptable but his hypothesis is considered to be
significant on the ground that T aylor raised his voice
very forcefully through deductjve postulation against
the prevalent concept o f the perm anency of the
continents and ocean basins and forcefully objected
to the ‘con traction th eo ry ’ and show ed a new
direction to solve the problem o f the origin o f the
continents and ocean basins. A. holm es has rightly
rem arked, ‘but T aylor m ust be given credit for
m aking an independent and slightly an earlier start in
this precarious field ’.
tor ward m ovem ent and (ii) w estw ard m ovem ent but
the driving force responsible for both types o f m ove­
ment was tidal force o f the m oon.
L auratia started m oving aw ay from the north
pole because o f enorm ous tidal force o f the moon
tow ards the equator in a radial m anner. This m ove­
ment o f landm ass resulted into tensional force near
the north pole w hich caused stretching, splitting and
rupture in the landm ass. C onsequently, B affin Bay,
Labrador Sea and D avis S trait w ere form ed. S im i­
larly, the displacem ent o f the G ondw analand from
the south pole tow ards the equator caused splitting
and disruption and hence the G ondw analand was
split into several parts. C onsequently, G reat A us­
tralian Bight and R oss Sea w ere form ed around
A ntarctic continent. A rctic sea was form ed betw een
G reenland and Siberia due to equatorw ard m ove­
m ent o f Lauratia. A tlantic and Indian coeans were
supposed to have been form ed because o f filling of
gaps betw een the drifting continents with water.
T aylor assum ed that the landm asses began to m ove
in lobe form w hile drifting through the zones o f
lesser resistance. T hus, m ountains and island arcs
w ere form ed in the frontal part o f the m oving lobes.
The H im alayas, C aucasus and A lps are considered
to have been form ed during equatorw ard m ovem ent
o f the L auratia and G ondw analand from the north
and south poles respectively w hile the Rockies and
A ndes w ere form ed due to w estw ard m ovem ent of
the landm asses.
6.4 CONTINENTAL DRIFTTHEORYOF WEGENER
Aim of the Theory
Professor A lfred W egener o f G erm any was
prim arily a m eteorologist. He propounded his con­
cept on continental drift in the year 1912 but it could
not com e in light till 1922 w hen he elaborated his
concept in a book entitled ‘D ie E n tstehung der
K ontinente and O zea n e’ w hich w as translated in
English in 1924. W egener's displacem ent hypoth­
esis w as based on the w orks and findings o f a host
o f scientists such as geologists, palaeo-clim atologists, palaeontologists, geophysicists and others.
The main problem before W egener, w hich needed
explanation, was related to clim atic changes. It may
be pointed out that there are am ple evidences which
indicate w idespread clim atic changes throughout
the past history o f the earth. In fact, the continental
drift theory o f W egener ‘grew out o f the need of
explaining the m ajor variations o f clim ate in the
p ast’. The clim atic changes w hich have occurred on
the globe may be explained in tw o w ays.
Criticism s
Since F.B. T alor's m ain aim was to explain
the origin o f the Tertiary folded m ountains and
hence he m ade the continents to m ove at a very large
scale. In fact, som e sort o f horizontal m ovem ent of
the landm asses w as essential for the origin o f m oun­
tains but the displacem ent o f landm asses upto 32-64
km would have been sufficient enough for the pur­
pose. C ontrary to this, T aylor has described the
displacem ent o f the landm asses for thousands of
kilom etres. Secondly, the m ode of drift as suggested
by Taylor has also been erroneous. If the tidal force
of the moon was so enorm ous during C retaceous
period that it could displace the landm asses for
thousands o f kilom etres apart then it m ight have also
put a break on the rotatory m otion o f the earth and
thus the rotation o f the earth m ight have stopped
within a year. A ccording to A. H olm es neither tidal
(1) If the continents rem ained stationary at
their places .throughout geological history of the
earth, the clim atic zones m ight have shifted from one
region to another region and thus a particular region
m ight have experienced varying clim atic conditions
from tim e to time.
(2) If the clim atic zones rem ained stationary,
the landm assesm ighthavebeendisplacedanddrifted.
W egener opted for the second alternative as
he rejected the view o f the perm anency o f c o n t i n e n t s
and ocean basins. Thus, the main
o f Wegener
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CONTINENTS AND OCEAN BASINS
119
behind his displacement hypothesis9 was to ex­
plain the global clim atic changes w hich are reported
to have taken place during the past earth history.
geological, clim atic and floral records, he claimed
that all the present-day continents could be joined to
form Pangaea. The following evidences support the
concept o f the existence o f Pangaea during C arbon­
iferous period.
Basic Prem ise of th e Theory
Follow ing E dw ard Suess, W egener believed
in three layers system o f the earth e.g. outer layer o f
‘sial’, interm ediate layer o f ‘s im a ’ and the lower
layer o f n ife . A ccording to W egener sial was
considered to be lim ited to the continental masses
alone w hereas the ocean crust was represented by
the upper part o f sim a. C ontinents or sialic masses
were floating on sim a w ithout any resistance offered
by sima. He assum ed, on the basis o f evidences of
palaeo-clim atology. palaeontology, palaeobotany,
geology and geophysics, that all the landmasses
were united together in the form of one landmass,
which he nam ed P a n g a e a , in C arboniferous period.
There were several sm aller inland seas scattered
over the Pangaea w hich was surrounded by a huge
water body, w hich w as nam ed by W egener as
‘P a n th a la s a ’, representing primaeval Pacific Ocean.
Lauratia consisting o f present North America, Eu­
rope and A sia form ed northern part o f the Pangaea
while G ondw analand consisting of South America,
Africa, M adagascar (now M alagasy), Peninsular
India, A ustralia and A ntarctica represented the south­
ern part o f the Pangaea. South pole w as located near
present D urban (near N atal in southern Africa) dur­
ing C arboniferous period. Thus, W egeners theory
of continental drift begins from Carboniferous pe­
riod, he does not describe the conditions during preCarboniferous tim es ‘but the postulation of a Car­
boniferous Pangaea does not mean that he disbe­
lieves in p r e - C a r boniferous drift; events before this
time are know n w ith m uch less certainty, and the
distribution o f plants and anim als can largely be
explained by m ovem ents which have taken place
since the C arboniferous’ (J.A . Steers. 1961. p. 160).
The Pangaea was disrupted during subsequent peri­
ods and broken landm asses drifted away from each
other and thus the present position o f the continents
and ocean basins became possible.
(1) A ccording to W egener there is geographi­
cal sim ilarity along both the coasts o f the Atlantic
Ocean. Both the opposing coasts o f the A tlantic can
be fitted together in the sam e way as tw o cut off
pieces o f wood can be refitted (jig-saw fit) (fig. 6.3).
(2) Geological evidences denote that the C al­
edonian and Hercynian m ountain system s o f the
western and eastern coastal areas o f the A tlantic are.
similar and identical (fig. 6.4). The A pplachians o f
the north-eastern regions o f N orth A m erica are com ­
patible with the mountain system s o f Ireland, W ales
and north-western Europe.
Fig. 6.3: Jig-saw fitting (juxtaposition) o f South
America and Africa,
(3) Geologically, both the coasts o f the Atlan­
tic are also identical. Du Toit, after detailed study o f
the eastern coasts o f South America and western
coast o f Africa, has said that the geological struc­
tures of both the coasts are more or less similar.
According to Du Toil both the landmasses (i.e.
South America and Africa) cannot be actually brought
together but near to each other because a gap o f 400-
Evidences in Support of The Theory
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W egener has successfully attempted to prove
the unification o f all landmasses in the form of a
single landmass, the Pangaea, during Carboniferous
period. On the basis o f evidences gathered from
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120
GEOMORPHOLOGY 1
800 km w ould sep arate them d u e to the ex isten ce o f
continental shelves and slopes o f th ese tw o landm asses.
lan d m asses w ere u n ited in th e a n cien t tim es and the
an im als used to m ig rate to far o f f places in the
w estern d irectio n .
(7) T h e d istrib u tio n o f g lo sso p teris flora in
India, S outh A frica, A u stralia, A n tarctica, Falkland
islands etc. p ro v es the fact that all the landm asses
w ere prev io u sly united an d co n tig u o u s in the form of
P angaea.
(8) T h e ev id en ces o f C a rb o n ife ro u s glacia­
tion o f B razil, F alk lan d , S o u th A frica, Peninsular
India, A u stralia and A n tarctica fu rth er prove the
u n ific a tio n o f all la n d m a sse s in o n e landm ass
(P an g aea) durin g C a rb o n ife ro u s p eriod.
P r o c e s s of th e T heory
A s stated earlier the m ain aim o f W egener
behind the p o stu latio n o f his ‘d rift th e o ry ’ was to
explain m ajor clim atic ch an g es w hich are reported
to have taken p lace in the p ast geo lo gical history o f
the earth, such as C arb o n ifero u s glaciation o f m ajor
parts o f the G o n d w an alan d . B esides, W egener also
attem pted to solve o th er p ro b lem s o f the earth e.g.
origin o f m o u n tain s, island arcs and festoons, origin
an ev olution o f continenLs and ocean basins etc.
Fig. 6.4 : Geological similarity on the eastern coast o f
South America and the western coast o f A f­
rica.
(4) T here is m arked sim ilarity in th e fossils
and vegetation rem ain s found on the eastern co ast o f
South A m erica and the w estern coast o f A frica.
(5) It has been reported from geodetic ev i­
dences that G reenland is d riftin g w estw ard at the
rate o f 20 cm p er year. T h e evid en ces o f sea floor
spreading after 1960 have co n firm ed the m o v em en t
o f landm asses w ith resp ect to each other.
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(6) T h e lem m ings (sm all sized an im als) o f the
northern part o f S can d in av ia have a tendency to run
w estw ard w hen th eir p o p u latio n is en o rm o u sly in­
creased but they are foundered in the sea w ater due
to absence o f any la n d beyond N o rw ag ian coast.
This behaviour o f lem m ings proves the fact that the
(1)
F o rc e R esp o n sib le fo r th e D rift—
cording to W eg en er the co n tin en ts after breaking
aw ay from the P an ag aea m oved (d rifte d ) in two
directio n s e.g. (i) cq u ato rw ard m o v em en t and (ii)
w estw ard m ovem ent. T h e eq u ato rw ard m ovem ent
o f sialic blocks (co n tin en tal blo ck s) w as caused by
gravitational differen tial force and force o f buoy­
ancy. A s already stated the co n tin ental blocks, ac­
cording to W egener, w ere fo rm ed o f lighter sialic
m aterials (silica and alu m in iu m ) and were floating
w ithout any friction on relatively denser ‘sima’.
T hus, the equ ato rw ard m o v em en t o f the sialic blocks
(continental b lo c k s) w ould d ep en d on the relation of
the cen tre o f gravity and the cen tre o f buoyancy of
the floating con tin en tal m ass. G en erally, these two
types o f forces o perate in o p p o site directions. ‘But
because o f the ellip so id al form o f the earth, these
forces are not in d irect o p p o sitio n , but are so r e la t e d
that, if the buoyancy p o int lies u nder the centre of
gravity, the resultant (fo rce) is directed towards the
e q u a to r’ (J.A., Steers, 1961, p. 164).
The westward m ovem ent o f the continents ;
was caused by the tidal force o f the sun and the j
moon. According to W egener the attractional force |
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c 0 MTINENTS a n d o c e a n b a s i n s
(2) A ctual D riftin g o f the C ontinents— The
disruption, rifting and ultim ately drifting o f the
continental blocks began in C arboniferous period.
The m ovem ent o f the continental blocks away from
the poles w as dram atically called by W egener as
‘the flight from the p o les’. Pangaea was broken
into two parts due to differential gravitational force
and the force o f b u o y an cy . The northern part became
Lauratia (A n garalan d ) w hile the southern part was
called by W egener as G ondw analand. The inter­
vening space betw een these tw o giant continental
blocks was filled up w ith w ater and the resultant
water body was called T ethys Sea. This phase o f the
disruption o f P angaea is called ‘O pening o f T ethys’.
G ondwanaland was disrupted during Cretaceous
period and Indian peninsula, M adagascar, Australia
and A ntarctica broke aw ay from Pangaea and drifted
apart under the im pact o f tidal force o f the sun and
the m oon. N o rth A m e ric a b ro k e aw ay from
Angaraland and drifted w estw ard due to tidal force.
Similarly, South A m erica broke away from Africa
and m oved w estw ard under the im pact o f tidal force.
Due to northw ard m ovem ent o f Indian Peninsula
Indian O cean was form ed while the Atlantic Ocean
was form ed due to w estw ard m ovem ent of two
Americas. It may be m entioned that N orth and South
Americas w ere drifting w estw ard at different rates
and hence ‘S ’ shape o f the A tlantic Ocean could be
possible. A rctic and N orth Sea were lorm ed due to
flight of the continental blocks from north pole. The
size of the Panthalasa (prim itive Pacific Ocean)
was rem arkably reduced because o f the movement
of continental blocks from all sides towards Panthalasa.
Thus, the rem aining portion o f Panthalasa became
the Pacific Ocean. It may be mentioned that disrup­
tion, rifting and displacem ent (d riftin g )o f continen­
tal blocks continued from C arboniferous period to
Pliocene period w hen the present pattern and ar­
rangem ent o f the continents and ocean basins was
attained (fig. 6.5). T here have been frequent changes
in the positions o f the equator and the poles as given
in table 4.1.
Table 6.1 : Shifting of the Position® of the Poles
Period
N orth Pole
Silurian
14°N latitude to the n orth-w est o f
124°W IonM ad ag ascar
S ou th Pole
gitude
C arboniferous 16°N latitude near D urban in
147°W IonN atal
gitude
Tertiary
51^N latitude near 53®S latitude to
153°W Ionthe south o f A frica
gitude
Equator was located at the m ost northerly
location during Silurian period as it passed north o f
Norway. It passed through London during C arbon­
iferous period and through present locations o f the
European Alpine m ountains during T ertiary period
(fig. 6.6). T h e south Pole and E quator obviously
moved into accordant positions. The prevailing w est­
ward and equatorw ard m ovem ents jnust be referred
to these positions’ (J.A. Steers, 1961, p. 166).
(3)
M ountain B uilding— A .G . W eg en eralso
attempted to solve the problem o f the origin o f
folded m ountains o f Tertiary period on the basis o f
his continental drift theory. The frontal edges o f
westward drifting continental blocks o f N orth and
South Americas were crum pled and folded against
the resistance o f the rocks o f the s e a -flo o r (sim a) and
thus the western C ordilleras o f the tw o Americas
(e.g. Rockies and A ndes and other m ountain chains
associated with them ) were form ed. Sim ilarly, the
A lpine ranges o f E u rasia w ere fo ld ed due to
equatorward m ovem ent o f E ruasia and Africa to­
gether with Pennisular India (equator was passing
thorough Tethys sea at that tim e). Here, W egener
postulated contrasting view points. According to
W egener sial (continental blocks) w as floating upon
sim a w ithout any friction and resistance but during
the later part of his theory he pointed out that mountains
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of the sun and the m oon, w hich was m axim um when
the moon was nearest to the earth, dragged the outer
sialic crust (continental blocks) over the interior of
the earth, tow ards the w est. It m ay be pointed out that
in any drift theory the w eak est point and the m ost
difficult problem is related to the com petent force
responsible for the m o v em en t o f the continents.
‘Such a force (tidal fo rce/ attractional force o f the
sun and the m oon) is extrao rd in arily sm all, but, as in
the case o f other forces, the question o f time is all
important; given su fficien t tim e, it is claim ed that
even these very sm all forces are able to cause m ove­
ments’ (J.A. S teers, 1961, p. 164).
121
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g eo m o rph o lo g y
122
Fig. 6.5 : Disruption o f Pangaea and drifting o f continents. The dotted lines denote the present position o f continents
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and ocean basins.
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CONTINENTS a n d o c e a n b a sin s
123
Jertiury N p
-JS
C a r l j o n i le r oa s N V
• Silurian S Pole
ertiary V
C arboniferous
S Pole
/
• Tertian* S Pole
Fig. 6.6 : Different positions o f Poles and Equator.
were form ed at the frontal edges o f floating and
drifting continental blocks (sialic crust) due to fric­
tion and resistance offered by sima. How could it be
possible ? The question rem ains unansw ered. Inspite
of this serious flaw in the continental drift theory of
W egener, S.W . W ooldridge and R.S. M organ have
remarked, ‘certainly the problem o f m ountain build­
ing is one in w hich the hypothesis o f continental drift
solves m ore difficulties than it creates’.
Australia, Antarctica etc. were extensively glaci­
ated. According to W egener all the continental blocks
were united together in the form o f one landm ass
called as Pangaea. South Pole was located near the
present position of Durban in Natal. Thus, south
pole was located in the middle o f Pangaea. Conse­
quently, ice sheets might have spread from south
pole outward at the time o f glaciation and the afore­
said land areas, which were closer to south pole,
might have been covered with thick ice sheets. At
much later date, these land areas m ight have parted
away due to disruption o f Pangaea and related con­
tinental drift. G lossopteris flora m ight have also
been distributed over the aforesaid areas when these
were united together.
(4) O rigin o f Island A rc s — W egener has
related the process o f the origin o f island arcs and
festoons (o f eastern A sia, W est Indies and the arc of
the southern A ntilles betw een Tierra del Fugo and
A ntarctica) to the differential rates o f continental
drift. W hen the A siatic block (part of Angaraland)
was m oving w estw ard, the eastern m aigin of this
block could not keep pace w ith the westward m ov­
ing m ajor landm ass, rather lagged behind, conse­
quently the island arcs and festoons consisting of
Sakhalin, Kurile, Japan, P h ilippines etc. were formed.
Similarly, some portions of N orth and South Am eri­
cas, while they w ere m oving w estward, were left
behind and the island arcs o f W est Indies and south­
ern A ntilles were formed.
(5) C arboniferous Glac;itation— There are
ample evidences to dem onstrate that there was largescale glaciation during Carboniferous period when
Brazil, Falkland, Southern Africa, Peninsular India,
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Evaluation of the Theory
It may be pointed out that W egener’s conti­
nental drift theory widely departed from the con­
temporary orthodox geological ideas o f the nine­
teenth century and the tim e-honoured thermal con­
traction theory o f the m ountain building and thus it
was obvious that the believers of contraction theory
should not only critisize the new theory o f horizontal
displacement of the continents but should also
discard it. ‘It is now widely agreed that he (W egener)
handled his case as an advocate rather than as an
impartial scientific observer, appearing to ignore
evidences unfavourable to his ideas and distort other
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124
GEOMORPHOLOGY
pre-C arboniferous tim es. M any questions remain
unansw ered such as, W hat kept P angaea together till
its disruption in M esozoic e ra ? ’ W hy did the process
o f continental drift not start before M esozoic era ?
etc. Som e w riters argue that 4it is not a fair criticism
to say that any pre-C arboniferous m ountain building
cannot be explained on W egener's hypothesis merely
because he does not develop his schem e in earlier
geological tim es’ (J.A. Steers, 1961, pp. 161-161).
ev id en ces in harm o n y w ith the th e o ry ' (S.W .
W ooldridge and R.S. M organ, 1959, p. 40). The
critics o f W egener’s continental drift theory fall in
tw o broad categoreis e.g. (i) the critics and w riters
who alw ays attem pted to search errors and dis­
crepancies in W egener's original synthesis and (ii)
the scientists w ho attem pted to m odify, enlarge and
correct the original theory o f W egener w hile retain­
ing its basic tenet. T he follow ing flaw s and defects
have been pointed out by different scientists in
W egener's theory o f continental drift.
It may be concluded that ‘even if all the m atter
of his theory is w rong, geologists and others can but
rem em ber that it is largely to him that we ow e our
more recent view s on w orld te cto n ic s’ (J.A . Steers,
1961, p. 174). Though m ost p o in t o f W egener's
theory was rejected but its central them e o f horizon­
tal displacem ent was retained. In fact, the postula­
tion o f plate tectonic theory after 1960 is the result o f
this continental drift theory o f W egener. W egener is,
thus, given credit to have started th inking in this
precarious field.
(1) The forces applied by W egener (differen­
tial gravitational force and the force o f buoyancy and
tidal force of the sun and the m oon) are not sufficient
enough to drift the continents so apart, T h e tidal
force as invoked by W egener to account for the
supposed w esterly drift o f the continents would need
to be 10,000 m illion tim es as pow erful as it is at
present to produce the required effects, and, if it had
such a value, it w ould stop the earth's rotation com ­
pletely in a year’ (S.W. W ooldridge and R.S. M organ,
1959, p. 40). Sim ilarly, the differential gravitational
force and the force o f buoyancy are also not adequate
to cause equatorw ard m ovem ent o f the continents,
instead the force, if so enorm ous, m ight have caused
the concentration o f the continents near the equator.
6.5 P LA T E T EC T O N IC T H E O R Y
The rigid lithospheric slabs o r rigid and solid
crustal layers are te ch n ic a lly called ‘p la tes’. The
whole m echanism o f the evolution, nature and m o ­
tion of plates and resultant reactions is called ‘plate
te c to n ic s ’. In other w ords, the w hole process o f
plate motions is referred to as plate tectonics. ‘M ov­
ing o v er the w eak a s th e n o s p h e re , in d iv id u a l
lithospheric plates glide slow ly ov er the su rfa c e o f
the globe ; much as a pack o f ice o f the A rctic Ocean
drifts under the dragging force o f currents and w inds’
(A.N. Strahler and A.H. S trahler, 1978, p. 373).
Plate tectonic theory, a great scientific ach iev em en t
o f the decade o f 1960s, is based on tw o m ajor
scientific concepts e.g. (i) the c o n c e p t o f continental
d rift and (ii) the concept o f s e a -flo o r spreading.
L ithosphere is internally m ade o f rigid p lates (fig.
6.7). Six m ajor and 20 m inor plates hav e been
identified so far (Eurasian p late , In d ian -A u stralian
plate, A m erican plate, Pacific Plate, A frican plate
and A ntarctic p late , fig. 1 1 . 1 ).
(2) W egener has described several contrast­
ing view points. Initially, sialic m asses (continents)
w ere considered by W egener as freely floating over
‘s im a ’ w ithout any friction offered by ‘s im a ’ but in
later part o f his theory he has described forceful
resistance offered by ‘sim a’ in the free m ovem ent of
sialic continents to explain the origin o f m ountains
along the frontal edges o f floating continents. M oreo­
ver, ‘it is difficult to show how the sial blocks, in
their passage through the sim a, would crum ple at
their frontal edges and produce m ountains’ (J.A.
Steers, 1961, p. 195). A ccording to W ills no com ­
pression could be possible to form the Rockies and
the A ndes if the ‘sim a’ is m ore rigid than the ‘sial’.
Bow ie has m aintained that sim a has no strength to
crum ple sial to form m ountains.
It m ay be m entioned th at the term ‘plate’ was
first used by C anadian g eo physicist J.T. Wilson in
1965. M ckenzie and P arker discu ssed in detail the
m echanism o f plate m otions on the basis of Eulers
geom etrical theorem in 1967. T hey postulated **
paving stone’ hypothesis w herein the oceanic crust
(3) Both the coasts o f the A tlantic O cean
* cannot be com pletely refitted. Thus, the concept o f
juxtaposition’ or ‘jig-saw fit’ cannot be validated.
(4) Wegener has not elaborated the direction
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and chronological sequence of the displacement of
the continents. He did not describe the situations of
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C0NTINENTS a n d OCEAN b a s i n s
125
CONVERGENCE
DIVERGENCE
Sub duct ion
zone
Mid oceanic
Ridge
Trench
TraTisform*—
rjo u tt^
:ontine
ntol pfc
oceanic
plate
lA sth en o sp h e re l
| L ith o sp h e re r
Fig. 6.7 : Diagramatic presentation o f main aspects o f plate tectonics (based on A.N. Strahler 1971).
(1) Constructive Plate M argins— These are
was considered to be new ly form ed at mid-oceanic
ridges and destroyed at the trenches. Isacks and
Sykes confirm ed the ‘paving stone hypothesis’ in
1967. W.J. M organ and Le Pichon elaborated the
various aspects o f plate tectonics in 1968. Now the
continental drift and displacem ent are considered a
reality on the basis o f plate tectonics.
also called as ‘divergent plate m argins’ or‘accreting
plate margins’. C onstructive p late m arg in s (bounda­
ries) represent zones o f div erg en ce w h e re there is
continuous upw elling o f m olten m aterial (la v a ) and
thus new oceanic crust is c o n tin u o u sly fo rm e d . In
fact, oceanic plates split apart along the m id -o c ea n ic
ridges and m ove in opposite d ire c tio n s (fig. 6 . 8).
It may be highlighted that tectonically plate
boundaries or plate m argins are m ost important
because all tectonic activities occur along the palte
margins e.g. seism ic events, vulcanicity, mountain
building, faulting etc. Thus, the detailed study of
plate margins is not only desirable but is also nec­
essary. Plate m argins are generally divided into
three groups, as follow s :
(2) Destructive Plate M argins— These are
also called as ‘consum ing plate m argin s’ or ‘co n ­
vergent plate m argins’ b ecau se tw o plates m ove
tow ards each other or tw o plates co n v e rg e along a
line and leading edge o f one p late o v e rrid es the other
plate and the overridden plate is su b d u cted or thrust
into the m antle and thus part o f the c ru st (plate) is
lost in the m antle (fig. 6 .8 ).
DIVERGENCE
^
, V?hC2.nn’C
Continental chain
cr ust
CONVERGENCE
Trench
Sea flo o r
T,oor
Dceamc crust (basalt)/ ---- ------------------ K
LITHOSPHERE
Magma
x-v
V-a ;
- \ \ r
Soft layer
*v A.
ASTHENOS
PHERE
^
ASTHENOSPHERE
S u b d u c tio n
Rising
m antle
rock
Melting
M A N T L E
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Fig. 6.8:Diagramatic presentation o f different types o f plate margins.
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GEOMORPHOLOGY
126
(3)
Conservative Plate M argins arc also
called as shear plaic margins. Here, two plates pass
or slide past one another along transform faults and
thus crust is neither created nor destroyed.
H.
Hess postulated the concept of ‘plate tec­
tonics’ in I960 in support of continental drift. The
continents and occans move with the movement of
these plates. The present shape and arrangement of
the continents and ocean basins could be attained
because of continuous relative movement of differ­
ent plates of the second Pangaea since Carbonifer­
ous period. Plate tectonic theory is based on the
evidences o f ( 1 ) sea-flo o r spreading and (ii)
palaeomagnctism.
Sea-Floor Spreading
The concept o f sea floor spreading was first
propounded by professor Hary Hess of the Princeton
University in the year 1960. His concept was based
on the researcn findings of numerous marine geolo­
gists, gcochcmists and geophysicists. Mason of the
Scripps Institute of Oceanography obtained signifi­
cant information about the magnetism of the rocks of
sea-floor of the Pacific Ocean with the help of
magnetometer. Later on he surveyed a long stretch
of the sea-floor of the Pacific Ocean from Mexico to
British Columbia along the western coast of North
Amrica. When the data of magnetic anomalies ob­
tained during the aforesaid survey were displayed on
a chart, there emerged well defined patterns of
stripes (fig. 6.9). Based on these information Hary
Hess propounded that the mid-oceanic ridges were
situated on the rising thermal convection currents
coming up from the mantle (fig. 6.10). The oceanic
crust moves in opposite directions from mid-oceanic
ridges, 'l’hese molten lavas cool down and solidify to
form new crust along the trailing ends of divergent
plates (oceanic crust). Thus, there is continuous
creation of new crust along the mid-oceanic ridges
and the expanding crusts (plates) are destroyed along
the oceanic trenchcs. These facts prove that the
continents and ocean basins are in constant motion.
Fig. 6.9 : Patterns o f positive magnetic anomalies off
the coast o f Sanfransisco.
profiles ot magnetic anomalies plotted on the basis
of actual data obtained during the survey, he found
sizeable difference between the two profiles. When
he plotted the magnetic profiles on the basis of
alternate bands of normal and reverse magnetism in
separate stripes o f 20 km width on either side of the
ridge, he found com plete parallelism between the
computed profiles and observed profiles.
Vine and M attheus have opined on the basis
ol the evidences of temporal reversal in the geo­
magnetic field and the concept o f sea-floor spreading
as propounded by Deitz and Hess that when molten hot
lavas come up with the rising thermal convection
current along the mid-oceanic ridges and get cooled
and solidified, these (lavas) also get magnetized, at
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W.G. Vine and M attheus conducted the mag­
netic survey of the central part of Carlsberg Ridge in
the Indian Ocean in 1963 and computed the magnetic
profiles on the basis of general magnetism. When he
compared the computed magnetic profiles with the
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127
CONTINENTS A N D O C E A N BA SIN S
the sam e tim e , in a c c o rd a n c e w ith th e then
geom agnetic field and thus altern ate bands or stripes
o f the earth (known as geocentric d ipole m agnetic
field), (ii) normal and reverse m agnetic am om alies
are found in alternate manner on either side o f the
m id-oceanic ridges, (iii) there is com plete parallel­
ism in the magnetic anom alies on either side o f the
mid-oceanic ridges and (iv) there is parallelism in
the time sequence o f palaeom agnetic epochs and
events calculated for 4.5 m illion years on the basis o f
magnetism o f basaltic rocks or sedim entary rocks.
Fig. 6.11 depicts the position o f m agnetic stripes on
either side o f the m id-oceanic ridge along the tim escale o f their formation.
of m agnetic an o m alie s are form ed on either side o f
the m id-oceanic ridge. In o th er w ords, when m olten
lavas are u p w elled along the m id-oceanic ridges,
these divide th e e arlier b asaltic layer into two equal
halves and these basaltic layers slide horizontally on
either side o f the m id -o cean ic ridges. The findings of
Cox, D oell and D alrym pal (1964). O pdyke (1966)
and H eritzler (1966) have validated the following
facts - (i) there is rev ersal in the m ain m agnetic field
Mid-Oceanic
ridge
Ascending
c u rre n ts
Fig. 6.10 : P attern o f therm al convective currents and pla te movements.
m
i
i T
It m ay be co n clu d ed , on the basis o f above
discussion, that there is c o n tin u o u s sp re a d in g o f seafloor. N ew basaltic cru st is c o n tin u o u sly formed
along the m id-oceanic ridges. The n ew ly formed
basaltic layer is div id ed into tw o eq u a l halves and is
thus displaced aw ay from the m id -o c e a n ic ridge.
A lternate stripes o f p o sitiv e an d n e g a tiv e magnetic
anom alies are found on e ith e r sid e o f the midoceanic ridges. S u ch m ag n etic a n o m a lie s (positive
and negative) ‘are form ed b ecau se o f temporal re­
versal in the g eo m ag n etic field. The ro c k s formed
during reverse p olarity (re v e rse d geom agnetic field)
denote negative m ag n etic a n o m a lie s ’.
]
The age of magnetic stripes* the rate o f seafloor spreading and the time o f drifting o f different
continents are calculated on the basis o f above facts.
The dating o f the magnetic stripes formed upto 4.5
million years before present has been com pleted on
the basis of information obtained from the survey o f
palaeomagnetism o f the sea-floors o f different oceans.
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Fig. 6.11 : Diagramatic presentation o f magnetic stripes
on either side o f the mid-oceanic ridges
according to Vine and Matthe us. The periods
o f the formation o f these stripes have been
named after known scientists (e.g. Gilbert.
Gass, Matuyama and Bruhnes).
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GEOMORPHOLOGY
128
netism and sea-flooi spreading is available only for
the last 200 m illion years but on the basis o f general
m echanism o f plate tectonics and the evidences
from the continents the sequence o f earlier events
may be reconstructed. V alentine and M oors (1970)
and H allam (1972) have attem pted to reconstruct the
chronological sequence o f the continents and ocean
basins from the beginning to the present tim e. About
700 million years ago all the landm asses w ere united
The rate o f sea-floor spreading is calculated on two
bases e.g. (i) on the basis o f the age o f isochrons
(isochrons are those lines which join the points o f
equal dates o f the m agnetic stripes plotted on the
m ap) and (ii) on the basis o f distance between two
isochrons. Thus, the rates o f spreading (drifting) o f
different oceans have been determ ined on the basis
of above principles. The Atlantic and Indian O ceans
are spreading (expanding) very sluggishly i.e. at the
rate o f 1.0 to 1.5 cm per year while the Pacific Ocean
is expanding at the rate o f 6.0cm per year. It m ay be
pointed out that the rate o f seafloor spreading
alw ays m eans the rate o f expansion only on one side
o f the m id-oceanic ridge. For exam ple, if the rate of
sea-floor is reported to be 1.0 cm per year, the total
spreading of the concerned ocean would be 1 + I =
2 cm per year. The recent studies have shown that (i)
the maximum spreading o f the Pacific Ocean is 6 to
9 cm per year (total expansion 12 to 18 cm /year)
along the eastern Pacific ridge between equator and
30°S latitude, (ii) the southern A tlantic Ocean is
spreading along the southern Atlantic ridge at the
rate of 2 cm per year (total expansion 4 cm /year) and
(iii) the Indian Ocean is expanding at the rate o f 1.5
to 3 cm per year (total expansion being 3 to 6 cm/
year).
000 000 years B P (Before Present)
f t Hercynian
Mountain
Pangaea I
%
Caledonian
Mountains
Pangaea II
present
CDn linents
Fig. 6.12 : The probable pattern o f continental move­
ment during the last 700 million years (based
on Valentine and Moors. 1970).
together in the form o f one single giant landmass
known as ‘P a n g a e a I ’. A bout 600-500 m illion years
before present first Pangaea was broken because of
thermal convective currents com ing from w ithin the
earth, most probably from the m antle and different
landmasses drifted apart. These landm asses were
again united together due to plate motions*in one
land mass known as ‘P a n g a e a I I ’ about 300-200
million years before present. A ccording to A. Hallam
Second P an g aea began to break during early Jurassic
period and N. W. A frica broke away from N. America
and drifted away. The zone o f sea-floor spreading
continued to extend tow ards north and south. The
separation o f South A m erica and A frica was accom­
plished during m iddle C retaceous period, and North
A m erica and Europe began to m ove aw ay from each
other (fig. 6. 1 2 ).
Plate Tectonics and Continental Displacement
On the basis of the evidences of palaeomagnetism and sea-floor spreading it has been now
validated that the continents and ocean basins have
never been stationary' or perm anent at their places
rather these have always been mobile thorughout the
geological history' of the earth and they are still
m oving in relation to each other. The scientists have
discovered ample evidences to demonstrate the open­
ing and closing of ocean basins. For exam ple, the
M editerranean Sea is the residual of once very vast
ocean (Tethys Sea) and the Pacific Ocean is continu­
ously contracting because o f gradual subduction of
A m erican Plate along its ridge. On the other hand,
the Atlantic Ocean is continuously expanding for the
last 200 million years. Red Sea has started to open (to
expand). It may be mentioned that continental masses
com e closer to each other when the oceans begin to
close while continents are displaced away when the
oceans begin to open (expand).
The opening of N orth A tlantic was accom­
plished in many phases. A fter the separation of
North A m erica from Africa, Europe and Greenland
broke away from Labrador during late Cretaceous
period (about 80 million years before present) and
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Though the sequence o f events o f continental
displacem ent based on the evidences o f palaeomag-
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CONTINENTS AND O CEA N BASINS
Na
” America
Fault
J
129
thus Labrador sea was formed. This newly form ed
sea continued to remain for som e time as northern
extension o f the A tlantic O cean. Rockall plateau
was separated from G reenland during Tertiary pe­
riod (about 60 million years before present). L abra­
dor Sea and North A tlantic continued to expand
between Europe and G reenland upto m iddle M iocene
period because the European and A m erican plates
continued to move eastw ard and w estw ard respec­
tively. The spreading o f Labrador Sea stopped by
middle M iocene period (about 47 m illion years
before present) but North A tlantic continued to ex­
pand.
Africa/ Europe
Granite
Indian Ocean did not exist before C retaceous
period. Indian plate began to m ove tow ards A siatic
plate through ‘Tethys S ea’ and A ustralian-A ntarctic
plates after breaking away from African plate began
to move southward during C retaceous period. Dan
M ackenzie and John Sclater have presented the
chronological sequence of the evolution o f Indian
Ocean on the basis of the study o f m agnetic anom a­
lies. According to them Indian plate began to move
northward at the rate of 18 cm per year during early
Tertiary period but the m ovem ent stopped during
Eocene period. At the same time A ntarctica broke
away from Australia. Thus, the Pacific O cean began
to shrink in size because o f expansion of the A tlantic
and Indian Oceans. Fig. 6.13 depicts the chronologi­
cal events of the Atlantic O cean during past 700
million years. The A tlantic O cean began to open
about 700 million years before present because o f
breaking o f F irs t P an g aea when the A m erican and
Africa-European plates began to m ove in divergent
directions and thus the A tlantic continued to expand
till 400 million years before present when the A tlan­
tic again began to close. Because o f the closing o f the
A tlantic Ocean A pplachian m ountains o f N orth
A m erica were formed. The A tlantic O cean again
began to open up about 150 m illion years before
present when Second Pangaea was broken into sev­
eral landm asses and it still continues to expand
because o f the m ovem ent o f A m erican and E uro­
pean plates in opposite directions. It m ay be pointed
out that the A tlantic O cean is continuously expand­
ing for the past 200 m illion years but the Pacific
O cean is contracting in size because o f w estw ard
4
Atlantic
5
Mi E| M2 E2
Mlogeocline (M)
Eugeocline (E)
Evolutionary' history o f the Atlantic Ocean
during the past 700 million years. 1. Forma­
tion o f new ocean basins 700 million years
ago. 2. D eposition o f m iogeocline and
eugeocline on the margins about 500 million
years ago. 3. Closing o f the Atlantic Ocean
and the form ation o f part o f the Applachians
due to convergence o f Eurasian and Ameri­
can plates about 400 million years ago. 4.
Atlantic closed completely and the formation
o f the Applachians o f North America and
Hercynian mountains o f Europe was com­
pleted about 300 million years ago. 5. Reo­
pening o f the Atlantic due to plate motion
about 150 million years ago. 6. Present situ­
ation, beginning o f the form ation o f new
geosync lines (After Dietz, 1973).
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Fig. 6.13
Atlantic
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GEOMORPHOLOGY
130
A n ta rc tic a
Fig. 6.14
The evolution o f the continents and ocean basins on the basis o f plate tectonics since Triassic period and the
probablefuture pattern ofevents uplo 50 million years hence. I. Triassic period, 200 million years ago, 2. Late
Triassic period, I HO million years ago, 3. Late Jurassic period, 135 million years ago, 4. Late Cretaceous
period, 65 million years a%o, 5. Present position and 6. 50 million years hence Arrows indicate the directions
of movement o f the continents (after Dietz and Holden, 1973).
movement of the A mericas Fig. 6.14 depicts the
probable situation of (he continents and ocean
basins during 50 million years hence.
veyed magnetic anom alies in this area show, as
observed by A.W. G irdler, the pattern o f stripe and
these arc sim ilar to the m agnetic anom alies of the
ocean basins. F.J. Vine calculated the rate of the
spreading of the Red Sea on the basis o f the data of
magnetic anom alies in the year 1966. A ccording to
him the Red Sea is spreading at the rate of one
centimetre per year (total spreading 2 cm /year) since
the past 3-4 million years. Alen and M orelli calcu­
lated the spreading rate in 1969 as 1.1 cm /year (total
'Die following examples demonstrate the trends
and patterns of continental displacem ent, sea-floor
spreading and contraction in ihe si/e of the oceans.
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Red Sea and the G ulf o f Aden— Red Sea is
an example o f axial trough which is located between
Africa and Arabian peninsula (fig. 6.15). I he sur­
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CONTINENTS AND OCEAN BASINS
131
plate. Nubian and Somali plates arc separated by
Ethiopian fault. Fig. 6 . 15 denotes the location o f Red
Sea, G ulf of Aden, A rabian, Nubian and Somali
plates and the pole o f rotation.
The G ulf o f C alifornia— The Pacific Ocean
is a waning ocean because it is continuously being
contracted in its size because of gradual encroachm ent
o f westward moving A m erican plates. It is believed
that like m id-Atlantic ridge there might have been a
mid-oceanic ridge in the Pacific Ocean but it has
now been remarkably deform ed due to plate m ove­
ment. The magnetic survey of the G u lf o f C alifornia
revealed the presence of stripped m agnetic anom aly.
This situation validates two facts viz. (i) East Pacific
Rise (ridge) is also located in the G u lf o f C alifornia
and there has been continuous spreading o f the g u lf
along the ridge since the past four million years and
(ii) Baja, the Californian peninsula, was previously
united with the mainland of North A m erica but later
on it broke away from the continent due to spreading
o f sea floor.
Fig. 6.15 : Diagramatic presentation o f separation o f
Africa and Arabia due to spreading o f Red
Sea and g u lf o f Aden. Arrows indicate direc­
tions ofthe movement ofthe plates and spread­
ing o f Red Sea and G ulf o f Aden. A and B
denote the poles o f rotation (after A.M.
Quennel, 1958).
Evaluation
It is commonly agreed by the m ajority o f the
scientists that plate tectonics has validated the con­
cept o f continental drift, rather continental drift has
now become a reality. The only point of argum ent
and question is related to the com petent force re­
sponsible for the drifting of the continents. M ost of
the scientists still rely on the thermal convective
currents com ing from the mantle as the probable
adequate force to move the plates (continents) in
different directions. See chapter 1 1 for detailed
description on plate tectonics.
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spreading 2.2 cm/year). Similarly, the rate of spreading
of the G ulf o f Aden has been calculated on the basis
o f stripped m agnetic anom alies as 0.9 to 1.1 cm /year
(total spreading 1.8 to 2.2 cm /year). The Red Sea and
the G ulf o f Aden are located at the junction of three
plates viz. N ubian plate, Somali plate and Arabian
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:
132-139
TH EO RY OF ISO STA SY
In tro d u c tio n ; d isc o v e ry o f th e c o n c e p t ; c o n c e p t o f A iry ; c o n c e p t o f
P r a t t ; c o n c e p t o f H ay fo rd a n d B o w ie ; c o n c e p t o f J o ly ; c o n c e p t o f
H o lm e s ; g lo b a l iso sta tic a d ju stm e n t.
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CHAPTER 7
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7
THEORY OF ISOSTASY
7.1 INTRODUCTION
7.2 D IS C O V E R Y O F T H E C O N C E P T
D ifferent relief features o f varying m agnitudes
e.g. m ountains, plateaus, plains, lakes, seas and
oceans, faults and rift valleys etc. standing on the
earth’s surface are probably balanced by certain
difinite principle, otherw ise these w ould have not
been m aintained in their present form. W henever
this balance is disturbed, there start violent earth
m ovem ents and tectonic events. Thus, ‘isostasy sim ­
ply m eans a m achanical stability between the up­
standing parts and low lying basins on a rotating
earth ’.
T hough the co ncept o f isostasy cam e in the
m ind o f geologists all o f sudden but its concept grew
out o f gradual thinking in term s o f gravitational
attractio n o f g ian t m o u n tain o u s m asses. Pierre
B ouguer during his expedition o f the A ndes in 1735
found that the tow ering volcanic peak o f Chim borazo
was not attracting the plum b line as it should have
done. He thus m aintained that the gravitational at­
traction of the A ndes ‘is m uch sm aller than that to be
expected from the m ass represented by these m oun­
tain s’. Sim ilar discrepencies w ere noted during the
geodetic survey o f the Indo-G angetic plain for the
determ ination o f latitudes under the supervision of
Sir G eorge Everest, the then Surveyor G eneral of
India, in 1859. The difference oflatitu d e o f Kalianpur
and K aliana (603 km due northw ard) was deter­
m ined by both direct triangulation m ethod and astro­
nomical m ethod. K aliana was only 96 km away from
the H im alayas. The difference betw een two results
am ounted to 5.23 seconds as given below —
T he word isostasy, derived from a German
w ord ‘isostasios’ (m eaning thereby ‘in equipoise’),
w as first proposed by A m erican geologist Dutton in
1859 to express his view to indicate ‘the state o f
balance w hich he thought m ust exist between large
upstanding areas o f the earth's surface, m ountain
ranges and plateaus, and contiguous low lands, etc.’
(S.W . W ooldridge and R.S. M organ, 1959). A ccord­
ing to D utton the upstanding parts o f the earth
(m ountains, plateaus, plains and ocean basins) must
be com pensated by lighter rock m aterial from be­
neath so that the crustal reliefs should rem ain in
m echanical stability. A ccording J.A. Steers (1961),
‘this doctrine states that w herever equilibrium exists
on the earth's surface, equal mass m ust underlie
equal surface area s.’
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Result obtained through triangulation = 5° 23' 42.294”
Result obtained through
astronomical method
= 5° 23' 37.058”
Difference
= 5.236"
This discrepancy betw een tw o m ethods was
attributed to the attraction o f the H im alayas due to
which the plum b-bob used in the astronom ical deter­
m ination of latitude was deflected.
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THEORY
o f is o s t a s y
133
Gravitational deflection at Kaliana
= 27.853"
Gravitational deflection at Kalianpur = 11.968”
difference
= 15.885"
Thus, the difference of 15.885" was in fact
more than 3 tim es the observed deflection o f 5.236"
during the survey. Pratt's calculation of the differ­
ence of the gravitational deflections brought another
fact before the scientists that the Himalaya was not
exerting the attraction according to its enormous
mass. This interpretation gave birth to another problem-What reason is behind low attractional force of
the H im alayas ? The follow ing explanations were
offered for this question.
(1) The H im alayas are hollow and are com ­
posed o f bubbles and not the rocks. Due to this fact
the weight and density o f the Himalayas would be
low and thus their gravitational force would also be
low. This was the reason for the difference in the
results o f tw o locations as referred to above. This
explanation cannot be accepted because such a high
mountain, if com posed of bubbles, cannot stand on
the earth's surface.
(2) If the m ountains are not hollow, the visible
mountain m ass m ust be com pensated by defficiency
of mass from below. In other words, the density of
the rocks o f the m ountains ‘must be relatively low
down to considerable depth.’ Thus, the total weight
would be low and consequently the attractional
force would also be low.
(3) The rocks o f the Himalayas are o f low
density in them selves and thus their attraction is also
low.
(4)
It was suggested ‘that there is such a level
below the surface o f the earth below which there is
no change in the density o f the rocks’, density varies
only above this level. Thus, all colum ns have equal
mass along this level. It was therefore suggested on
this basis that ‘bigger the colum n, lesser the den­
sity, and sm aller the colum n, greater the density.9
Thus, the debate on the discrepancies o f the
gravitational deflections o f the plum b-line and nu­
merous explanations for these discrepancies resulted
into the postulation o f the concept o f isostasy by
different scientists, the views o f a few o f them are
presented below.
7.3 THE CO N CEPT O F SIR G E O R G E A IRY
According to Airy the inner part o f the m oun­
tains cannot be hollow, rather the excess w eight o f
the mountains is com pensated (balanced) by lighter
materials below. A ccording to him the crust o f
relatively lighter material is floating in the substra­
tum of denser material. In other words, ‘siaP is
floating in ‘sim a ’. Thus, the H im alayas are floating
in denser glassy magma. A ccording to A iry ‘the
great mass of the Him alayas was not only a surface
phenomenon : the lighter rocks o f which they are
composed do not merely rest on a level surface o f
denser material beneath, but, as a boat in water, sink
into the denser m aterial’ (J.A. Steers, 1961). In other
words, the Himalayas are floating in the denser
magma with their maximum portion sunk in the
magma in the same way as a boat floats in w ater with
its maximum part sunk in the water. This concept in
fact involves the principle of floatation. For exam ­
ple, an iceberg floats in w ater in such a way that for
every one part to be above w ater-level, nine parts q f
the iceberg remain below w ater level. If we assume
the average density of the crust and the substratum to
be 2.67 and 3.0 respectively, for every one part o f the
crust to remain above the substratum , nine parts of
the crust must be in the substratum . In other words,
the law of floatation dem ands that ‘the ratio o f
freeboard to draught is 1 to 9 .’ It may be pointed out
that Airy did not mention the exam ple o f the floata­
tion of iceberg. He simply m aintained that the crustal
parts (landm asses) were floating, like a boat, in the
m agma of the substratum .
If we apply the law o f floatation, as stated
above, in the case o f the concept o f A iry, then we
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This interpretation, thus, brought the fact be­
fore the scientists that the enorm ous m ass o f the
Himalaya w as responsible, through its attractional
force, for the difference in the results of two m eth­
ods. Later on the m atter w as referred to Archdeacon
Pratt for further investigation and clarification. He
attempted to estim ate the am ount o f attraction o f the
Himalayas on the basic assum ption that all the m oun­
tains had the average density o f 2.75. Thus, Pratt
based on m inim um estim ate of the mass o f the
Himalayas calculated the gravitational effects on the
plumbob at two places (K aliana and Kalianpur) and
to his dism ay he discovered that the difference was
surprisingly m ore than actually worked out during
the survey.
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GEOMORPHOLOGY
134
have to assum e that for the 8848 m height o f the
H im alaya there m ust be a root, 9 tim es m ore in
length than the height o f the H im alaya, in the sub­
stratum. Thus, for 8848-m part o f the H im alaya
above, there m ust be dow nw ard projection o f lighter
m aterial beneath the m ountain reaching a depth o f
79,632 m (roughly 80,000 m).
Joly applied the principle o f floatation for the
crust of the earth taking the freeboard to draught
ratio as 1 to 8. A ccording to him ‘for every em ergent
part o f the crust above the upper level o f the substra­
tum there are eight parts subm erged1 (J.A. Steers,
1961). If we apply Joly's view o f flotation to the
concept o f Airy, there would be dow nw ard projec­
tion of the H im alaya upto a depth of 70,784 m (8848
m x 8) in the substratum .
Fig. 7. J : Illustration o f the concept o fA iry on isostasy.
freeboard to draught ratio as 1 to 9) or 70,784 m (if
the freeboard to draught ratio is taken as 1 to 8). It
would be w rong to assum e that the H im alaya would
have a dow nw ard projection o f root o f lighter mate­
rial beneath the m ountain reaching such a great
depth o f 79,632 m or 70,784 m because such a long
root, even if accepted, w ould m elt due to very high
temperature prevailing there, as tem perature increases
with increasing depth at the rate o f 1°C per 32 m.
Thus, according to Airy the H im alayas were
exerting their real attractional force because there
existed a long root o f lighter material in the substra­
tum which com pensated the m aterial above. Based
on above observation Airy postulated that 4if the
land column above the substratum is larger, its
greater part would be subm erged in the substratum
and if the land colum n is lower, its sm aller part
would be subm erged in the substratum .’ A ccording
to A iry the density o f different colum ns o f the land
(e.g. m ountains, plateaus, plains etc.) rem ains the
same. In other words, density does not change with
depth, that is, ‘uniform density w ith varying thick­
n ess.’
This m eans that the continents are made o f
rocks having uniform density but their thickness or
length varies from place to place. In order to prove
this concept A iry took several pieces o f iron o f
varying lengths and put them in a basin full of
m ercury. These pieces o f iron sunk upto varying
depths depending on their lengths. The same pattern
m ay be dem onstrated by taking w ooden pices o f
varying lengths. If put into the basin o f w ater these
w ould sink in the w ater according to their lengths
(fig. 7.1).
7.4 T H E C O N C E P T O F A R C H D EA C O N PRATT
W hile studying the differen ce o f gravitational
deflection o f 5.236 seconds d u rin g the geodetic
survey o f K aliana and K alianpur A rchdeacon Pratt
calculated the g ravitational force o f the Himalaya
after taking the average den sity o f the H im alaya as
2.75 and cam e to know that the d ifference should
have been 15.885 seconds. H e, then, studied the
rocks (and their d en sities) o f the H im alaya and
plains and found that the density of
co mneighbouring
­
each h igher part is less than a lo w er part. In other
w ords, the density o f m ountains is less than the
density o f p lateaus, that o f plateau is less than the
density o f plain and the density o f plain is less than
the density o f oceanic flo o r and so on. This means
that there is inverse relatio n sh ip betw een the height
o f the reliefs and density.
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•
T hough the concept o f S ir G eorge Airy
m ands great respect am ong the scientific com m u­
nity but it also suffers from certain defects and
errors. If we accept the A iry's view s o f isostasy, then
every upstanding part m ust have a root below in
accordance with its height. Thus, the H im alaya would
have a root equivalent to 7 9 ,6 3 2 fn (if we accept the
“Q uite recently, how ever, the fundamental
concept o f A iry, the continental m asses floating as
lighter (sial) blocks in a h eav ier (sim a) substratum,
has been rejuvenated, largely through the influence
o f H eiskanen's w ork, so that it is now probably true
to say that m ost geologists favour A iry's explana­
tion” (J.A. Steers, 1961, p. 75).
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THEORY o f is o s t a s y
135
A ccording to P ratt there is a level o f com pen­
sation above w hich there is variation in the density
of different colum ns o f land but there is no change in
density below this level. D ensity does not change
within one colum n but it changes from one column
to other colum ns above the level o f com pensation.
Thus, the central them e o f the concept of Pratt on
isostasy may be expressed as ‘uniform depth with
varying density. A ccording to Pratt equal surface
area must underlie equal m ass along the line of
com pensation. T his statem ent may be explained
with an exam ple (fig. 7.2).
Fig. 7.3 : Explanation o f the concept o f Prat ton isostasy.
Bowie has opined that though Pratt does not
believe in the law of floatation, as stated by Sir
George Airy but if. we look, m inutely, into the
concept of Pratt we certainly find the glim pse o f law
of floatation indirectly. Sim ilarly, though Pratt does
not believe directly in the concept o f ‘root form a­
tion’ but very close perusal of his concept on isostasy,
does indicate the glimpse of such idea (root form a­
tion) indirectly. W hile m aking a com parative analy­
sis of the views of Airy and Pratt on isostasy Bow ie
has observed that ‘the fundam ental difference be-
L in e o f C om pensation
Fig. 7.2: Line o f compensation according to Archdea­
con Pratt.
There are tw o colum ns, A and B, along the
line o f com pensation. Both the colum ns, A and B,
have equal surface area but there is difference in
their height. Both the colum ns m ust have equal mass
along the line o f com pensation, so the density of
column B should be m ore than the density of column
A so that the w eight o f both the colum ns become
equal along the line o f com pensation. Thus, the
Pratt's concept o f inverse relationship between the
height o f different colum ns and their respective
densities m ay be expressed in the follow ing m an­
ner— ‘bigger the colu m n , lesser the density and
sm aller the colu m n , g rea ter the d en sity.’ A ccord­
ing to Pratt density varies only in the lithosphere and
not in the pyrosphere and barysphere. Thus, P ratts
concept o f isostasy w as related to the ‘law o f com ­
pensation’ and not to ‘the law o f floatation .’ A c­
cording to Pratt d ifferen t re lie f features are standing
only because o f the fact that their respective m ass is
equal along the line o f com pensation because o f
their varying densities. T his co n cep t may be ex ­
plained w ith the help o f fig. 7.3.
Uniform Density
V arying Density
Density
3.0 3.0
3.0
3.0 4.0
5.0
Line of Compensation
SUB S T R A T U M
AIRY
PR A T T
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Fig. 7.4 : Comparison o f the views o f Airy and Pratt on
isostasy.
tw een A iry's and Pratt's view s is th at the form er
p ostulated a u niform d ensity w ith varying thick*
ness, and the latter a u n iform d ep th w ith varying
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GEOMORPHOLOGY
136
d e n s ity .’ Fig. 7.4 explains the fundam ental d iffer­
ence betw een the concepts o f Airy and Pratt on
isostasy.
7.5 T H E C O N C E P T O F H A YFO RD AND BOW IE
H ayford and Bowie have propounded their
concepts o f isostasy alm ost sim ilar to the concept of
Pratt. A ccording to them there is a plane w here there
is com plete com pensation of the crustal parts. D en­
sities vary with elevations o f colum ns o f crustal
parts above this plane o f com pensation. The density
o f the m ountains is less than the ocean floor. In other
w ords, the crust is com posed of lighter material
under the m ountains than under the floor of the
oceans. There is such a zone below the plane of
com pensation where density is uniform in lateral
direction. Thus, according to Hayford and Bowie
there is inverse relationship between the height of
colum ns of the crust and their respective densities
(as assum ed by A rchdeacon Pratt) above the line of
com pensation. The p lan e of co m p en sa tio n (level of
com pensation) is supposedly loeated at the depth of
about 100 km. The colum ns having the rocks of
lesser density stand higher than the colum ns having
the rock o f higher density. This statem ent may be
understood with the help of fig. 7.5.
they exert equal dow n w ard pressu re at the level of
com pensation and thus b alance one another (S.W.
W ooldridge and R.S. M organ, 1959). Fig. 7.6 ex­
plains the above concept. It is ap p aren t from fig. 7.6
that different colum ns o f equal cross-section cut
from various m etals and ores having varying densi­
ties are seen floating in a basin o f m ercury but all of
them reach the sam e line (level o f com pensation)
and thus exert equal w eight along the line o f com­
pensation.
B ow ie m ade a co m p arativ e study o f the views
o f Airy and Pratt on isostasy and concluded that
there was a great deal o f sim ilarity in their views. In
fact, ‘both the view s appeared to him sim ilar but not
the sam e’. Bowie could observe a glim pse of the
concept of root form ation and law o f floatation of
Airy, though indirectly, in the view s of Pratt. The
concept of H ayford and B ow ie, that the crustal parts
(various reliefs) are in the form o f vertical columns,
is not tenable because the crustal features are found
in the form of horizontal layers.
CJ
N
OJ c
C
‘,
1
c
©
Im U&
CJ c 1
z U
•o
3m\
L
I
Level o f C c> m p en sa tio ii^ ^ ^ J ^ - : ^ ^ j ; -
Fig. 7.6 : Illustration o f the concept o f Bowie on isostasy.
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Fig. 7.5 : Explanation o f views o f Hayford and Bowie
on Isostasy. The densities mentioned in the
different columns (e.%. inland plain, plateau,
coastal plain and off shore region) are imagi­
nary.
There are four im aginary colum ns (interior
plain, plateau, coastal plain and off shore region) in
fig 7.5 which reach the level o f com pensation.
Their height varies but they are balanced by their
varying densities. ‘The assum ption is that the vary­
ing volume of m atter in the several colum ns is
com pensated by their density, in such a fashion that
7.6 TH E C O N C E P T O F J O L Y
Joly, while presenting his view s on isostasy in
1925, contradicted the concept of Hay ford and Bowie.
He disapproved the view o f H ayford and Bowie
about the existence o f level o f com pensation at th*
depth of about 100 km on the ground that the tem­
perature at this depth w ould be so high that it would
cause com plete liquefaction and thus level of com­
pensation w ould not be possible. He further refuted
the concept of H ayford and Bowie that ‘denisity
varies above the level o f com pensation but remain*
uniform below the level o f co m pensation’ on the
ground that such condition w ould not be possible in
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THEORY o f i s o s t a s y
137
practice because such condition would be easily
disturbed by the geological events and thus the level
of com pensation w ould be disturbed. A ccording to
Joly there exists a layer o f 10-m ile (16 km) thickness
below a shell o f uniform density. The density varies
in this zone o f 10-mile thickness. It, thus, appears
that Joly assum ed the level o f com pensation as not a
linear phenom enon but a zonal phenom enon. In
other words, he did not believe in a ‘line (level) of
com pensation9 rather he believed in a ‘zone of
com pensation’ (o f 10-mile thickness). Thus, we
also find a glim pse o f the law o f floatation (it may be
remembered that Joly did not m ention this, we only
infer the idea o f floatation from Joly's concept) in
Joly's concept w hich is closer to the Airy's concept
rather than the concept o f H ayford and Bowie.
£
Sea Level
Sea F lo o r . */.#
SIM A Density 3.0.
- — —_ —
^
Fig. 7.8: Diagramatic presentation o f the earth's crust
and the upper part o f the mantle to illustrate
the relationship between surface features and
crustal structure and the concept o f isostasy
(based on A. Holmes and D .L Holmes , 1978).
‘This is in close agreem ent with floatation
idea; the areas of low density in the 10-mile layer
correspond w ith dow nw ard projections of the light
continental crust, w hile those of high density repre­
sent the intervening areas filled with material of the
heavier understratum ’ (S.W . W ooldridge and R.S.
M organ, 1959). (Fig. 7.7).
A. Holmes and D.L. H olm es (1978) have
tried to explain and illustrate the concept o f isostasy
through a diagram (fig. 7.9) w hich show s ch aracter­
istic exam ples of crustal colum ns, each o f w hich has
the same area and extends dow nw ard to the sam e
depth below sea-level, the sam e depth at w hich the
weight of each colum n exerts approxim ately the
pressure on the underlying m aterial, irrespective o f
its surface elevation’ (A. H olm es and D .L. H olm es,
1978, P. 21). They have taken the depth o f 50 km for
isostatic com pensation in those areas w hich have not
been disturbed by geological events fo r fairly longer
duration. A Holmes and D.L. H olm es have attem pted
to explain and illustrate the concept o f equal w eight
along the ‘level o f e q u a l p ressu re’ through the
exam ples o f 4 colum ns of equal cross-section through
characteristic parts of the continents and ocean floor
(fig. 7.9). T hese four colum ns are (i) p lateau, 4 km
high ; (ii) plateau, 1 km hig h ; (iii) p lain at sea level
and (iv) ocean, 5 km deep. Each colum n has a
thickness of 50 km. The figures to the right o f each
colum n d en o te d en sity (a v e ra g e ). M indicates
M ohorovicic D iscontinuity. T he w eight o f each co l­
umn along the level o f equal pressure is alm ost the
sam e, ranging betw een 150.00 to 151.2. According
to H olm es and H olm es the total w eight o f each
colum n along the level o f equal p ressure can be
obtained by sum m ing up the product o f the density
and corresponding thickness dow n to the depth o f 50
km as given below .
/ *.*.• * Uniform Density Zone16 kilom etre Compensation Zone
Fig. 7.7:
M ountain
Compensation zone o f 10-mile thickness (af­
ter Joly). Finer dots indicate lighter materials
while larger dots represent denser materials.
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7.7 THE CONCEPT OF HOLMES
The view s o f A rthur H olm es on isostasy, to a
greater extent, are com patible with the views of
Airy. Follow ing A iry H olm es has also assum ed that
upstanding crustal parts are m ade o f lighter m ateri­
als and in order to balance them m ajor portions o f
these higher colum ns are subm erged in greater depth
of lighter m aterials (o f very low density). A ccording
to Holmes the higher colum ns are standing because
of the fact that there is lighter m aterial below them
for greater depth w hereas there is lighter material
below the sm aller colum ns upto lesser depth (fig.
7.8).
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GEOMORPHOLOGY
138
adjustment does not occur at local level, it does exist
at extensive regional level. It is necessary that th a t
must be balance at local level, it maybe and it may
not b e. The endogenetic forces and resultant tectonic
events cause disturbances in the ideal condition of
isostasy but nature always tends towards the isostatic
adjustment.
(i) For the plateau (4 km high from sea level
(fig. 7.9 A V 5 4 x 2.8 (average density) = 151.2
(the whole section is continental crust)
(ii) For the plateau (1 km high) (fig. 7.9B)- 36
x 2.8 (continental crust) + 15.33 (m antle sima,
probably basaltic rock) = 150.3
(iii) For the plain near the sea level (fig. 7.9 C)-
For exam ple, a new ly form ed m ountain due to
tectonic activities is subjected to severe denudation.
C onsequently, there is continuous low ering o f the
height o f the m ountain. On the other hand, eroded
sedim ents are deposited in the oceanic areas, with
the result there is continuous increase o f weight of
sedim ents on the sea-floor. Due to this mechanism
the m ountainous area gradually becom es lighter and
the oceanic floor becom es heavier, and thus the state
o f balance or isostasy betw een these two areas gets
disturbed but the balance has to be maintained. It
may be stated that the superincum bent pressure and
weight over the m ountain decreases because of con­
tinuous removal o f m aterial through denudational
processes. This m echanism leads to gradual rise in
the mountain. On the other hand, continuous sedi­
mentation on the sea-floor causes gradual subsid­
ence o f the sea-floor. Thus, in order to maintain
isostatic balance between these two features there
30 x 2.8 (continental crust) + 20 x 3.3 (mantle
sima) = 150.0
(iv) For the ocean (5 km deep. fig. 7.9 D)5 x 1.03 (sea water) + 1 x 2 . 4 (sediments) + 5
x 2.9 (crustal sima, probably basaltic rock) + 39 x 3.3
(mantle sima)
f = 150.75.
7.8 G LO B A L ISO STATIC ADJUSTMENT
It may be pointed out that there is no com plete
isostatic adjustment over the globe because the earth
is so unresting and thus geological forces (endogenetic
forces) com ing from within the earth very often
disturb such isostatic adjustment. M oreover, recently
a few scientists have even questioned the concept of
isostasy. Even there is disagreement among the
scientists about local or regional nature o f isostasy.
It appears from the result of various expeditions,
experiments and observations that if the isostatic
Thickness
A
Ploteou
k km high
B
Ploteou
1 km high
Ploin neor
sea level
Oceon
5km deep
27
Av
28
5
1
4
Seo level
1 03
2 U
2 9 . m — 10
—
2-9
M
39
33
20
— 30
— 1*0
33
Hi
/Vpprox
Level of
Equol Pressure
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Fig. 7.9: Columns o f equal cross section through characteristic parts o f the continents and ocean floor. White portion
(unshaded) denotes continental crust while larger dots represent mantle sima. Broken line shows sea **tur>
dense tiny dots reveal sediments and sparse tiny dots indicate crustal sima. probably basaltic rock. After A- I
Holmes and D.L. Holmes, 1978.
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t h e o r y o f ’is o s t a s y
139
must be slow flow age o f relatively heavier materials
of substratum (from beneath the seafloor) towards
the lighter m aterials o f the rising column o f the
mountain at or below the level o f compensation (fig.
7.10). Thus, the process o f redistribution of materi­
als ultim ately restores the disturbed isostatic condi­
tion to com plete isostatic balance. Commenting on
the validity o f the above mechanism of the isostatic
adjustm ent, W ooldridge and M organ (1959) have
remarked, “that som e such mechanism operates is
indeed very likely ; geologists have irrefutable evi­
dence that sedim ents can depress the floor of a
loaded sea to a lim ited extent, and some species of
sub-crustal flow has been invoked on many other
grounds. But clearly we are not justified in regarding
the crust as com posed of columns, moving up and
down independently ; such a conception flouts the
facts of observation, and even it did not, it would, on
the geological side, create many more problems than
it solved’ (S.W. W ooldridge and R.S. Morgan, 1959,
p. 26).
Denunciation of M ountain Range
Fig. 7.10 : Mechanism o f isostatic adjustment at global
scale (based on A. Holmes).
example, extensive parts o f North A m erica and
Eurasia were subsided under the enorm ous w eight o f
accumulation o f thick ice sheets during Pleistocene
glaciation but the landmasses began to rise suddenly
because of release of pressure o f superincum bent
thick load of ice sheets due to deglciation and conse­
quent melting of ice sheets about 25,000 years ago
and thus the isostatic balance was disturbed. A c­
cording to an estimate major parts o f Scandinavia
and Finland have risen by 900 feet. The land masses
are still rising at the rate of one foot per 28 years
under the process of isostatic recovery. The isostatic
adjustment in these areas could not be achieved till
now.
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Some times the endogenetic forces act so
suddenly and violently that the state of isostatic
balance is thrown out o f gear all of sudden and hence
the isostatic adjustment through the process of flowage
o f materials from the substratum is not maintained.
Similarly, some times climatic changes occur at
such an extensive global scale that there is accumu­
lation of thick ice sheets on the land surface and thus
increased burden causes isostatic disturbance. For
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ROCKS
140-157
I n tr o d u c tio n ; c la s s if i c a t io n o f r o c k s ; i g n e o u s r o c k s ; s e d i m e n t a r y r o c k s ;
m e tm o r p h ic r o c k s .
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CHAPTER 8
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9
8
ROCKS
am m onium , table 8.1) co n stitu te 99 p e r c e n t o f the
total m ass o f the earth w h ereas o n ly fo u r elem en ts
(iron, oxygen, silicon and m a g n esiu m ) a c c o u n t for
90 per cent o f total m ass o f the earth. O n the o th er
hand, the eight m o st ab u n d an t elem en ts w h ich co n ­
stitute 99 per cent o f total m ass o f the c ru s t are
oxygen, silicon, alum inium , iron, m ag n esiu m , c a l­
cium, potassium and sodium (tab le 8. 1 ).
8.1 INTRODUCTION
The m aterials o f the crust or lithosphere are
generally called as rocks. T he word lithosphere, in
fact, m eans ‘r o c k s p h e re ’ as the literal m eaning o f
‘lith o s’ is rock. The sm allest com ponent o f the crust
or the lithosphere is elem ent. As regards the whole
earth eight m ost abundant elem ents (iron, oxygen,
silicon, m agnesium , nickel, sulphur, calcium and
Table 8.1 : Important Elements of the Whole Earth and the Crust
Earth's C rust
W hole Earth
Elements
Percentage
Elem ents
P ercen tag e
1.. Iron
35
1. Oxygen
46
2.
Oxygen
30
2. Silicon
28
3.
Silicon
15
3. A lum inium
8
4.
M agnesium
13
4. Iron
6
5.
Nickel
2.4
5. M agnesium
4
6. Sulphur
1.9
6. C alcium
2.4
7.
Calcium
1.1
7. Potassium
2.3
8. A lum inium
1.1
8. Sodium
2 .1
1.0
M ore than one elem ent o f the earth's crust are
organized to form com pounds w hich are know n as
m inerals and m inerals are organized to form rocks.
The im portant m ineral groups are silicates, carbon-,
ates, sulphides, m etal oxide etc.
O thers, less than
1.0
(1)
T he silica te m in era ls are very im portant
rock m aking m inerals. T he m o st o u tstan d in g rockform ing silicate m ineral g ro u p s are q u artz, feldspar,
and ferrom agnesium . Q u artz is co m p o sed o f two
elem ents viz. sillicon and o x y g en and is generally a
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Others, less than
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rocks
141
hard and re sista n t m in e ra l. T h e m o st ab u n d an t and
most im p o rtan t ro c k fo rm in g silicate m ineral is
feldspar w h ich is a lso very im p o rta n t eco n o m ically
because it is u sed in c e ra m ic s and g lass industry.
F eldspar is very w e a k m in e ra l and is easily broken
down and d e c o m p o se d d u e to ch em ical w eathering
and is c h a n g e d in to c la y s as h y d rate d alum ino
silicates. W h en silic o n an d o x y g en co m bine w ith
iron and m a g n e siu m , ferro m a g n esiu m m inerals are
form ed. F e rro m a g n e siu m m in erals are easily w eath ­
ered and e ro d e d aw ay and are easily altered and
rem oved. T he rock s having abundant ferrom agnesium
m inerals p ro v id e w eak stru ctu re fo r the construction
of b u ild in g s, ro a d s , d a m s, re serv o irs, tunnels etc.
books o f earth history and fo ssils are the p a g e s’.
S. W . W o o ld rid g e and R .S. M o rg an (1 9 5 9 ) have
aptly rem arked, ‘R ocks w h eth er igneous or sedi­
m entary, co n stitu te on the one hand the m anuscripts
o f the past earth history, on the other, th e basis for
co ntem porary sc en ery .’
8.2 CLASSIFICATION OF ROCKS
The crustal rocks are classified on several
grounds e.g. m ode o f form ation, physical and chem ical
properties, locations etc.
(2) C a r b o n a te grou p o f m in erals is very
m uch su c c e p tib le to ch em ical w eathering and ero ­
sion in h u m id areas. C alcite is the m ost im portant
m ineral o f th is group. L im esto n es and m arbles hav­
ing a b u n d a n t calcite are co rro d ed by the surface and
g ro u n d w ater and ex ten siv e caves are form ed below
the gro u n d surface. S u ch areas provide very w eak
structures for construction sites e.g. construction of
buildings, roads, dam s, reservoirs, air-strips, tunnels etc.
(3) S u lp h id e m in era ls include pyrites, iron
su lp h id es etc. W hen these m inerals com e in contact
w ith w ater o r air, th ese form ferric hydroxides and
sulfuric acids w hich cau se serious environm ental
problem s.
(4) M eta llic elem en ts like iron, alum inium
etc. after re ac tin g w ith atm ospheric oxygen form
m etal oxides w h ich are co m m ercially very im por­
tant.
(i) Ign eou s rocks, form ed due to coolin g,
solidification and cry stalization o f m o lten earth m a­
terials know n as m agm a (below th e earth 's surface)
and lava (on the earth's surface), e.g. b asalt, granites
etc.
(ii) S ed im en tary rock s, fo rm ed th ro u g h the
lithification and com pression and cem en tatio n o f the
sedim ents deposited in a p articu la r p lace m ainly
aquatic areas, e.g. sandstones, lim esto nes, co n g lo m ­
erates etc.
(iii) M etam orp h ic ro ck s, fo rm ed due to
change either in the form or co m p o sitio n o f either
igneous or sedim entary rocks pro v id ed that there is
no disintegration o f p re-existing rocks, e.g. slate,
quartzite, m arble etc.
8.3 IGNEOUS ROCKS
The w ord igneous has been derived from a
Latin Word ‘ig n is’, m eaning there by fire. It does not
m ean that the origin o f igneous rocks is associated
w ith fire in any way. In fact, the igneous rocks are
form ed due to cooling, solid ificatio n and crystaliza­
tion o f hot and m olten m aterials know n as magmas
and lavas. Since the m agm as and lavas are so hot that
they look like red pieces o f fire but this is not the
case. Igneous rocks are also called as prim ary rocks
because these w ere originated first o f all the rocks
d uring the form ation o f upper crust o f the earth on
cooling, solidification and crystallization o f hot and
liquid m agm as after the origin o f the earth. Thus, all
the subsequent rocks w ere form ed, w hether directly
or indirectly, from the igneous rocks in one way or
the other. T his is why ig n eo u s rocks are also called
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R ocks, th u s, rep resen tin g the geom aterials o f
the earth's crust, are co m p o sed o f tw o or m ore
m inerals. R o ck s play very im portant role in d eter­
m ining the c h ara cteristic features o f several types of
erosional lan d fo rm s b ecau se the nature and m ag n i­
tude o f ero sio n largely d ep en d s on the structure and
com position o f rocks. T h e fu ndam ental dictum o f
fam ous A m erican g e o m o rp h o lo g is t W .M . D avis that
‘the landscape is a fu n ctio n o f stru ctu re, process and
tim e (stag es)’ lays m o re em p h asis on the dom inant
role o f rocks in the ev o lu tio n o f landform s. A cco rd ­
ing to A .K . L o b eck ‘a rock sh o u ld be conceived as a
product o f its en v iro n m en t. W hen the environm ent
is changed, the rock c h a n g e s’. R ocks are also very
helpful in dating the age o f the earth as ‘rocks are the
C lassification on th e b asis o f m o d e o f fo r ­
m ation — T he rocks are d iv id ed into th ree broad
categories on the basis o f th eir m o d e (m eth o d ) o f
form ation.
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GEOMORPHOLOGY
142
ering and thus the ro ck s are easily d isin tegrated and
decom posed.
as p a re n t ro ck s. It is believed that the igneous rocks
were formed during each period o f the geological
history of the earth and these are still being form ed.
(5) Igneous rocks d o not co n tain fossils be­
cause (i) w hen the an cien t igneous ro ck s w ere formed
due to cooling and so lid ificatio n o f m olten rock
m aterials at the tim e o f the o rigin o f the earth, there
was no life on new ly born earth and (ii) since the
igneous rocks are form ed due to co o lin g and solidi­
fication o f very hot and m olten m aterials and hence
any rem ains o f plants or an im als (fo ssils) are de­
stroyed because o f very high tem p eratu re.
Characteristics of Igneous Rocks
(1) In all, igneous rocks are roughly hard
rocks and water percolates with great difficulty
along the joints. Some tim es the rocks becom e so
soft, due to their exposure to environm ental co n d i­
tions for longer duration, that they can be easily dug
out by a spade (e.g. basalt).
(2) Igneous rocks are granular or crystalline
rocks but there are much variations in the size, form
and texture of grains because these properties largely
depend upon the rate and place of cooling and
solidification o f m agmas or lavas. For exam ple,
when the lavas are quickly cooled down and solidi­
fied at the surface o f the earth, there is no sufficient
time for the developm ent of grains/crystals. C onse­
quently, either there are no crystals in the resultant
basaltic rocks or if there are some crystals at all, they
are so minute that they cannot be seen without the
help of a microscope. Contrary to this, if magmas are
cooled and solidified at a very slow rate inside the
earth, there is sufficient time for the full develop­
m ent of grains, and thus the resultant igneous rocks
are characterized by coarse grains.
(6) The num ber o f jo in ts in creases upw ard in
any igneous rock. The jo in ts are fo rm ed due to (i)
cooling and contraction, (ii) ex p an sio n and contrac­
tion during m echanical w eathering, (iii) decrease in
superincum bent load due to rem oval o f m aterials
through denudational processes and (i v) earth m ove­
ment caused by isostatic d isturbances. W henever
these joints are plugged by m inerals, the rocks be­
come quite hard and resistant to w eathering and
erosion.
(7) Igneous rocks are m ostly asso ciated with
the volcanic activities and thus they are also called as
volcanic rocks. Igneous rocks are generally found in
the volcanic zones.
Classification of Igneous R ocks
(3) Igneous rocks do not have strata like
sedimentary rocks. W hen lava flows in a region
occur in several phases, layers after layers of lavas
are deposited and solidified one upon another and
thus there is some sort of confusion about the layers
or strata but actually these are no strata rather these
are layers of lavas. Such examples may be seen
anywhere in the W estern Ghats where several lava
flows during Cretaceous period resulted into the
formation of thick basaltic cover having numerous
layers of lavas of varying compositions. One can see
such lava layers near Khandala or along the deeply
enterenched valleys of the Koyna river, the Krishna
riv er, the S arasw ati riv er etc. in and around
M ahabaleshw ar plateau.
There are vast variations in the igneous rocks
in terms of chem ical and m ineralogical characteris­
tics, texture of grains, form s and size o f grains, m ode
of origin etc. Thus, the igneous rocks are classified
on several grounds in a variety of w ays as follow s—
(1) The m ost traditional m ethod o f the classi­
fication of the igneous rocks is based on the am ount
of silica ( S i0 2). Thus, the igneous rocks are divided
into two broad categories e.g. (i) acidic igneous
rocks having more silica, e.g. granites, and (ii) basic
igneous rocks having low er am ount o f silica, e.g.
gabbro. It may be pointed out that silica content is
not a m easure of acidity.
(2) On the basis o f the chem istry and minera­
logical com position (light and dark m inerals) the
igneous rocks are classified into two dom inant groups
e.g. (i) felsic igneous rocks com posed o f the domi­
nant m inerals of the light group such as quartz and
feldspar having rich content o f silica. The word
‘felsic’ has been derived from fell(s), feldspar plus
ic, m eaning thereby the dom inance o f feldspar min-
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(4) Since water does not penetrate the rocks
easily and hence igneous rocks are less affected by
chem ical w eathering but basalts are very easily
weathered and eroded away when they come in
constant touch with water. Coarse grained igneous
rocks are affected by mechanical or physical weath­
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143
rocks
cral, (ii) m afic ign eou s rocks com posed o f the
dominant m ineral o f dark group such as pyroxenes,
amphiboles and olivines, all o f w hich have rich
contents o f m agnesium and iron. The word ‘m afic’
has been derived from m agnesium and f (ferrous) for
iron and ic m eaning thereby the dom inance o f m ag­
nesium and ferrous (iron) and (iii) ultram afic ign e­
ous rocks are characterised by the abundance of
pyroxenes and olivine minerals, examples, periodotite
(rich in pyroxene and olivine), dunite (rich in olivine)
etc.
into two m ajor groups o f plutonic intrusive igneous
rocks and hypabyssal intrusive igneous rocks on the
basis o f the depth o f the place o f cooling o f magmas
from the earth's surface. W hen the m agm as are
cooled and solidified very deep w ithin the earth, the
resultant rocks becom e plutonic but w hen the m ag­
mas are cooled ju st below the earth's surface, the
rocks are called as hypabyssal igneous rocks.
(i) Plutonic igneous rocks are formed due to
cooling o f m agm as very deep inside the earth. Since
the rate o f cooling o f m agm as is exceedingly slow
because o f high tem perature prevailing there and
(3)
T he igneous rocks are also classified on
the basis o f texture o f grains into 5 m ajor groups—
hence there is sufficient tim e lo r the full develop­
ment of large grains. Thus, the plutonic igneous
(i) P egm atitic igneous rocks (very coarse­
rocks are very coarse-grained (pegm atites) rocks.
grained igneous rocks) include very large crystals
Granite is best representative exam ple o f this cat­
several m etres across. Exam ples, granites.
egory.
(ii) P haneritic igneous rocks (coarse grained
(ii) H ypabyssal igneous rocks are formed
igneous rocks). The word phaneritic has been de­
due to cooling and solidification o f rising m agm a
rived from G reek w ord ‘phanero’, meaning thereby
during volcanic activity in th e cracks, pores, crev ­
visible.
ices, and hollow places ju st beneath the earth ’s
(iii) A phanitic igneous rocks (fine grained
surface, the resultant rocks are called as hypabyssal
igneous rocks). The word aphanitic has been derived
igneous rocks. The m agm as are solidified in differ­
from the G reek word, ‘aph an ’, meaning thereby
ent forms depending upon the hollow places such as
invisible, that is the grains of the aphanites are so
batholiths, loccoliths, phacoliths, lopoliths, sills,
m inute that they cannot be seen by bare eyes.
dikes etc. It should be rem em bered that these should
(iv) G lassy igneous rocks (w ithout grains of
not be taken as the types o f igneous rocks because
any size).
these are different shapes of solidified m agm as.
(v) Porphyritic igneous rocks (mix-grained
(A)
Batholiths are long, irregular and undu­
igneous rocks).
lating forms of solidified intruded m agmas. They are
usually dom e-shaped and their side walls are very
(4)
The igneous rocks are more commonly
steep, almost vertical. The upper portion of batholiths
classified on the basis o f the m ode of occurrence into
are seen when the superincum bent cover is rem oved
two m ajor groups.
due to continued denudation but their bases are
(i) Intrusive Igneous Rocks
(a) Plutonic igneous rocks
(b) Hypabyssal igneous rocks
(ii) E xtrusive Igneous Rocks
(a) Explosive type
(b) Q uiet type
1. Intrusive Igneous Rocks
W hen the rising m agm as during a volcanic
activity do not reach the earth s surface rather they
are cooled and solidified below the surface of the
earth, the resultant igneous rocks are called intrusive
igneous rocks. T hese rocks are further subdivided
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Fig. 8.1 : D iagram atic presentation o f a granitic
batholith.
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GEOMORPHOLOGY
144
never seen because they are buried deep w ithin the
earth. W hen exposed to the surface they are sub­
IMiucolitli
Anticline
jected to intense w eathering and erosion and hence
their surfaces becom e highly irregular and corru­
gated. N um erous batholithic dom es w ere intruded in
the the D harw arian sedim entaries in m any parts of
the peninsular India during pre-C am brian period.
S y n c lin e
M any o f such batholithic dom es have now been
Fig. 8.3 : An example o f phacoliths.
exposed well above the surface in m any parts o f the
(D) L op olith s— The w ord lopolith has been
derived from G erm an w ord ‘lo p a s’ m ean ing thereby
a shallow basin or bow l shape body. W hen m agm a
B atholiths. M urha pahar near Pithauriya village, to
is injected and solidifed in a co n cav e sh allow basin
the north-w est o f Ranchi city, is a typical exam ple o f
w hose central part is sagged d o w nw ard, the resultant
exposed Ranchi batholithic dom es.
form o f solidified m agm a is called a lopolith. The
(B)
L accoliths— The w ord laccolith has been
rocks of lopoliths are generally co arse-g rain ed be­
derived from German word, ia c c o s ’ m eaning thereby
cause o f slow process o f co o lin g o f m agm as.
‘lith o s’ or rocks. Laccoliths are form ed due to injec­
(E ) Sills— The w ord ‘s ills ’ has been derived
tion (intrusion) o f m agm as along the bedding planes
from an A nglo-Saxon w ord ‘sy F m eaning thereby a
ledge. The sills are usually parallel to the bedding
planes o f sedim entary rocks. In fact, sills are formed
due to injection and solidification o f m agm as be­
tween the bedding planes o f sendim entary rocks.
Thick beds o f m agm as are called sills w hereas thin
beds of m agm a are term ed as ‘s h e e ts ’. The thickness
o f sills ranges betw een a few centim etres to several
m etres. W hen sills are tilted to g eth er w ith the
sendim entary beds due to earth m ovem ents and are
exposed to exogenous denudational processes, they
form significant landform s like cuesta, hogbacks
Fig. 8.2 : Diagramatic illustration o f a typical laccolith.
and ridges (fig. 8.4).
C hotanagpur plateau o f India m ainly Ranchi plateau
w here such batholithic dom es are called as R anchi
o f horizontally bedded sendimentary rocks. Laccoliths
are o f m ushroom shape having convex sum m ital
form . T he ascending gases during a volcanic eru p ­
tion force the upper starta o f the flat layered sedi­
m entary rocks to arch up in the form o f a convex arch
o r a dom e. C onsequently, the gap between the arched
up or dom ed upper starta and the horizontal low er
starta is injected w ith m agm a and other volcanic
m aterials (fig. 8.2 ).
(C )
Phacoliths are form ed due to injection o f
m agm a along the anticlines and synclines in the
Intrusion of Lavu
Fig. 8.4 : Intrusion o f sills between the horizontal bed­
ding planes o f sedimentary rocks.
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regions o f folded m ountains (fig. 8.3).
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rocks
145
(F)
D y k es rep resen t w all-like form ation
s o lid i f y m agm as. T h ese are m ostly perpendicular
to the beds o f se d im e n ta ry ro ck s. T he th ic k ­
ness o f dykes ran g es from a few centim etres to
R esistant
E ro sio n
of
when filled up with w ater, is called a ‘d y k e la k e ’
(fig. 8 .5 ); (ii) If the rocks o f dykes are more resistant
than the country-rocks, upstanding ridges and hills
are formed because o f m ore erosion o f the countryrocks (fig. 8.6) and (iii) If the rocks o f dykes and
country-rocks are of uniform resistance, both are
uniformly dissected and hence no significant landform
is developed but the height is gradually reduced (fig.
8.7).
2. Extrusive Igneous rocks
The igneous rocks form ed due to co oling and
solidification of hot and m olten lavas at the earth’s
surface are called extrusive igneous rock s. Gener­
ally, extrusive igneous rocks are form ed during
fissure eruption o f volcanoes resulting into flood
b asalts. These rocks are also called as voican ic
Fig. 8.5 : The resultant feature on dyke after erosion
(the rocks o f dykes being less resistant than
the surrounding country-rocks).
several h u n d r e d m e tr e s but the length extends from
a few m e tre s to se v era l k ilom etres. A well defined
dyke is o b s e rv a b le a c ro ss the palaeochannel and
valley o f the N a r m a d a riv e r n ear D hu n w a d h ar Falls
(B heraghat) n e a r J a b a lp u r city. T h e relative resist­
ance o f d y k e s in c o m p a ris o n to the surrounding
c o u n tr y - r o c k s g iv e s b irth to a few in te resting
landform s e.g. (1) I f the ro ck s o f dykes are w eaker
and less r e s is ta n t th an the country rocks, the upper
portion o f d y k e s is m o re ero d e d than the countryrocks, w ith the resu lt a de p re ssio n is formed, which,
Fig. 8.6 : Form o f a dyke after erosion (when the rocks
o f dyke are more resistant than the country rocks).
Erosion
Fig. 8 .7 :
Form o f a dyke after erosion {when the rocks o f dyke and country-rocks are o f uniform resistance).
rocks; E xtrusive igneous rocks are generally finebasalts because lavas after com ing
over the earth's surface are quickly cooled and soo r
lidified due to com paratively extrem ely low ternperature of the atm osphere and thus there is no
enough time lor the developm ent o f grains or
g l a s s y
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g r a i n e d
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146
GEOMORPHOLOGY
crystals. B asalt is the m ost sig n ifican t rep resen tativ e
ex am p le o f ex tru siv e igneous rocks. G ab b ro and
obsidian are the o th e r im p o rtan t ex am p les o f this
group. E xtrusive igneous ro ck s are fu rth er divided
into tw o m a jo r su b catc g o ries on the basis o f the
nature o f the ap p earan ce o f lavas on the earth's
surface e.g. (i) ex p lo siv e ty p e and (ii) q u iet type.
(i) A cid ign eou s rocks are those which carry
silica co ntent betw een 65 to 85 per cent. T he average
density varies from 2.75 to 2.8. Q uartz and white and
pink feldspar are the do m in an t m inerals. Acid igne­
ous rocks generally lack in iron and m agnesium . On
an average acid igneous rocks are hard and relatively
resistant to erosion. G ranite is the m ost significant
exam ple o f this group o f rocks. T hese rocks are light
in w eight and are used as building m aterials because
o f their less erosivity.
(i) E x p lo s iv e T y p e — T h e ig n e o u s ro ck s
form ed due to m ix tu re o f volcan ic m aterials ejected
d u rin g ex p lo siv e ty p e o f v io len t volcanic eruptions
are called expolosi ve type o f extrusive igenous rocks.
V olcanic m aterials include ‘b o m b s’ (big fragm ents
o f ro ck s), i a p i l l i ’ (fragm ents o f the size o f a peas)
and volcanic dusts and ashes. Fine volcanic m ateri­
als, when d ep o sited in aquatic co ndition, are called
‘tu ffs ’. T he m ixture o f larg er and sm aller particles
after deposition is called ‘b r e c c ia ’ or ‘ag g lo m er­
a te .’ T hese are m ore susceptible to erosion because
these are not w ell con solidated.
(ii) B asic ign eou s rock s contain silica content
betw een 45 to 60 per cent. T h eir average density
ranges from 2.8 to 3.0. Such igneous rocks are dom i­
nated by ferro-m agnesium m inerals. T here is very
low am ount o f feldspar. T he rock is heavy in weight
and dark in colour because o f the dom inance o f iron
content. Basic igneous rocks are easily eroded away
when these com e in regular co n tact with water. These
rocks are fine-grained igenous rocks. B asalt, gabbro,
dolerite etc. are the typical exam ples o f this group.
(ii) Q u iet T yp e— T h e ap pearance o f lavas
th rough m inor cracks and o penings on the earth's
surface is called ‘lava flo w ’. T hese lavas after being
cooled and solidified form basaltic igneous rocks.
F lood basalts resulting from several episodes o f lava
flow d u ring fissure flow s o f volcanic eruption form
ex ten siv e ‘la v a -p la te a u ’ and ‘la v a -p la in s’ w herein
several layers o f basalts are deposited one upon
another.
T he thickness o f lavas o f the C olum bia pla­
teau o f the states o f W ashington and O regon (U SA ),
spread over an area o f about 6 .4 5 ,0 0 0 km 2 (2,50,000
square m iles), m easures ab o u t 1,216 m (4,000 feet).
T he ex ten siv e lava flow s during C retaceous period
co vered an area o f about 7 ,7 4 ,0 0 0 km 2 (3,00,000
sq uare m iles) o f P en in su lar India. Several beds o f
basaltic lavas are clearly observable all along the
exposed sectio n s o f the W estern G hats m ainly near
K handala (betw een B om bay and Pune) and over
M ah ab alesh w ar plateau.
Classification of Igneous Rocks on the Basis of
Chemical Composition
(iv) U ltra-basic ign eou s rocks carry silica
content less than 45 per cent but their average density
varies from 2.8 to 3.4. P eridotite is the typical exam­
ple o f this group o f rocks.
Classification of Igneous Rocks on the Basis of
The Texture of Grains
The texture o f the cry stals (grains) o f igneous
rocks depends on 3 basic factors viz. (i) source
region o f the origin o f m ag m as and lavas and places
of their cooling and so lid ificatio n ; (ii) rate o f cool­
ing and so lidification o f m ag m as and lavas and (iii)
quantity o f w ater and gases (vapour) w ith hot and
m olten m agm as and lavas. If m ag m as and lavas are
cooled slow ly and g radually the g rains are well
developed but if they are co o led and solidified at a
very faster rate, grains are not w ell developed. The
rate o f co o lin g o f m agm as and lavas also depends
upon several factors viz. (i) W hen magmas are
cooled deep w ithin the earth, the rate o f cooling is
exceedingly slow because o f very high temperature
prevailing there and hence very large and coarse
grains are form ed, (ii) If lavas are cooled at the
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T hough the chem ical com position o f igneous
rocks varies significantly from one group to another
group but each type o f igneous rock contains som e
am ount o f silica. T hus, on the basis o f silica content,
igneous rocks are divided into the follow ing four
types.
(iii) Iterm ediate ign eou s rocks are those in
which silica content is less than the am ount present in
the acid igneous rocks but m ore than the basic igne­
ous rocks. The average density ranges betw een 2.75
and 2.8. D iorite and andesite are the representative
exam ples o f this group o f rocks.
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147
rocks
rocks and (6) fragm ental igneous rock s (consisting
.o f bom bs, lapilli, breccia, volcanic dusts and ashes,
surface o f the earth, the rate o f cooling is very fast
because o f very low tem perature (in com parison to
the tem perature of lavas) of outside environm ent
and hence either grains are not formed at all, or if
they are form ed, they are so minute that they cannot
be seen w ithout the aid o f a m icroscope, (iii) If
magmas and lavas are associated with larger propor­
tion o f w ater vapour and gases, the rate o f their
cooling and consequent solidification is slowed down
and hence larger grains are formed.
tuffs etc.).
It is desirable to discuss in b rief the major
characteristics o f granites and basalts w hich repre­
sent the intrusive and extrusive igneous rocks re­
spectively.
Granites
G ranites are the m ost significant exam ple o f
the plutonic intrusive igneous rocks w hich are form ed
deep within the earth. Since the rate o f cooling and
solidification of m agm as inside the earth is very
slow because of very high tem perature prevailing
underground and hence granites becom e co arse­
grained due to full developm ent o f large-sized grains.
Granites are com posed essentially o f the m inerals o f
quartz, feldspar, and m ica but the m ost abundant
mineral is feldspar, mainly orthoclase. Som e tim es,
the minerals are uniform ly distributed and all o f
them are almost o f the sam e size. B esides, albite,
biotite, m uscovite and hornblende are also found in
granite rocks.
On the basis of the size o f grains (texture)
igneous rocks are generally divided into (i) coarse­
grained igneous rocks (plutonic igneous’rocks come
under this category, grainite is the exam ple), (ii)
fine-grained igneous rocks (extrusive igneous rocks
fall under this group, basalt is the exam ple) and (iii)
m edium -grained igneous rocks (hypabyssal rocks
are generally m edium grained igenous rocks).
A lternatively, igneous rocks are divided into
six sub-types on the basis of textural characteristics
o f the rocks e.g. ( 1 ) pegm atitic igenous rocks (or
very coarse-grained igneous rocks ; examples : plu­
tonic igneous rocks e.g. pegmatitic granites, pegmatitic
diorite, pegm atitic sym te e tc .) ; (2 ) phaneritic igne­
ous rocks (or coarse-grained igneous rocks ; plu­
tonic igenous rocks ; exam ples, granites, diorties
e tc .) ; (3 ) aphanitic igneous rocks (or fine-grained
igneous rocks ; grins are so minute that they cannot
be seen w ithout the help o f a m icroscope ; examples:
basalt, felsite and the rocks of sills and dykes) ; (4)
glassy igenous rocks (or grainless igneous rocks;
usually there is general absence of grains; examples:
pitch stones, obsidians, pumice, perlite etc.) ; (5)
porphyritic igneous rocks (or mix-grained igneous
The granite family includes num erous types
of rocks. These granitic rocks are differentiated on
the basis o f their texture and m ineral com position,
for example, hornblende granite (w hen hornblende
mineral is most dom inant), rhyolite granite, pum ice
granite, absidian granite, pitch-stone granite etc.
From the standpoint o f chem ical com position gran­
ites are acidic rocks wherein silica content ranges
between 65 to 85 per cent. G ranites are generally
light in weight as their density varies from 2.75 to
2.8. Table 8.2 denotes percentage com position o f
different minerals in granites.
Table 2 : Mineral Composition of Granites
Minerals
Percentage
Feldspar
52.3
Quartz
31.3
Mica
11.5
Hornblende
2.4
Iron
2.0
others
0.55
Granites are generally resistant to erosion but when
the rocks are well jointed, they are easily weathered
and a very peculiar landform , ‘t o r \ is formed.
There is also wide range of colour variation in
different types of granites. The colour variation is
caused mainly because o f the number ol different
minerals present in the rocks and the size of grains.
Generally, granites are of light colour but if orthoclase
mineral is present in abundance, the granites become
pink to yellow or slightly reddish in colour. If dark
coloured hornblende or biotitc is a dominant mineral,
the granites bccomc of dark black or dark grey colour.
Basalts
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Basalt is a very fine-grained, dark-coloured
extrusive igneous rock w hich is form ed due to co o l­
ing and solidification o f m olten lavas at the surface
o f the earth. Some tim es, the cooling o f lavas takes
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GEOMORPHOLOG'
148
and cones. T he fo llo w in g are th e m ain characteris­
tics o f sed im en tary rocks.
place so rapidly that no tim e is available for the
crystallization o f basalt and hence no grains are
form ed, with the result the rock becom es glassy
b a s a lt. B asalts having grains, though very sm all
rather m inute, are called ap hanitic basalts. C hem i­
cally, basalts contain 45 to 65 per cent o f silica
content. T hough the rock is heavy in w eight but is
m ore susceptible to chem ical w eathering and fluvial
erosion. T he dark colour o f basalts is because o f the
abundance o f iron. F eldslpar is the m ost dom inant
m ineral (46.2 per cent). B esides, augite (36.9 per
cent), olivine (7.6 per cent), m ineral iron (9.5 per
cent) etc. (others - 2.4 per cent) are other constituent
m inerals o f basalts. Som e tim es, polygonal cracks
are developed in basalts due to contraction on co o l­
ing o f lavas. C olum nar jointings in basalts give birth
to peculiar landform s characterized by uneven ter­
rain surfaces.
(1) Sedim entary rocks are form ed o f sediments
derived from the o ld er ro ck s, p lan t an d anim al re­
m ains and thus these ro ck s co n tain fo ssils o f plants
and anim als. T he age o f the fo rm atio n o f a given
sedim entary rock m ay be d eterm in e d on the basis of
the analysis o f the fossils to be found in th at rock.
(2) S ed im en tary ro ck s are fo u n d o ver the
largest surface areas o f the g lo b e. It is b eliev ed that
about 75 p er cent o f the su rface area o f th e globe is
covered by sedim entary ro ck s w h ereas ig n eo u s and
m atam orphic rocks co v er the rem a in in g 25 per cent
area. Inspite o f th eir larg est co v era g e the sedim en­
tary rocks co n stitu te only 5 p e r c e n t o f th e com posi­
tion o f the crust w h ereas 95 p e r c e n t o f the crust is
com posed o f igneous and m e tam o rp h ic ro ck s. Thus,
it is obvious that ‘the sed im en tary ro ck s are im por­
tant for extent, not for dep th in the ea rth 's c ru s t.’
8.4 SED IM EN TA R Y R O C K S
(3) T he d ep osition o f sed im en ts o f various
types and sizes to form sed im en tary ro ck s tak e place
in certain sequence and system . T h e size o f sedim ents
decreases from the littoral m arg in s to th e centre of
the w ater bodies or sed im en tatio n b asin s. D ifferent
sedim ents are co nsolidated and co m p a c te d by d if­
ferent types o f cem enting elem en ts e.g . silica, iron
com pounds, calcite, clay etc.
Sedim entary rocks, as the w ord im plies, are
form ed due to ag g reg atio n and com paction o f
sedim ents. The w ord ‘sed im en ta ry ’ has been de­
rived from Latin word ‘sed im en tu m ’ which means
‘settlin g d ow n ’. Sedim entary rocks are also called
as stratified or layered rocks because these rocks
have different layers or strata o f different types of
sedim ents. Som e tim es, layers are absent in some
sedim entary rocks, for exam ple loess. The sedim ents
and debris derived through the disintegration and
decom position o f the rocks by the agents of w eath­
ering and erosion are gradually deposited in w ater
bodies. Thus, layers after layers o f sedim ents and
debris are regularly deposited. C ontinuous sedim en­
tation increases the w eight and pressure and thus
different layers are consolidated and com pacted to
form sedim entary rocks.
(4) S edim entary rocks co n tain sev eral layers
or strata but these are seldom cry stallin e rocks.
(5) L ike igneous rocks sed im en tary ro ck s are
not found in m assiv e form s such as b atholiths,
laccoliths, dykes etc.
(6) L ayers o f sed im en tary ro ck s are seldom
found in original h o rizo n tal m anner. S edim entary
la y e rs are g e n e ra lly d e fo rm e d d u e to la te ra l
com pressive and tensile forces. T h e b ed s are folded
and found in an ticlin al and sy n clin al form s. Tensile
and co m p ressiv e forces also create fau lts due to
dislocation o f beds.
A ccording to P.G. W orcester (1948) ‘sedi­
m entary rocks, as sedim ent im plies, are com posed
largely o f fragm ents o f older rocks and m inerals,
that have been m ore or less thoroughly consolidated
and arranged in layers and strata.’
(7) S ed im en tary ro ck s m ay be w ell consoli­
dated, poorly co n so lid ated and ev en unconsolidated.
The com p o sitio n o f the ro ck s d ep en d s upon the
nature o f cem en tin g elem en ts and ro ck forming
m inerals.
Characteristics of Sedimentary R ocks
Though m ost o f sedim entary rocks are d ep o s­
ited due to continuous deposition o f sedim ents in
w ater bodies (lakes, ponds, basins, rivers and seas)
but som e tim es these are also form ed at the land
surface, e.g. loess, rocks o f sand dunes, alluvial fans
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(8) S edim entary rocks are ch aracterized by
d illeren t sizes o f jo in ts. T h ese are g en erally perpen­
dicular to the b edding planes.
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149
rocks
(9)
T h e connecting plane betw een two con­
inclined layers are called cross lam ination or cross
secutive beds or layers o f sedim entary rocks is called
bedding.
‘bedding p lan e’. T he uniform ity o f tw o beds along
a bedding plane is called conform ity (i.e. when beds
are sim ilar in all respect). W hen tw o consecutive
beds are not uniform or conform al, the structure is
called u n con form ity. In fact, ‘an unconform ity is a
break in a stratigraphic sequence resulting from a
change in conditions that caused deposition to cease
for a considerable tim e’ (J.D. C ollison and D.B.
Thom pson, 1 9 8 2 ). T h ere are several types o f
unconform ity e.g. (i) non-conform ity (where sedi­
mentary rocks succeed igneous or matamorphic rocks),
(ii) angular un con form ity (w here horizontal sedi­
mentary beds are deposited over previously folded
or tilted strata), (iii) disconform ity (where two
conform able beds are separated by mere changes o f
sedim ent type), (iv) paraconform ity (where two
sets o f conform able beds are separated by same
types o f sedim ents) etc.
(11) Soft m uds and alluvia deposited by the
rivers during flood period develop cracks w hen
baked in the sun. T hese cracks are generally o f
polygonal shapes. Such cracks are called as m ud
cracks or sun cracks.
(12) M ost o f the sedim entary rocks are p er­
m eable and porous but a few o f them are also nonporous and im perm eable. The porosity o f the rocks
depends upon the ratio betw een the voids and the
volume o f a given rock mass.
Classification of sedimentary rocks
1. ON THE BASIS OF THE NATURE OF SEDIMENTS
(1) M echanically form ed or clastic rocks
(i) Sandstones
(ii) Conglom erates
(iii) Clay rock
(iv) Shale
(v) Loess
(2) Chem ically form ed sed im en tary rocks
(i) Gypsum
1—
T
T—
(ii) Salt rock
T
(3) O rganically form ed sedim entary rocks
U nconform ity j
(i)
(ii)
(iii)
(iv)
D isconform ity
1
Lim estones
Dolomites
Coals
Peats
2. ON THE BASIS OF TRANSPORTING AGENTS
(1) Argillaceous or aqueous rocks
(i) M arine rocks
(10)
Sedim entation units in the sedim entary
(ii) Lacustrine rocks
rocks having a thickness o f greater than one centi­
(iii)
Riverine rocks
metre are called beds. The upper and lower surfaces
(2) Aeolian sedim entary rocks
of a bed are called bedding planes or bounding
(i) Loess
planes. Som e tim es, the low er surface o f a bed is
called sole w hile the upper surface is known as
(3) Glacial sedim entary rocks
upper bedding surface. There are further sedim en­
(i) Till
tary units w ithin a bed. The units having a thickness
(ii) M oraines
of more than one centim etre are called as layers or
Mechanically Formed Sedimentary Rocks
strata w hereas the units below one centim etre thick­
Previously form ed rocks are subjected to m e­
ness are known as lam inae. Thus several strata and
chanical or physical disintegration and thus the rocks
laminae m ake up a bed. W hen the beds are deposited
are broken into fragm ents o f different sizes. T hese
at an angle to the depositional surface, they are
called cross beds and the general phenom ena o f
are called fragm ental rock m aterials o r clastic m ate­
(A ) d isc o n fo rm ity
unconformity.
and
(B) angular
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Fig. 8.8:
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GEOMORPHOLOGY
150
(2) C o n g lo m era tes— C o n g lo m erate s are
form ed due to cem entation and consolidation of
pebbles o f various sizes together with sands. ‘The
term conglom erate is applied to cefnented fragmen­
tal rocks containing rounded fragm ents such as peb­
bles and boulders; if the fragm ents are angular or sub
angular, the rock is called b re c c ia ’ (A. Homes and
D.L. H olm es, 1978). P olished and rounded frag­
m ents are called pebbles having a diam eter upto 4
mm while those fragm ents w hich have the diameter
upto 256 mm are called boulders. The rock frag­
ments after being cem ented by clay form gravels.
Though gravels are found in layers but there is
general absence o f uniform ity. W hen the rounded
fragm ental m aterials are cem ented by quartz, the
resultant rocks becom e conglom erates. If conglom ­
erates are form ed due to their cem entation by silica,
they becom e very hard rocks and resistant to erosion.
rials which becom e source m aterials for the form a­
tion o f clastic sedim entary rocks. These m aterials
are obtained, transported and deposited at suitable
places by different exogenous processes (geological
agents) like running w ater (rivers), wind, glaciers,
and sea waves. T hese m aterials are further broken
dow n into finer particles due to their mutual colli­
sion during their transportation. These materials
after being deposited and consolidated in different
w ater bodies (sedim entation basins, lakes, seas, riv­
ers etc.) form sedim entary rocks known as clastic
sedim entary rocks. Sandstones, conglom erates, silt,
shale, clay etc. are im portant m em bers o f this group.
(1) S a n d sto n es— S andstones are form ed
m ostly due to deposition, cem entation and consoli­
dation o f sand grains. Sand grains are divided into
five categories on the basis of their size. W hen sand
Table 8.3 : Classification of Sands by Grain size
Sand Types
(3) C lay R ock a n d S h ale— C lay rocks are
formed due to deposition and cem entation of fine
sedim ents. The rocks form ed o f the sedim ents hav­
ing the grain size o f 0.03 mm to 0.004 m m are called
silts w hereas clays are form ed when the sediments
o f the grain size o f 0.004 mm to 0.00012 mm are
cem ented and consolidated. Silt and clay are soft and
weak rocks but they are definitely im pervious. Clay
rocks are form ed exclusively o f kaolin minerals.
Since clay rocks are not soluble and hence these are
least affected by chem ical w eathering but these are
easily eroded away. Pure clay rocks are o f white
colour but they change in colour w hen they are
mixed with the im purities o f other m aterials. Shales
are formed due to consolidation o f silt and clay.
Shales are form ed o f thin lam inae w hich are easily
separated. Shales are im perm eable rocks and there­
fore they hold m ineral oil above them.
Grain Size
(mm)
(i) Very coarse sand
(ii) Coarse sand
(iii) Medium sand
(iv) Fine sand
(v) Very fine sand
1.0 to 2.0
0.5 to 1.0
0.25 to 0.5
0.125 to 0.25
0.0625 to 0.125
grains are deposited in w ater bodies and are aggre­
gated and consolidated by cem enting elem ents (e.g.
silica, calcium , iron oxiae, clay etc.), sandstones are
formed. The colour o f sandstones varies according
to the nature and am ount o f cem enting elem ents and
m inerals. Sandstones becom e red or gray when ce­
mented by iron oxides but these becom e white or
gray when calcium carbonate dominates. Sandstones
becom e hard and resistant to erosion when ce­
mented by silica. On an average sandstones are
porous rocks and w ater easily percolates through
them. On the basis of textural and m ineralogical
characteristics sandstones are classified into (i) quartz
arenites (arenite from Latin word arena, m eaning
thereby sand) com posed entirely of quartz grains,
(ii) arkose sandstones (feldspar being the dom inant
mineral), (iii) lithic arenites (com posed o f fine­
grained rock fragm ents, m ostly derived from shales,
slates, schists and volcanic rocks) and (i v) graywacke
sandstones (com posed o f quartz, feldspar and rock
fragments surrounded by a fine-grained clay wttftrix).
Chem ically Formed Sedimentary R ocks
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Running w ater contains chem ical materials
in suspension. W hen such chem ically active water
com es in contact w ith the country rocks in its way,
soluble m aterials are rem oved from the rocks. Such
m aterials are called chem ically derived or formed
sedim ents. These chem ical m aterials after being
settled down and com pacted and cem ented form
chem ical sedim entary rocks such as gypsum and salt
rocks.
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ROCKS
The sedim ents derived from the disintegra­
tion or decom position o f plants and animals are
called organic sedim ents. These sedim ents after
being deposited and consolidated form organic sedi­
mentary rocks. On the basis o f lime and carbon
content these rocks are divided into 3 categories e.g.
(i) clacareous rocks, (ii) carbonaceous rocks and (iii)
siliceous rocks.
(1) C a lc a re o u s ro ck s are formed due to depo­
sition and consolidation of sedim ents derived from
the skeletons and rem ains o f those animals and
plants w hich contain larger portion o f lime. Lim e­
stone is the m ost significant characteristic example
of calcareous rocks. Lim estones are formed in the
following m anner—
(i) Calcium oxide (CaO) reacts with water
(H20 ) to form calcium hydroxide (Ca(OH)2)
CaO + H 20 —» C a(O H )2
(ii) Calcium hydroxide reacts with carbon di­
oxide ( C 0 2) to form calcium carbonate (C aC 0 3)
C a(O H )2 + C 0 2 —> Ca CO 3 (limestone) + H20
The calcareous rocks are collectively called
as c a rb o n a te ro ck s or simply carb o n ates. Lime­
stones (C aC O ,) or calcium carbonate, magnesium
carbonate (M g C O ,) and dolom ite (CaMg (C 0 3)2)
are im portant carbonate rocks. Limestones are found
in both the form s-thinly bedded and thickly bedded.
They are form ed o f both fine sedim ents as well as
coarse sedim ents. The most dominant minerals are
calcite (o f hexagonal shape) and argonite (of
orthorhom bic shape). Since limestones are formed
of chem ically soluble m aterials and hence these are
most susceptible to chem ical weathering as fol­
lows—
(i) Carbon dioxide ( C 0 2) after being dissolved
in water form carboric acid (H2 CO 3)
c o 2 + h 2 o ^ h 2c o 3
(ii) Carbonic acid reacts with limestone (C aC 03)
to form calcium bi-carbonate (C a(H C 0 3)2)
H2C 0 3 4- C A C 0 3 -> Ca (H C 0 3)2
Though lim estoens are very weak rocks in
humid regions because these easily dissociate when
they com e in contact with w ater but these become
resistant rocks in hot and dry clim ate because of the
fact that lim estones have uniform and homogeneous
structure and hence these are not affected by differ­
ential expansion and contraction due to tem perature
changes. The rocks having the carbonates o f both
calcium and m agnesium are known as dolom ites
which are less soluble than limestones. These car­
bonate rocks, after w eathering and chem ical erosion,
give birth to karst topography-C halk is another
form of carbonate rocks but it is softer and m ore
porous than limestone. Chalks are form ed due to
precipitation of carbonate materials which are de­
rived from m icro-organism s like foram inifera.
(2) Carbonaceous rocks are dom inated by
carbonic materials which represent vegetation re­
mains. These rocks are formed due to transform ation
of vegetations because of their burial during earth
movements and consequent weight and pressure o f
overlying deposits. The initial form o f carbonaceous
rocks is peat which is o f a dark gray colour. Vegeta­
tion remains canT>e seen with the help o f m icro­
scope. The other subsequent forms o f carbonaceous
sedimentary rocks are lignite, bitum inous and an­
thracite coals with greater proportion o f carbon and
darker colour. Coals are also found in stratified form
wherein coal layers are known as coal seam s.
Carbonaceous rocks are more im portant econom i­
cally than geomorphologically.
(3) Siliceous rocks are form ed due to dom i­
nance of silica content. Siliceous rocks are formed
due to aggregation and com paction of wastes de­
rived from sponge and radiolarian organism s and
diatom plants. Geyserites are also deposits o f silica
around geysers. Geyserites have different colours
e.g. white, gray or pink due to im purities o f deposi­
tion of various types of sediments.
Classification on the Basis of Transporting Agents
Sedimentary rocks are aiso classified on the
basis of transporting agents or geological agents
(e.g. running water or rivers, wind, glaciers, oceanic
currents and sea waves). These agents o f transporta­
tion obtain different types o f sedim ents and deposit
them in suitable places where sedim ents are consoli­
dated and cem ented to form sedim entary rocks of
various sorts. Based on m ajor transporting agents,
sedimentary rocks are divided into argillaceous,
aeolian and glacial rocks.
(1)
A rgillaceous rocks are also called as
aqueous rocks because these are formed in water
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Organically Formed Sedimentary Rocks
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152
GEOMORPHOLOGY
areas. Aqueous word has been derived from Latin
word ‘aq u a’ which m eans ‘w a ter’. A queous rocks
are called as argillaceous rocks because o f the d om i­
nance of clay in the rocks. In fact, the word argillaceous
has been derived from Latin w ord ‘a rg y ll’ or ‘a rg ill’
meaning thereby clay. A rgillacous rocks are charac­
terized by their general softness. T hese are essen­
tially im pervious rocks. A rgillaceous rocks are fur­
ther divided into 3 sub-types on the basis o f the
places o f their form ation, (i) M arine argillaceous
sedim entary rocks are form ed due to deposition
and consolidation o f sedim ents in the oceans and
seas mainly in their littoral zones. The process o f
sedim entation in m arine environm ent is well or­
dered and sequential in character. In other w ords, the
size of particles deceases progressively from the
coastal lands tow ards the seas or the oceans e.g. the
order of the particles from the coast lands tow ards
the sea is of boulders, cobbles, pebbles, granules,
sands, silts, clay and lime. It is evident that as we go
away from the coast lands tow ards the sea, the size
of sedim ents becom es so fine that they are kept in
suspension with oceanic water. Sandstones, lim e­
stones, dolom ites and chalk are the m ost im portant
exam ples of marine argillaceous sedim entary rocks,
(ii) Lacustrine argillaceous sedim entary rocks
are formed due to deposition and consolidation o f
sedim ents in lake en v iro n m en t. G enerally, the
sedim ents are deposited at the floor o f the lakes. The
lacustrine rocks may be seen in 3 conditions viz. (a)
if the lake becom es dry, (b) if the floor o f the lake is
raised due to earth m ovem ents and (c) if the whole
lake'is filled up w ith sedim ents. It may be pointed out
that there is no ordering in the size o f sedim ents as is
the case with the seas and the oceans, (iii) R iverine
argillaceous sed im entary rocks are those which
are form ed due to deposition o f sedim ents in the
riverine environm ent. The sedim ents m ay be depos­
ited in the beds o f the rivers and in the flood plains.
Such deposition includes alluvia w hich are dom i­
nated by clay. A lluvia are deposited on either side o f
the alluvial rivers during floods. It may be pointed
out that alluvial deposits are renewed alm ost every
year. Alluvial deposits develop polygonal cracks
due to their exposure to insolation.
due to m echanical w eathering in the hot and dry
regions. This process results in the form ation of
im m ense quantity o f sands o f d ifferent sizes. Winds
pick up these sands and deposit them at various
places. The particles are further com m inuted into
finer particles due to attrition w hile they are being
transported from one place to another. Continuous
deposition o f sands results in the form ation o f differ­
ent layers but these layers are not w ell consolidated
as is the case with the argillaceous rocks. Som e­
tim es, there is com plete absence o f layers in the
airborn or aeolian sedim entary rocks. Loess is the
m ost im portant m em ber o f this group. L oess is, in
fact, the heaps o f unconsolidated fine materials.
There is general absence o f lam inae and layers in the
loessic formation. These are soft and porous rocks.
W ater can easily infiltrate in the loessic deposits.
Thus, loess is easily eroded away. Thus, m ost out­
standing characteristic feature o f loess is that the
entire loessic m ass may stand like a vertical c liff or
wall. The best exam ple is observable on the left and
right banks o f the palaeochannel and valley o f the
N arm ada river at D hunw adhar falls (B heraghat)
near Jabalpur (M .P.) where the loessic banks rise 20
to 25 m from the valley floor and form com plete
vertical free-face cliff section. The sedim ents are so
loosely arranged that they can be rem oved even by
using fingers. The m ost extensive loessic deposits
are found in north C hina w here the thickness of
sedim ents is o f several hundred m etres. The deposits
are of yellow colour and are rich in lim e and hence
these look like fine loam soils. T he Y ellow river
(form erly Hwang Ho) and its tributaries easily erode
the loessic deposits and hence the river becomes
overloaded and causes frequent severe floods. It
may be pointed out that the Y ellow river o f China
carries the largest am ount o f sedim ents (1640 m il­
lion tonnes per year) in the w orld. T he river is called
‘Y ellow ’ because o f the yellow colour o f the sediments
w hich are derived through the erosion o f yellow
coloured C hinese loess.
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(3)
G lacial rocks— T he m aterials deposited
by glaciers are called glacial drifts w hich are depos­
ited in four conditions and therefore there are four
types o f m orainic deposits viz. (i) laterlal m o r a in e
(2)
A eolian sedim entary rocks are formed
w hen glacial m aterials are deposited on e i t h e r side of
due to deposition o f sands brought down by the
a glacier, (ii) m edial m oraines, when glacial sedi­
wind. Pre-existing rocks are greatly disintegrated
mentary materials are deposited along the joining
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153
rocks
glaciers ( ‘The lateral m o rain es o f jo in in g ice stream s
merge and form a sin g le m ed ia l m o ra in e in the
middle o f larger flo w s— ’ F. Press and R. Siever,
1978), (iii) g rou n d m o ra in es, w hen the glacial
materials are d ep o sited in th e bed o f the g lacier and
^iv) term inal m o ra in es (th ese are form ed w hen the
elacier is ablated an d m aterials are deposited therein)
(fig. 8.9).
and the w eight and pressure o f o v erlying rocks
becom es enorm ous due to oro g en etic m ovem ents.
W hen the rocks are m etam orphosed to the greatest
intensity, the process is know n as in ten se m etam orphism . D harw arian sedim entary rocks o f peninsular
India have suffered intense m etam orphism .
The change in the form o f th e rocks d u rin g the
process o f m etam orphism takes p lace in tw o w ays
viz. (i) physical m e ta m o rp h ism pertaining to changes
in textural com position o f the rocks and (ii) ch e m i­
cal m e ta m o rp h ism , leading to ch an g es in the c h em i­
cal com position o f the rocks. S om e tim es, b o th the
processes o f m etam orphism beco m e o p erativ e to­
gether. It m ay be pointed out again that du rin g the
process o f m etam orphism there m ay be co m p lete
alteration in the form o f the rocks, the form and
nature o f m inerals m ay change, old m in erals m ay be
rearranged and changed in new m in erals, new m in ­
erals may be added, p re-existing m in erals m ay be
transform ed into other form s due to m eltin g cau sed
by very high tem perature, p re-ex istin g c ry stallin e
rocks may be recrystallized but th ere w ould be no
disintegration and decom position o f th e ro ck s in any
circum stance.
M edial M oraine
C
03
C
I
L a tera l M oraine
V
H
Fig. 8.9 : Different types o f moraines.
8.5 M ETAM O RPHIC R O C K S
Meaning and C h a ra cteristics
‘M etam orphic rocks include rocks that have
been changed eith er in form or com position w ithout
disintegration’ (P.G . W orcester, 1948). M etam orphic rocks, as the w ord ‘m etam o rp h ism ’ im plies, are
formed due to chan g es in the form s o f other rocks.
Originally, the w ord m etam orphism has been de­
rived from the w ord ‘m e ta m o r p h o s e ’ which means
change in form . In fact, m etam orphic rock means
complete alteration in the appearance o f pre-existing
rocks due to change in m ineral com position and
texture through tem p eratu re and pressure. M etam or­
phic rocks are generally form ed due to change in
form o f sedim entary and igenous rocks. Som e times,
even previously form ed m etam orphic rocks are again
m etam orphosed.
It may be m entioned that the process o f m eta­
morphism sim ply m eans change in form but in
geology this is used for specific m eaning and condi­
tion. For exam ple, the form and com position o f a
rock may change during the process o f m etam or­
phism but there is no disintegration and decom posi­
tion of the rock. W hen already form ed m etam orphic
rocks are again m etam orphosed, the process is known
as rem etam orphism . T his becom es possible only
when the tem perature becom es exceedingly high
Some times, the form o f the rocks is so changed
due to intense m etam orphism that it b ecom es d iffi­
cult to find out the original form o f the rocks. S om e
rocks, after m etam orphism , becom e h ard er than
their original form s, such as m arb les from lim e­
stones and quartzites from sandstones. M arb les and
quartzites are relatively m ore resistan t to ero sio n
than their parent rocks, lim estones and san d sto n es.
Fossils o f sedim entary rocks are also d estro y ed
during the process o f m etam orphism .
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‘U nlike igneous rocks, the tex tu re o f m e ta­
m orphic rocks is the result o f recry sta llizatio n o r
conversion o f one m ineral to an o th er in the solid
state* (Press and Siever, 1978). F o lia tio n , d efined as
streaking or parallel arrangem ent o f the co n stitu en t
crystals (o f the m etam orphic rocks) w hich generally
‘cut the rocks at an angle to the bedding planes o f the
original sedim ents o f the p aren t ro c k s’ is the m ost
com m on characteristic feature o f m etam orphic rocks.
The coarse-grained m etam oprhic rocks are im per­
fectly foliated (e.g. gneisses from g ran ites) w hile
fine-grained m etam orphic rocks are perfectly fo li­
ated (e.g. schists from shales). T he property o f
m etam orphic rocks to part o r split along the bedding
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GEOMORPHOLOGY
(ii) Regional metamorphism (involving
larger area)
planes is known as ftattilUy. The structure o f the
presence o f numerous closely spaced parallel planes
o f splitting is known as cleavagc. In fact, cleavage
is a special type o f foliation which denotes the
tendency o f a rock to cleave or break or split into
moderately thin sheets or laminae. Schistocity reef­
ers to the growth o f larger crystals and segregation o f
some minerals into lighter and darker bands.
(3) C om posite classification
(i) Contact or thermal metamorphism
(ii) Dynamic and regional metamorphism
(iii) H ydro-m etam orphism
(iv) H ydro-therm al m etam orphism
Agents of Metamorphism
Contact Metamorphism
(i) H eat is the most im portant factor for the
■development o f m etam orphic rocks from pre-exist­
ing parent rocks. It may be pointed out that mineral
com position is entirely changed due to intense heat
but the rocks are seldom melted. The required heat
for m etam orphism is available during vulcanicity
w hen hot and m olten m agm as ascend through the
crustal rocks.
C ontact m etam orphism takes place w hen the
mineral com position o f the surrounding rocks known
as aureoles is changed due to intense h eat o f the
intruding m agm as. This process o f m etam orphism is
called contact m etam orphism because o f the fact
that m etam orphism occurs w hen the rocks com e in
contact with the intruding m agm as. This process is
also called as therm al m etam orphism because the
rocks are changed in their form s due to high tem ­
perature of the introduing m agm as. Such m etam or­
phism occurs during volcanic activity w hen the
physical properties o f the surrounding rocks are
changed due to intense heat o f the rising m agm as o f
dykes (fig. 8.10). Som e tim es, the rocks com ing in
contact with the intruding m agm as are also changed
in their chem ical com position due to som e water and
water vapour associated w ith the intruding m agm as.
Lim estones are changed to m arbles due to contact
metam orphism .
(ii) C om pression resulting from convergent
horizontal m ovem ents caused by endogenetic forces
causes folding in rock beds. Thus, the resultant
pressure from com pressive forces and consequent
folding changes the form and com position of parent
rocks. This factor becom es operative during m oun­
tain building.
(iii) Solution— Chem ically active hot gases
and water while passing through the rocks change
their chem ical com position. M agmatic w ater and
w ater confined in the beds o f sedim entary rocks also
help in introducing chem ical changes in the rocks.
Types of Metamorphisms
The agents and factors of metamorphism some
tim es operate separately and some times work to­
gether. The processes of m etam orphism may be
classified on the bases o f (i) the nature o f the agents
o f m etam orphism and (ii) place and area involved in
m etam orphism .
(1) On the basis o f the nature o f agents
(i) Therm al m etamorphism (due to heat)
(ii) D ynam ic m etamorphism (due to pres­
sure)
(iii) H ydro-m etam orphism (due to hydro­
static pressure)
(iv) H ydro-therm al-m etam orphism (due to
water and heat)
Fig. 8.10 : Illustration o f contact or thermal metamor­
phism.
(2) On the basis o f place or area
Contact metamorphism (localized in
area)
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(i)
As stated above, the rocks surrounding the
igneous intrusions are altered due to intense heat o f
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155
rocks
characteristic feature o f m ountainous area. Regional
m etam orphism is further divided into tw o sub-types
viz. (i) D ynam ic regional m etam orphism , when
the rocks are m etam orphosed due to com pressive
forces and resultant high pressure caused by conver­
gent horizontal m ovem ents (fig. 8.11), and (ii) Static
regional m etam orphism , when the rocks are m eta­
m orphosed at greater depth due to intense pressure
and w eig h to f overlying rocks (superincum bent load)
magmas. The m argins o f the altered rock around
igneous intrusions are called aureoles the w idth of
which (i.e. the dim ension o f m etam orphosed rocks)
depends upon m ainly tw o factors e.g. (i) tem pera­
ture of intruding m agm a, and (ii) the depth o f m agm a
intrusions in the curst.
Regional Metamorphism
W hen the rocks are altered in their forms in
extensive area the process is called regional m eta­
morphism. Such m etam orphism is also known as
dynam ic m etam orp h ism because pressure plays
dominant role in the alteration o f the form o f the
rocks though tem perature is also an im portant factor.
The sedim entary rocks are folded due to com pressive
forces during the period o f m ountain building. This
process results in intense pressure and heat which
ultim ately alter the original form of the concerned
rocks. Dynamic metamorphism leads to crystallization
(fig. 8.12).
Fig. 8.12 : Example o f static regional metamorphism.
Hydro-Metamorphism
S edm im entary Rock
C om pression
The alteration in the com position o f the rocks
due to hydrological factor takes place in.a num ber o f
ways e.g. (i) W hen the chem ically active w ater
(solvent) passes through the country rocks, there
occur several chem ical changes in the rocks due to
varied chem ical reactions, (ii) T he storage o f im ­
mense volum e o f w ater in big reservoirs exterts high
pressure on the underlying rocks and thus the rocks
are altered in their form s due to pressure o f overlying
huge volum e o f water. Such type o f m etam orphism
is known as h ydrostatic m etam orp h ism .
C om pression
Hydro-Thermo-Metamorphism
The m inor alteration in the physical and chem i­
cal com position o f the rocks cau sed by the w eight
and pressure o f w ater m ass and chem ically active
hot gases and w ater vapour is called hydro-therm om etam orphism w hich is, in fact, geographically less
im portant.
Metainorpliisiii
Classification of Metamorphic R o ck s
in the rocks and if the rocks are already crystallized
they are recrystallized. R egional m etam orphism is a
Theclassification o f metamoprhic rocks is easier
and less com plicated because generally these are
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Fig. 8.11 : Example ofdynamic regional metamorphism.
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GEOMORPHOLOGY
156
the Narmada river at Bheraghat near Jabalpur (M .P.)
show different grades o f colour though w hite and
classified on the basis o f those original o r parent rocks
from which they have been formed. It is obvious that
the parent rocks in relation to m etam orphic rocks are
sedimentary and igneous rocks. Som e tim es, the
process of m etam orphism becom es so intense and the
parent rocks are so greatly m etam orphosed that it
becom es very difficult to trace the true nature o f the
rocks before their m etam orphism . Besides such con­
ditions, m etam oprhic rocks are divided into 2 broad
categories.
pink colours dom inate.
D o lo m ites and ch alk s are also m etam orphosed
to m arbles due to ex cessiv e h eat b u t th ese have only
local im portance. M arb les are m o re resistan t to
erosion than th eir p aren t lim esto n e s. B esides, they
are econom ically v alu ab le ro ck s b ecau se they are
used as bu ild in g m aterials for the c o n stru ctio n o f
very' im portant b u ild in g s as m o n u m en ts. F o r ex am ­
ple, T ajm ahal o f A g ra and D ilw ara te m p le o f M ount
A bu (R ajasthan) have been bu ilt o f m arbles.
(1) M eta-sedim entary or para-m etam orphic rocks are those m etam orphic rocks w hich are
form ed due to alteration o f the form s o f sedim entary
rocks, e.g. m arbles from lim estones, quartzites from
sandstones, slates from shales and clays etc.
S chists are fin e-g rain ed m e tam o rp h ic rocks
and are ch aracterized by w ell d ev elo p ed foliation .
The w’ord schist has been d eriv ed from F ren ch w ord
‘sch iste’ and G erm an w ord ‘s c h is to se ’ w hich means
to split. W hen shale sed im en tary ro ck s are subjected
to intense com pressive force and co n seq u en t fo ld ­
ing and pressure, the clay and o th er m in erals o f the
original shale rocks are ch an g ed to m ica minerals
due to high pressure and tem p eratu re and thus shales
are changed to schists. D uring the p ro cess o f re ­
gional m etam orphism the schists get foliated. S chists
are nam ed on the basis o f d o m in an t m inerals, e.g.
m ica-schists, h orn b len d e sch ists, q u a rtz sch ists
etc. M ica schist is the co m m o n est type o f sch ist
rocks because it is form ed from arg illac eo u s shale
sedim entary rock w hich is a very co m m o n rock and
is abundantly found on the earth 's su rface. M icaschist is com posed o f m uscovite, biottle, p lag io clase
and some tim es garnet. H ornblend sch ists are form ed
from b a s a ltic ro c k s an d c o n ta in h o rn b le n d e ,
plagioclase and som e q u artz m inerals. G r een sch ists
are com posed o f green m in erals such as hornblende
and chlorites, provided that the ro ck s are w ell foli­
ated. If the schists rich in green m in erals are poorly
foliated, they are called g ree n sto n es. The term
m etabasite is used to nam e th o se sch ists w hich are
form ed from basalts or d o lerites.
(2) M eta-igneous or ortho-m etam orphic
rocks represent these m etam orphic rocks which are
form ed due to changes in the form o f igneous rocks,
e.g. gneisses from granites, serpentine from gabbro,
basic granulites from am phibolites, eclogite from
basaltic rocks etc.
M etam orphic rocks are also classified on the
basis of foliation into (i) foliated m etam oprhic
rocks e.g. slates, gneisses and schists and (ii) n o n ­
foliated m etam orphic rocks, e.g. quartzites, m ar­
bles, serpentines etc.
Important Metamorphic R ocks
M a rb le s are generally formed due to changes
in lim estones because o f tem perature changes. Lim e­
stones are transform ed into m arbles due to contact
therm al m etam orphism during volcanic activity.
L im estones are also m etam orphosed due to dynamic
regional m etam orphism wherein calcium carbon­
ates and other finer particles are changed into calcite. In fact, the m etam orphism o f lim estones to
m arble involves a num ber o f changes in the mineralogical characteristics o f lim estoens. For exam ple,
the reaction between calcium carbonate o f lim estone
during the process o f m etam orphism produces a new
m ineral known as w ollastonite or calcium silicate.
The colour o f m arbles depends upon the nature o f
parent lim estones. If the original lim estones are
devoid o f any im purities, the resultant m arbles be­
com e pure white in colour. The colour changes due
to im purities o f other m aterials in the parent lim e­
stones. The m arbles o f C arrara region o f Italy are
pure w hite w hile the m arbles exposed along both the
banks o f the m agnificent and stupendous gorge o f
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Slates are form ed d ue to d y n am ic regional
m etam orphism o f shales and o th er argillaceous rocks.
Slates are ch aracterized by the ‘p resen ce o f num er­
ous closely-spaced parallel p lan es o f splitting or
cleavage but the splitting planes o f slates are not
parallel to the bedding planes rath e r they form angle
w ith the bedding planes. S om e tim es, the angle
betw een the sp littin g planes and b edding planes
becom es obtuse angle. Such structure o f slates is
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ROCKS
157
known as slaty clea v a g e (fig. 8.13) w hich is form ed
due to com pressive p ressu re exerted on the rocks.
granites (igneous). F eldspar is the m ost dom inant
m ineral o f gneisses. Like schists, gneisses are also
foliated rocks but the foliation is open and is som e
tim es absent. T here are several types o f banded
gneisses, w hich som e tim es pass into au gen gneiss.
The process o f gran itization or gran itification
m eans the transform ation o f m ica-schist to gneiss.
G neissic rocks produce, after w eatheirng and ero­
sion, rounded topography.
Q u a r tz i te s are g e n e ra lly fo rm e d fro m
sandstones w hich are dom inated by the abundance
o f quartz m ineral. D uring the p rocess o f m e tam o r­
phism the voids w ithin the san d sto nes are co m ­
pacted due to excessive com pression and h eat and
are also filled with silica, with the resu lt q u artzites
becom e very hard and resistant to erosion. W hen
quartzites lie over w eaker sed im en tary rocks like
shales or lim estones as c a p ro c k s , they form stu p e n ­
dous wall-like escarpments. K aim ur escarpm ent along
the left bank o f the Son river (in M .P. and B ihar),
B hander escarpm ents (S atna and P anna d istricts o f
M .P.), Rew a escarpm ents facing the G an g a p lain s
etc. have been form ed due to resistan t cap ro ck s o f
quartzitic snadstones resting over shale lithology.
‘The term quartzite is also exten d ed to sandy ro ck s
which have been subjected to cem en tation by silica
deposited from solution. Such rocks are gen erally
softer than the true m etam orphic q u artzites and
often behave m ore like norm al san d to n es, b reaking
down into sandysoils’ (S.W . W oold ridge and R.S.
M organ, 1959).
Ilc(l(lini> i ’lu nc
Fig. 8.13 : Relationship between cleavage planes and
bedding planes o f slates.
The clavage is alw ays at right angle to the direction
of com pression. Slates, if subjected to further in­
tense m etam orphism due to im m ense com pression,
are changed to phyllites or fine-grained m ica-schist,
‘Slates, in fact, m ay be regarded as a special type of
fine-grained sch ist’ (S.W . W ooldridge and R.S.
Morgan). Slates are not as m uch resistant to erosion
as are schists and gneisses. They are o f varied
colours.
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G n eisses are coarse-grained m etam orphic
rocks which are form ed due to m etam orphism of
conglomerates (sedim entary rocks) and coarse gained
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158-169
EARTH'S MOVEMENT
Introduction ; endogenetic forces (sudden forces and m ovem ents,
diastrophic forces and movements - epeirogenetic movements, orogenetic
m o v e m e n ts); folds ; faults ; rift valleys ; exogenetic forces.
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CHAPTER 9
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9
EARTH'S MOVEMENT
9.1 INTRODUCTION
earth's surface (e.g. m o u n tain s, p la teau s, p lain s, lakes,
fau lts, fo ld s etc.). V o lc a n ic e ru p tio n s an d seism ic
ev en ts are also th e e x p re ssio n s o f e n d o g e n e tic fo rces.
S u ch m o v e m en ts are ca lle d s u d d e n m o v e m e n ts and
th e fo rces re sp o n sib le fo r th e ir o rig in a re called
su d d en fo rce s. W e d o n o t k n o w p re c ise ly th e m o d e
o f o rig in o f th e e n d o g e n e tic fo rc e s an d m o v e m e n t
b ecau se th ese are re la te d to th e in te rio r o f th e earth
a b o u t w h ich o u r sc ie n tific k n o w le d g e is still lim ited .
O n an av erag e, th e o rig in o f e n d o g e n e tic fo rc e s is
related to th erm al c o n d itio n s o f th e in te rio r o f the
earth . G en erally , the e n d o g e n e tic fo rces a n d re la te d
h o rizo n tal and v ertical m o v e m e n ts are c a u s e d d u e to
co n tra ctio n an d e x p a n sio n o f ro c k s b e c a u se o f v a ry ­
ing th erm al co n d itio n s an d te m p e ra tu re c h a n g e s
in sid e th e earth . T h e d isp la c e m e n t an d re a d ju stm e n t
o f g eo m aterials so m e tim e s ta k e p la c e so rap id ly that
e arth m o v e m en ts are c a u se d b e lo w th e cru st. T h e
en d o g en e tic fo rces and m o v e m e n ts a re d iv id e d , on
the b asis o f in ten sity , in to tw o m a jo r c a te g o rie s viz.
( I ) d ia stro p h ic fo rces an d (2) su d d e n fo rces.
T h e stu d y o f fo rc e s affe c tin g th e cru st o f th e
e a rth o r o f g e o lo g ic a l p ro c e sse s is o f p aram o u n t
sig n ific a n c e b eca u se th e se fo rces an d resu ltan t m o v e­
m e n ts a re in v o lv e d in th e c re a tio n , d estru ctio n , re c ­
re a tio n and m a in te n a n c e o f g eo m a te ria ls and n u m e r­
o u s ty p e s o f re lie f featu res o f v ary in g m ag n itu d es.
T h e se fo rc e s very o ften a ffe c t an d ch an g e the earth 's
su rface. In fact, the c h a n g e is law o f n ature. T he
g e o lo g ic a l c h a n g e s are g en era lly o f tw o ty p es e.g. (i)
lo n g p e r io d c h a n g e s an d (ii) sh o rt-p e rio d ch a n g es.
L o n g -p e rio d c h a n g e s o c c u r so slo w ly th at m an is
u n a b le to n o tice su ch c h a n g e s d u rin g his life-p erio d .
O n th e o th e r h an d , sh o rt-p e rio d ch an g es take place
so su d d e n ly th a t th e se are n o ticed w ithin few se c ­
o n d s to few h o u rs, e.g. seism ic ev en ts, v o lcan ic
e ru p tio n s etc. T h e fo rces, w h ich a ffe c t the cru st o f
the e a rth , are d iv id e d in to tw o b ro ad categ o ries on
the b asis o f th e ir so u rces o f o rigin e.g. ( l ) en d o g en etic
fo r c e s and (ii) e x o g e n e tic fo r c e s (fig. 9 . 1).
9 .2 ENDOGENETIC FO RCES
T h e fo rces c o m in g fro m w ith in th e earth are
called as e n d o g e n e tic fo rces w h ich cau se tw o ty p es
o f m o v e m en ts in the e arth viz. ( l ) h o riz o n ta l m o v e ­
m en ts and (ii) v e rtica l m o v e m e n ts. T h e se m o v e ­
m e n ts m o to red by th e e n d o g e n e tic fo rces in tro d u ce
v ario u s ty p e s o f v e rtic a l irre g u la ritie s w h ich give
b irth to n u m e ro u s v a rie tie s o f re lie f featu res on the
(1) SUDDEN FO R C ES AND MOVEMENTS
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S u d d e n m o v e m e n ts , c a u s e d by s u d d e n
e n d o g e n e tic fo rc e s c o m in g fro m d e e p w ithin the
earth , ca u se su ch su d d en an d ra p id ev en ts that these
c a u se m a ssiv e d e stru c tio n s at an d b elow the earth's
su rfaces. S u ch ev e n ts, lik e v o lc an ic eru p tio n s and
e a rth q u a k e s, are calle d ‘e x tr e m e e v e n ts ’ and be-
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g^gTffSMOVEMENT
159
FORCES WHICH AFFECT THE EARTH'S CRUST
ENDOGENETIC FORCES
EXOGENETIC FORCES
DIASTROPH 1C FORCES
EPEIROGENETIC FORCES
UPWARD MOVEMENT
(EMERGENCE)
SUDDEN FORCES
OROGENETIC FORCES
VOLCANIC ERUPTION
EARTHQUAKES
DOWNWARD MOVEMENT
(SUBMERGENCE)
TENSIONAL FORCES
COMPRESSION AL FORCES
CRUSTAL FRACTURE
CRUSTAL BENDING
CRACKING
FAULTING
(FAULTS)
WARPING
FOLDING
(FOLDS)
DOWNWARPING
UPWARPING
Fig. 9 1 : Schematic presentation o f forces (endogenetic) affecting the earth's crust.
(2) DIASTROPHIC F O R C E S AND M O VEM ENTS
com e d isa stro u s h aza rd s w hen they o ccu r in densely
p opulated lo c alities. T h ese forces w ork very quickly
and th e ir re su lts are seen w ithin m inutes. It is
im portant to n o te th a t th ese fo rces are the result o f
long-period p re p aratio n d eep w ith in the earth. O nly
their cu m u lativ e effects on the earth s surface are
quick and s u d d e n ’ (S a v n d ra S ingh, 1991, E n v iro n ­
m ental G eo g rap h y , p. 6 8 ). G e o lo g ic a lly , these su d ­
den forces are term ed as ‘c o n s t r u c t i v e fo rc e s b e­
cause these create certain re lie f features on the
earth's surface. F o r ex am p le, volcanic eruptions
result in the form ation o f volcanic cones and m o u n ­
tains w hile fissure flow s o f lavas form extensive lava
plateaus (e.g. D eccan p lateau o f India, C olum bian
plateau o f the U SA etc.) and lava plains. E arth ­
quakes create faults, fractures, lakes etc.
D iastrophic forces include b o th v ertical and
horizontal m ovem ents w hich are cau sed d ue to forces
deep w ithin the earth. T h ese d ia stro p h ic fo rc e s o p e r­
ate very slow ly and th eir effects b eco m e d iscern ib le
after thousands and m illio n s o f y ears. T h ese forces,
also term ed as co n stru ctiv e fo rces, affect la rg e r
areas o f the globe and p ro d u ce m e so -lev el reliefs
(e.g.) m ountains, p lateau s, plain s, lak es, b ig faults
etc.). T hese d iastro p h ic fo rces and m o v e m en ts are
f u r th e r s u b d iv id e d in to tw o g r o u p s v iz . ( i)
epeirogenetic m ovem en ts and (ii) o ro g en etic m ove­
m ents.
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(i)
E p eiro g en etic M o v em e n ts— E p e iro g e n
etic w ord co n sists o f tw o w o rd s viz. ‘epiros* (m e a n ­
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GEOMORPHOLOGY
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160
a ffects larger areas o f the crust w herein the crustal
parts are either warped (raised) upward or downward.
T he upward rise o f the crustal part due to com pressive
force resulting from con vergen t horizontal m ove­
m ent is called u p w a r p in g w h ile the bending o f the
crustal part dow nw ard in the form o f a basin or
depression is called d o w n w a r p in g . W hen the proc­
esses o f upwarping or d ow n w arp in g o f crustal rocks
affect larger areas, the resultant m ech an ism is called
b ro a d w a r p in g . W hen the co m p ressiv e horizontal
forces or con vergent fo rces and resultant m ovem ents
cause buckling and sq u eezin g o f crustal rocks, the
resultant m echanism is ca lled fo ld in g w hich causes
ing thereby continent) and ‘g e n e sis’ (m eaning thereby
origin). E p eirogen etic m ovem en t cau ses upliftm ent
and su bsid en ce o f continental m asses through up­
ward and dow nw ard m ovem en ts resp ectively. B oth
the m ovem ents are, in fact, vertical m ovem en ts.
T h ese forces and resultant m ovem en ts affect larger
parts o f the continents. T h ese are further divided into
tw o types viz. (i) u p w a r d m o v e m e n t and (ii) d o w n ­
w a r d m o v e m e n t . U p w a rd m o v e m e n t c a u s e s
upliftm ent o f continental m asses in tw o w ays e.g.(a)
the upliftm ent o f w h o le continent or part thereof and
(b) the upliftm ent o f coastal land o f the continents.
Such type o f upliftm ent is called em er g en ce.
D ow nw ard m ovem en t causes subsidence o f
continental m asses in tw o w ays viz. (i) subsidence o f
land area. Such type o f downward m ovem ent is
called as su b sid en ce , (ii) A lternatively, the land
area near the sea coast is m oved downward or is
subsided b elow sea-level and is thus subm erged
under sea water. Such type o f downward m ovem ent
is called as su b m erg en ce.
(ii)
O r o g e n e tic M o v e m e n t— T he
orogenetic has been derived from two Greek words,
‘o r o s’ (m eaning thereby m ountain) and ‘g e n e sis’
(m eaning thereby origin or form ation). O rogenetic
movement is caused due to endogenetic forces working
in horizontal manner. Horizontal forces and m o v e­
m ents are a lso ca lled as ‘t a n g e n tia l f o r c e s .’
O rogenetic or horizontal forces work in tw o w ays
viz. (i) in opposite directions and (iii) towards each
other. This is called ‘ten sio n a l fo r c e ’ when it oper­
ates in opposite directions. Such types o f force and
m ovem ent are also called as d iv er g en t fo rces and
m o v em en ts. Thus, tensional forces create rupture,
cracks, fracture and faults in the crustal parts o f the
earth. The force, when operates face to face, is called
co m p ressio n a l fo r c e or co n v e r g e n t fo rce. C om pressional force causes crustal bending leading to
the formation o f fold s or crustal warping leading to
local rise or subsidence o f crustal parts.
several types o f folds.
F o ld s
W a v e-lik e bends are form ed in the crustal
rocks due to tangential c o m p ressiv e force resulting
from horizontal m ovem en t cau sed by the endogenetic
force originating d eep w ithin the earth. Such bends
are called ‘f o ld s ’ w herein so m e parts are bent up and
som e parts are bent d ow n . T h e upfolded rock strata
in arch-like form are ca lled ‘a n t ic lin e s ’ w hile the
w ord
dow n folded structure form in g trou gh -lik e feature is
called ‘s y n c lin e ’ (fig . 9 .3 ). In fact, fold s are minor
forms o f broad w arping. T h e tw o sid es o f a fold are
called lim b s o f the fold. T he lim b w h ich is shared
betw een an an ticlin e and its co m p a n ion syn clin e is
called m id d le lim b . T he plane w h ich b isects the
angle betw een the tw o lim bs o f the anticline or
m iddle lim b o f the sy n clin e is ca lled the a x is of fold
C ru stal B en d in g — W hen horizontal forces
work face to face the crustal rocks are bent due to
resultant com pressional and tangential force. In other
words, when crustal parts m ove towards each other
under the influence o f horizontal or convergent forces
and m ovem ents, the crustal rocks undergo the proc­
ess o f ‘crustal bending’ in tw o w ays e.g. (i) w a rp in g
and (ii) fold in g. The process o f crustal warping
Synclinal Plane
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Fig. 9.2 : Different components o f a fold.
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EARTH S m o v e m e n t
m
or axial plane (fig. 9.2). On the basis of anticline and
syncline these axial planes are called as axis of
anticline and axis o f syncline respectively.
d irectio n o f any ho rizo n tal line along a bedding
p la n e ’ (A. H olm es and D .L. H olm es). The direction
o f d ip is alw ays at right an g le to the strik e (fig. 9.4).
Anticlines— T he upfolded rock beds are called
an ticlin es. In sim p le fold the rock strata o f both the
lim bs dip in op p o site d irectio n s. S om e tim es, fold­
ing becom es so acute that the d ip angle o f the
an ticlin e is accen tu ated and the fold b ecom es alm ost
vertical. W hen the slopes o f both the lim bs or sides
o f an an ticlin e are uniform , the an ticlin e is called as
‘symmetrical anticline’ but w hen the slopes are
unequal, the an ticlin e is called as ‘asymmetrical
anticline’. A nticlines are div id ed into tw o types on
the basis o f dip angle e.g. (i) g entle an ticlin e w hen
the dip angle is less than 40°, som e tim es 10 or 2° and
(ii) steep anticline w hen the dip an g le ran g es be­
tw een 40° and 90°.
Anticline
Fig. 9.3 : Anticlines and synclines.
It is d e s ira b le to e x p la in th e ch a ra c te ristic s o f
‘d ip ’ a n d ‘s t r i k e ’ as it b e c o m e s a b so lu tely n ece s­
sary to u n d e rs ta n d th e m in o rd e r to u n d erstan d the
s tru c tu ra l fo rm . T h e in c lin a tio n o f ro ck beds w ith
re s p e c t to h o riz o n ta l p la n e is term ed as ‘d ip ’ (fig.
9 .4 ). It is a p p a re n t th a t w e d e riv e tw o in form ation
a b o u t th e d ip e .g . (i) th e d ire c tio n o f m ax im u m slope
d o w n a b e d d in g p la n e an d (ii) th e an g le betw een the
m a x im u m s lo p e an d th e h o rizo n tal plane. T he d irec­
tio n o f d ip is m e a s u re d by its tru e b earin g in relation
to e a s t o r w e s t o f n o rth , e.g. 6 0 ° N .E .; w h ile the angle
o f d ip is m e a s u re d w ith an in stru m e n t called clin o m ­
eter. F o r e x a m p le , if an y ro c k bed is in clin ed at the
an g le o f 60° w ith re s p e c t to h o rizo n tal p lan e and the
d ire c tio n o f s lo p e is N , th en th e d ip w o u ld be ex ­
p re sse d as 6 0° N . ‘T h e s trik e o f an in clin ed bed is the
S y n clin es— D ow nfolded rock beds d ue to
com pressive forces caused by ho rizontal tangential
forces are called synclines. T hese are, in fact, tro u g h ­
like form in w hich beds on eith er side ‘incline
to g e th er’ tow ards the m iddle part. I f folded in­
tensely, the syncline assum es the form o f a canoe.
A n tic lin o riu m — A nticlinorium refers to those
folded structures in the reg io n s o f folded m ountains
w here there are a series o f m in o r anticlines and
synclines w ithin one exten siv e an ticline (fig. 9.5).
A n tic lin o riu m is fo rm e d w h en th e h o riz o n ta l
com pressive tangential forces do not w ork reg u ­
larly. C onsequently, due to differen ce in the in ten ­
sity o f com pressive forces such structures are form ed.
Such type o f folded structure is also called as fan fold.
Fig. 9.5 : Illustrationofanticlinoriurnandsynclinorium.
Synclinorium— Synclinorium represents such
a folded structure which includes an extensive syncline
having numerous minor anticlines and synclines.
Such structure is formed due to irregular folding
consequent upon irregular compressive forces (fig. 9.5).
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Fig. 9.4 : D ip a n d strike.
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GBOMQRnnijoerr
162
Types of Folds
p o ssib ility for th e sp littin g o f the lim b s o f su ch folds
b ecause o f in tense fo ld in g . S p littin g o f lim b s gives
birth to the form ation o f faults. It is a lso op in ed that
m onoclinal fo ld s are a lso form ed due to unequal
horizontal co m p ressiv e fo rces co m in g from both the
T he nature o f fo ld s depends on several factors
e.g . the nature o f rocks, the nature and intensity o f
com p ressive forces, duration o f the operation o f
com p ressive forces etc. T he elasticity o f rocks largely
affects the nature and the m agnitude o f fold in g
p rocess. The softer and m ore elastic rocks are sub­
jected to intense fold in g w h ile rigid and less elastic
rocks are on ly m oderately folded. The difference in
the intensity and m agnitude o f com p ressive forces
also cau ses variations in the characteristics o f folds.
N orm ally, both the lim bs o f a sim p le fold are more
or le ss o f equal inclination but in m ost o f the cases o f
different fold s the inclinations o f both the lim bs are
different. Thus, based on the inclination o f the lim bs,
fold s are d ivid ed into 5 types (fig. 9.6).
sides.
(4) I s o c lin a l f o ld s are form ed w hen the
co m p ressiv e forces are so strong that both the limbs
o f the fold b eco m e parallel but not horizontal.
(5 ) R e c u m b e n t fo ld s are form ed when the
com p ressive forces are s o stron g that both the limbs
o f the fold b eco m e parallel as w ell as horizontal.
(6) O v e r tu r n e d fo ld s are th ose fold s in which
o n e lim b o f th e fo ld is th ru s t upon another fold due
to in ten se c o m p re s s iv e fo rc e s. L im b s are seldom
h o riz o n ta l.
(7) P lu n g e fo ld s are form ed w h en the axis o f
th e fo ld in ste a d o f b e in g parallel to the horizontal
p la n e b e c o m e s tilte d a n d fo rm s p lu n g e angle which
is th e a n g le b e tw e e n th e ax is and the horizontal
p lan e.
(8)
b ro ad fo ld
sy n clin es.
also called
Fig. 9.6 : Types o f fo ld s -1, sym m etrical folds, 2. asym ­
m etrical folds, 3. m onoclinal folds, 4. isocli­
nal fo ld s and 5. recum bent folds.
F a n fo ld s r e p re s e n t an ex ten siv e and
c o n s is tin g o f s e v e ra l m in or an ticlin es and
S u ch fo ld re s e m b le s a fan. S u ch feature is
as a n ticlin o riu m o r synclinorium (fig. 9.5).
(9) O p en fo ld s are th o se in w h ich the angle
b etw een th e tw o lim b s o f th e fo ld is m ore than 90^
(1) S y m m etric a l fo ld s are sim ple folds, the
lim bs (both) o f w hich incline uniform ly. T h ese folds
are an exam ple o f open fold. Sym m etrical folds are
form ed w hen com p ressive forces work regularly but
with m oderate intensity. In fact, sym m etrical folds
are very rarely found in the field.
(2) A sy m m e tr ic a l fo ld s are characterized by
unequal and irregular lim bs. Both the lim bs incline
at different an gles. O ne lim b is relatively larger and
the inclination is m oderate and regular w hile the
other lim b is relatively shorter with, steep in clin a­
tion. Thus, both the lim bs are asym m etrical in terms
o f inclination and length.
Closed Fold
(3 ) M onoclinal folds are those in w hich one
lim b in clin es m oderately with regular slo p e w hile
the other lim b lin clin es steep ly at right angle and the
slo p e is alm ost vertical. It m ay be pointed out that
vertical force and m ovem ent are held responsible for
the form ation o f m on oclinal fold s. There is every
O pen Fold
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Fig. 9.7 : (A) Closed fo ld s and (B) open folds.
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EARTWS movement
Pit, ■" '
163
but less than 180° (i.e. obtuse angle between the
limbs o f a fold). Such open folds are formed due to
wave-like folding because o f moderate nature o f
com pressive force (fig . 9 .7 ).
o f overriding nappe, the resultant open structure is
called ‘structural window*. Several examples o f
‘com plete w indow ’ have been discovered in the
eastern A lp s.
(10)
C losed fold s are th o se fo ld s in w h ich the
angle b etw een th e tw o lim b s o f a fo ld is acute angle
Such folds are form ed b eca u se o f in tense com p ressive
force.
Direction of Force
Overturned Lrnt^
Nappes
. N a p p e s a re th e r e s u lt o f c o m p le x fo ld in g
m ech an ism c a u s e d b y in te n s e h o riz o n ta l m o v e m e n t
and re s u lta n t c o m p r e s s iv e fo rc e . B o th th e lim b s o f a
re c u m b e n t fo ld a re p a ra lle l an d h o riz o n ta l. D ue to
fu rth er in c re a s e in th e c o n tin u e d c o m p re ss iv e force
one lim b o f th e re c u m b e n t fo ld s slid e s fo rw ard and
o v errides th e o th e r fo ld . T h is p ro cess is called ‘thrust’
and th e p la n e a lo n g w h ic h o n e p a n o f the fold is
th ru st is c a lle d ‘th ru st p lan e’. T h e u p th ru st p art o f
th e f o ld is c a lle d ‘o v e r th r u s t f o ld '. W h en th e
c o m p re s s iv e fo rc e b e c o m e s so acu te th at it cro sses
th e lim it o f th e e la s tic ity o f th e ro ck b ed s, the lim bs
o f th e fo ld are so a c u te ly fo ld ed th a t th ese break at
th e a x is o f th e fo ld and the lo w er rock b ed s com e
u p w a rd . T h u s , th e re s u lta n t stru c tu re b eco m es re­
v e rse to th e n o rm a l s tru c tu re . D ue to co n tin u ed
h o riz o n ta l m o v e m e n t an d c o m p re ss iv e force the
b ro k e n lim b o f th e fo ld is th ro w n sev eral k ilo m etres
aw ay fro m its o rig in a l p la ce an d o v e rrid e s the rock
b ed s o f th e d is ta n t p la c e . S u ch ty p e o f stru ctu re
b e c o m e s u n c o n to rm a l to the o rig in a l stru ctu re o f the
p lace w h e re th e b ro k e n lim b o f the fold o f the o th er
p lace o v e rrid e s th e ro c k b ed s. S u ch b ro k en lim b o f
a ------------------Overturned Fold
the fo ld is c a lle d ‘n apple' (fig . 9 .8 ).
Fig. 9.8 : Formation o fn a p p le : (A) stage o f overturned
fold, IB) Overriding o f one limb o f the fo ld on
the other limb.
S everal e x a m p le s o f nappe are traceable in
the present fo ld ed m ountains. The nappes o f the
Alps have b een m ore sy stem a tica lly studied. Four
major nappes h ave b een id en tified in the A lp s m oun­
tains. The structure has b eco m e very' m uch com p lex
because o f su p erim p o sitio n o f on e nappe upon an­
other nappe. T he four m ajor groups o f A lp in e nappes
from b elow upward are (i) H e lv etic nappe, (ii) P en ­
nine nappe, (iii) A ustride n ap pe and (iv ) D inaride
nappe. In fact, th ese nappes are located like a series
of earthwaves. In m o st o f the lo c a lities the overrid­
e s nappes h ave been eroded aw ay b ecause o f d y ­
namic w h eels o f denudational p ro cesses and thus
burned basic structure has b een ex p o sed . W hen the
Portion o f lo w er nappe is seen b ecau se o f denudation
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A few exam ples o f nappes have also been
traced out in the H im alayas. T he ex isten ce o f nappes
has been d isco v ere d by W ad ia from K ashm ir
H im alaya, by Pilgrim from S im la H im alaya, by
A uden from Garhwal H im alaya and by H eim and
G ansser from Kumaun H im alaya. It is desirable to
m ention som e facts about nappe structure. W hen the
broken lim b o f a fold overrides the other fo ld near to
the broken fo ld , the resu ltan t nappe is c a lle d
autochthonous nappe. On the other hand, w hen the
lim b o f a fold, after being broken, overrid es the other
fold at 3 distant place (several kilom etres away), the
resultant nappe is called exotic nappe.
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GEOMORPHOLOGY
164
(2)
F au lt dip is the angle betw een the fault
plane and horizontal plane (fig. 9.9).
Crustal Fracture
Crustal fracture refers to displacem ent o f rocks
along a plane due to tensional and com pressional
forces acting either horizontally o r vertically or
som e tim es even in both w ays. C rustal fracture
depends on the strength o f rocks and intensity o f
tensional forces. T he crustal rocks suffer only cracks
w hen the tensional force is m oderate but w hen the
rocks are subjected to intense tensional force, the
rock beds are subjected to dislocation and d isplace­
m ent resulting into the form ation o f faults. G ener­
ally, fractures are divided into (i) jo in ts and (ii)
fau lts. A jo in t is defined as a fracture in the crustal
rocks w herein no appreciable m ovem ent o f rock
takes place, w hereas a fracture becom es fault when
there is appreciable displacem ent o f the rocks on
both sides o f a fracture and parallel to it.
(3) U p th ro w n sid e rep resen ts th e upperm ost
block o f a fault.
(4 ) D o w n t h r o w n s id e r e p r e s e n ts the
low erm ost block o f a fault. S o m e tim es, it becom es
difficu lt to find out, w h ich b lo c k h as really m oved
along the fault p lan e ?
(5) H a n g in g w a ll is th e u p p er w all o f a fault.
(6) F oot w a ll rep resen ts the lo w er wall, of a
fault.
Faults
A fault is a fracture in the crustal rocks wherein
the rocks are displaced along a plane called as fault
plane. In other w ords, when the crustal rocks are
displaced, due to tensional m ovem ent caused by the
endogenetic forces, along a plane, the resultant struc­
ture is called a fault. T he plane along w hich the rock
blocks are displaced is called fault plane. In fact,
there is real m ovem ent along the fault plane due to
w hich a fault is form ed (fig. 9.9). A fault plane may
be vertical, or inclined, or horizontal, or curved or o f
any type and form. T he m ovem ent responsible for
the form ation o f a fault may operate in vertical or
horizontal or in any direction. D uring the form ation
o f a fault the vertical displacem ent o f rock blocks
may occur upto several hundred m etres and ho rizo n ­
tally the rock blocks m ay be displaced upto several
kilom etres but it does not m ean that the total d is­
placem ent occurs at a single tim e. In fact, faultm ovem ent or the displacem ent o f rocks occurs only
upto a few m etres only at a tim e. Fault, in fact,
represents w eaker zones o f the earth w here crustal
m ovem ents becom e operative for lo n g er duration. A
few term s regarding an ideal fault should be u n d er­
stood before going into the d etails o f the m ode o f
form ation o f various types o f faults.
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Fig. 9.9 : Different components o f a fault.
(7)
F a u lt s c a r p is th e steep w all-lik e slope
caused by faulting o f the cru stal ro ck s. S om e tim es,
the fault scarp is so steep th a t it resem b les a cliff. It
may be po in ted o u t th a t scarp s are n o t alw ays form ed
due to faulting alo n e, rath e r th e se are also form ed
due to ero sio n , b u t w h e n e v e r th e se are form ed by
faulting (tecto n ic fo rces), th e se are called ‘faultsc a rp ts.T y p e s o f F a u lts - T h e d iffe re n t types o f fault­
ing o f the cru stal ro ck s are d e te rm in e d by the direc­
tion o f m o tio n alo n g th e fra c tu re p lan e. G enerally,
the rela tiv e m o v e m e n t or d isp la c e m e n t o f the rock
blo ck s or the slip o f th e ro ck b lo c k s o ccu rs approxi­
m ately in tw o d ire c tio n s viz. (i) e ith e r to the direc­
tion o f th e d ip o r (ii) to th e d irec tio n o f the strike o f
(1)
Fault plane is that plane along w hich the
th e fau lt plan e. T h u s, th e d isp la c e m e n t o r movem ent
rock blocks are displaced by tensional and co m p res­
sional forces acting vertically and horizontally to
o f ro ck b lo c k s m ay b e d istin g u ish e d as (a) dip slip
form a fault. A fault plane m ay be vertical, inclined,
m o v em en ts an d (b) strik e-slip m o v em en ts. Thus,
horizontal, curved o r o f any o th er form .
on th e b asis o f th e d irec tio n o f slip o r d isp lacem en t
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EARTH'S MOVEMENT
165
faults are divided in to (i) d ip -slip fa u lts and (ii)
strike-slip fau lts. A g ain , the d isp lacem en t o f rock
blocks m ainly upper b lo c k s m ay be eith er dow n the
direction o f the dip (then the resu ltan t fault is called
Do rm al fault) or up th e dip (the resu ltan t fault
becomes reverse or th ru st fa u lt). In the case of
strike-slip m o v em en t and fault, the relative d is­
placement o f the ro ck b lo ck s m ay be eith er to the
right (then the re su lta n t fau lt w ill be right-lateral or
dextral fault) or to the left side (the resultant fault
becomes left-la tera l or sin istra l fau lt). Strike slip
faults are also calle d as w ren ch fa u lts, tear faults or
transcurrent fa u lts. T h e co m b in atio n s o f normal
and wrench faults or rev erse and w rench faults are
called as ob liq u e slip fau lts.
area. It is, thus, also obvious th at som e sort o f
com pression is also involved in the form ation o f
reverse faults. R everse faults are also called as
thrust faults. Since the reverse fault is form ed due
to com pressive force resulting from horizontal m ove­
m ent and hence this is also called as com p ression al
fault. W hen the com pressive force exceeds the
strength o f the rocks, one block o f the fault overrides
the other block and the resultant fault is called as
overthrust fault w herein the fault plane becom es
alm ost horizontal.
A
(i) N orm al fa u lts are form ed due to the dis­
placement o f both the ro ck blocks in opposite direc­
tions due to fracture co n seq u en t upon greatest stress.
The fault plane is u sually betw een 45° and the
vertical. T he steep scarp resulting from norm al faults
is called fault-scarp or fau lt-lin e scarp the height of
which ranges betw een a few m etres to hundreds of
metres. It m ay be m en tio n ed that it becom es very
difficult to find out the exact height of the faultscarps in the field b ecau se the height is rem arkably
reduced due to c o n tin u ed denudation (fig. 9.10).
(ii) R everse fa u lts are form ed due to the
movement o f both the fractured rock blocks towards
each other. T he fault plane, in a reverse fault, is
usually inclined at an angle betw een 40 degree and
the horizontal (0 d egree). T he vertical stress is m ini­
mum w hile the h orizontal stress is m axim um . It may
be m entioned that in a reverse fault the rock beds on
the upper side are displaced up the fault plane rela­
tive to the rock beds below . It is apparent that reverse
faulting results in the shortening o f the faulted area
while norm al faults cause extension o f the faulted
B
Normal Fault
Fig. 9.10 : (A) Normal fault and (B) reverse fault.
(iii) Lateral or strike-slip faults are form ed
when the rock blocks are displaced horizontally
along the fault plane due to horizontal m ovem ent.
These are called left-lateral or sinistral faults when
the displacem ent of the rock blocks occurs to the left
on the far side o f the fault and right-lateral or
dextral faults when the displacem ent o f rock blocks
takes place to the right on the far side o f the fault (fig.
9.11). In m ajority of the cases there are no scarps in
such faults, if they occur at all, they are very low in
height.
B
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Fig. 9.11 : Formation o f strike-slip or transcurrentfaults- (A) right-lateral or dextral fault and (B) left-lateral or sinistral
fault {after A. Holmes and D.L. Holmes, 1978).
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GEOMORPHOLOGY
166
(iv)
S tep fa u lts- W hen a series o f faults o ccur
S im p le G r a b e n
in any area in such a w ay that the slopes o f all the
fault planes o f all the faults arc in the sam e d irectio n
H /iv. • I
•
—W t j **
V
the resultant faults are called as step faults (fig.
■"
**1
9.12). It is a prereq u isite con d itio n for the form ation
o f step faults that the d o w n w ard d isp lacem en t o f all
IW
"""%
the dow nthrow n blocks m u st o ccu r in the sam e
direction.
•. c / . Vv - * /'- •
•
•
•
•
S im p le H orst
W
*■*. *'1 w -".T*A"**1*
•
t:'? :
w i
'T **
•• .*
‘ • <’
Fig. 9.13 ■ Illustration o f rift valley a n d graben.
A rift v alley m ay be fo rm e d in tw o w ays viz.
(i) w hen the m id d le p o rtio n o f th e c ru s t betw een tw o
norm al faults is d ro p p e d d o w n w a rd w hile the tw o
b locks on e ith e r sid e o f th e d o w n dro p p ed block
rem ain stab le or (ii) w h en th e m id d le portion b e ­
tw een tw o n o rm al fau lts re m a in s stab le and the tw o
side b lo ck s on e ith e r sid e o f th e m id d le portion are
raised upw ard.
N o rm ally , a rift v alley is lo n g, narrow but
very d eep. R h in e rift v alley is th e b e st ex am p le o f a
w ell d efin ed rift v alley . It stre tc h e s fo r a distance of
320 km h av in g an a v e ra g e w id th b e tw e e n the cities
o f B asal and B in g en . T h e o n e sid e o f th is g reat rift
valley is b o u n d ed by V o sg e s an d H a rd t m ountains
(block m o u n ta in s-h o rst) an d th e o th e r side is bor­
dered by B lack F o re st an d O d e n w a ld m ountains.
T h e ex am p le o f the lo n g e st rift v a lle y is the valley
that runs from th e Jo rd o n riv e r v a lle y th ro u g h Red
S ea b asin to Z a m b ezi v alley fo r a d ista n c e o f 4,800
km . A few o f the rift v a lle y s a re so d e e p th at their
b o tto m /flo o r is b elo w the s e a -le v e l. D eath V alley o f
the so u th ern C a lifo rn ia (U S A ) is a g o o d ex am p le of
such g rab en . D ead S ea o f A sia p re se n ts an ideal
ex am p le o f ty p ical rift v alley . T h e flo o r o f the D ead
S ea is ab o u t 867 m b elo w s e a -le v e l. T h e flo o rs o f the
Jo rd o n rift v alley an d D eath V a lle y are also 433 m
Fig. 9.12 : Illustration o f step faults.
Rift Valley and Graben
Rift valley is a m ajo r re lie f feature resulting
from faulting activities. R ift valley rep resen ts a
trough, depression or basin betw een tw o crustal
parts. In fact, rift valleys are long and narrow troughs
bounded by one or m ore parallel norm al faults caused
by horizontal and vertical m ovem ents m otored by
endogenetic forces. R ift v alley s are actually form ed
due to displacem ent o f crustal parts and subsidence
o f m iddle portion betw een tw o norm al faults. R ift
valleys are generally also called as ‘g r a b e n ’ w hich
is a G erm an word w hich m eans a trough-like d ep res­
sion. T hese two term s are syno n y m o u sly used in
various parts o f the w orld. ‘T ensional crustal forces,
literally puling the crust apart, are resp o n sib le for
these dow n dropped fault b lo c k s’ (F. P ress and R.
Siever, 1974) (fig. 9.13). A few scien tists have
attem pted to differentiate a graben from a rift valley
on the basis o f size and d im ension. T hey believe that
a graben is relatively sm aller in size than a rift valley
but this m inor differen ce o f size is not accep tab le to
others. Thus, both the term s, graben and rift valley
should alw ays be co n sid ered as synonym.
belo w sea-lev el. T h e N a rm a d a v a lle y , the D am odar
valley an d so m e stre tc h e s o f th e S o n V alley , the Tapi
valley etc. are c o n sid e re d to b e e x a m p le s o f rift j
v alley s but th is v iew is still c o n tro v e rsia l and is not
acc ep tab le to all g eo lo g ists.
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It may be m entioned that the rift valleys are
not only con fin ed to the continental crustal surfaces
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EARTH'S
167
m ovem ent
but they are also found on sea-floor. In fact, the
deepest grabens are found in the form o f ‘ocean
ijeeps’ and tre n c h e s . T h e B o rtlet T rough located to
the south o f C uba is 4.8 km d eep w hile Java D eep is
6.4 km deep from the sea-floor. T he central plain of
Scotdand. S pencer B ay o f south A ustralia etc. are
examples o f rift valleys.
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crustal blocks. If this process is acceptcd then the
form ation o f the rill valley m ust be follow ed by
volcanic activities because the displaced magma
would try to ascend through the laults. Som e tim es,
the m echanism may be so sudden that there may be
sudden violent volcanic eruption, but the observa­
tions o f several deep rift valleys denote the fact that
rift valley formation is not necessarily alw ays asso­
Origin of Rift V alleys
ciated with volcanic eruptions. The observations
The riddle o f the problem o f the origin o f the
and several experim ents have revealed the lact that
rift valleys and graben s, typical topographic expres­
already existing volcanic activities and active volca­
sions o f faulting, still rem ains a m ystery. Though
noes ceased to operate at the time ol the lorm ation ol
many scientists have propounded their views re­
rift valleys. It might have becom e possible only
garding the origin o f the rift valleys based on their
when the exit o f the ascent ol m agm a would have
studies of respective rift valleys but their concepts
been plugged due to faulting activity. This explana­
and theories are still co n trovercial and no commonly
tion is also refuted on the ground that il wc accept the
acceptable theory could be propounded as yet. The
mode of formation o f a rift valley due to horizontal
hypotheses regardin g the origin o f the rift valleys are
tensional forces and resultant pulling ol bounding
generally grouped in tw o categories e.g. (1) tenfaulted side blocks of two normal faults apart, then
sional h y p o th e sis and (2) co m p re ssio n s! h y p o th ­
the upwelling of magma in the form ol lava cannot
esis.
be stopped, rather the pouring o f lava can be stopped
(1)
T e n s io n a l H y p o th e sis— The earlier hy­
due to com pressive forces. Thus, the tensional hy­
pothesis o f the origin o f the rift valleys was based on
pothesis of the origin o f the rift valleys is rejected on
the basic concept o f the ‘d ro p p e d keystone o f the
this ground.
a r c h ’ o f a building. A ccording to this concept the rift
(2)
Com pressional H ypothesis— In order to
valleys w ere related to the hollow space created by
remove the difficulties o f the tensional hypothesis o f
the dropping of the keystone o f an arch o f a building
the origin of the rift valleys com pressional hypoth­
dow nward. In otherw ords, an open space is formed
esis was postulated by a num ber o f scientists e.g.
at the m iddle portion of an arch o f a building when
Wayland, Baily Willis, Warcn D. Smith, E.C. Bullard
the keystone or keybrick falls dow nward due to
etc. W ayland through his studies o f Lake A lbert and
cracks developed in the arch. Similarly, when two
Ruwenzori section and Baily W illis based on his
parallel cracks develop in the crustal surface due to
studies of Dead Sea have postulated the concept that
tensional forces and when the bounding side blocks
the rift valleys are not formed by tensional forces but
on either side o f the two cracks or fractures are
are formed due to com pressional forces at greater
pulled apart due to tensional forces, the middle
depth. Due to intense com pression the side blocks
portion betw een tw o parallel normal faults moves
are thrown up along the thrust faults in the form of
dow nward and thus an open space is formed. This
horsts. These upthrown blocks are called overopen space becom es a rift valley.
thrusting rift blocks. The m iddle portion is forced
This ‘key sto n e h y p o th e sis’ was severely
to slip downward because o f the pressure resulting
criticized because it was based on erroneous con­
from the rising side blocks. Thus, the dow nw ard
cepts and beliefs. For exam ple, there is wide open
slipping middle portion betw een tw o faults is called
space below the arch o f a building and hence the
as rift block which is narrow upw ard but broader
keystone or keybrick, after the arch develops cracks,
downward.
In other words, the rift block gradually
can easily fall down but there is no open space
broadens out dow nw ard. Thus, the rift valleys are
beneath the crustal rocks and thus there would be
difficulty for the middle block between the two
formed due to slipping o f m iddle block or rift block
parallel normal faults to slip downward. The faulted
dow nward between tw o rising side blocks caused by
middle block can only be slipped downward when it
thrust faulting under the impact o f convergent
would be able todisplace the magma lying below the
com pressional forces.
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GEOMORPHOLOGY
168
S eco n d S ta g e, d ue to the form ation o f a crack
(3)
H ypothesis o f E .C . Bullard— E.C. Bullard,
(at A place, fig. 9 .1 4 ), one po rtio n overrides the
w hile conducting the gravity survey, p o stu lated his
oth
er portion. T his p ro cess is called as ‘th ru stin g .’
new concept o f the origin o f the rift valleys in 1933On the other hand, the second part is throw n down­
34. A ccording to him th e rift block cannot slip
w ard relative to the first part. T h is pro cess is called
dow nw ard under the im pact o f gravity, like a k ey ­
‘d o w n th ru stin g .’ A -C part (fig. 9 .1 4) has gone
stone o f an arch o f a building. T hus, the rift valley
upw ard bccausc of o v crth ru stin g . D ue to upthrusting
can be form ed only due to com pression com ing from
o f the side block (A -C ) upto a h eig ht o f a few
tw o sides. A ccordin g to B ullard the form ation o f a
thousand m etres the dow n th ru st block (A -D ) dev el­
rift valley is not com pleted during a single phase but
ops crack at a place (B) due to resu ltan t com pressive
is com pleted through a series o f sequential phases.
force. The place o f the crack is lo eated at the highest
First Stage, there is com pression in the crustal rock
point
o f dow nthrust block. T his new ly form ed crack
beds o f the rigid part o f a plateau due to active
continues to increase gradually.
horizontal m ovem ent. T he horizontal com pressive
T h i r d S t a g e , th e c r a c k d e v e lo p e d in
forces w ork face to face from both the sides o f the
dow
nthrust
block al B place (fig. 9 .1 4) becom es
land. This lateral com pression causes buckling o f
enlarged due to increased co m p ressio n w ith the
the crustal rocks. As the com pressive forces co n ­
result B-D part o f the dow nthrust block o v errid e s its
tinue to increase, the buckling and squeezing o f the
other part (A-B). Thus, the p o sition o f d o w n th ru st
crustal rocks also continue to increase. W hen the
A-B part betw een the two upthrust blocks (A -C and
com pression becom es so enorm ous that it exceeds
B-D) becom es a rift valley. A -B in fig. 9.14 d enotes
the strength o f the rocks, a crack is developed at a
the width o f the upper portion o f the rift valley.
place (A in fig. 9.14) in the crustal rocks. This crack
A ccording to E.C. B ullard the w idth o f the rift
is gradually enlarged due to continuous increase in
valley (A -B) depends upon the elasticity o f the
the com pressive force.
rocks, depth o f the rift valley and the density o f the
substratum . II the density o f the su b stratum is taken
to be 3.3, then the w idth o f the rift valley w ould be
I)
40
km if the depth o f the valley is 20 km . Sim ilarly,
iiiiiiiiiiifiiiiiii'iiiiiiiiiiiiiiiiiiiiiiiiiiiiiniiimiiiiiiiiiiiiiiiiiii
for a 40-km deep valley the w idth w ould be 65 km.
iiiiiiiiimiiiiiiMiiimiiminniimiiiiiiiiiiintiiniiiHiiniiiiii
iiiiiiiniiiiiiihiiiiiiniiiiiiiiiiitHitiijjjiijjjjjjjjjjjjlljifjjjjjjj}!
It may be concluded that n eith er the tensional
hypothesis nor the co m p ressio n al hy p o thesis could
be able to solve m any o f the in tricate problem s of the
origin o f the rift valleys.
iitiiiiiiiiiiii'jiiiiiiiiiiiiiiimimiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiu
iiiiiiiiiiiinjiiiiiiiiiiiiiiiiiiiim iiim iiiiifniiiim iiiiiiiiiiiniii
It
D
3&
ii 111.1111111111 m im m i n 1 1 1 1n nfTnTTTTTr*
iiiniiiiiimiiiiimiN
iimminniiiiniiMinnni«iinnimnmn»imnnnmniunni.
9.3 EXOGENETIC FORCES
The exogenetic forces or processes, also called
as denudational p ro c e sse s, or ‘d estru ction al forces
o r p ro c e s s e s ’ are o rig in ated from the atm osphere.
T hese forces are co n tin u o u sly en g ag ed in the de­
stru ctio n o f th e r e lie f fe a tu re s c re a te d by the
endogenetic forces through th eir w eathering, ero­
sional and dep o sitio n al activ ities. E x o genetic proc­
esses are, th erefo re, plan atio n p rocesses. Denuda­
tion includes both w eath erin g and erosion where
w eathering being a static p ro cess in clu d es the disin­
tegration and d eco m p o sitio n o f ro ck s in situ whereas
erosion is d y n am ic p ro cess which includes both,
removal o f materials and their transportation to
different destinations. W eathering is b asically o f
* IILLJjj|]TTT|
n
miiiiimiimiiiiiin
n fe?!niinrin7fm^
iiiiiiiiiiiiiiiiiniiMiiiimiiiiiiiiiiiiiiiiifi nnmiiiiiiiiiiiiiiii
Ijlllillthllimiilll IIIiliiiiiliillliiiillllliili liMilllinillllllilll
iHmjtiimiiiBH ninnnm)iinnninmintimnnmnnimm
^iiiiiiiii!n»* m iintiifiiiiiiii!iiiiiiiiinr.” a iu iiiin n i"
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Fig. 9.14 . Formation o f a rift valley according to E C
Bullard.
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EARTH’S MOVEMENT
K9
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three types viz. (i) physical or mechanical weather­
ing, (ii) chemical weathering and (iii) biological
weathering. Weathering is very important for the
biospheric ecosystem because weathering of parent
rocks results in the formation of soils which are very
essential for the sustenance o f the biotic lives in the
biosphere. The erosional processes include running
water or river, groundwater, sea-waves, glaciers,
periglacial processes and wind. These erosional proc­
esses erode the rocks, transport the eroded materials
(except periglacial processes) and deposit them in
suitable places and thus form several types o f ero­
sional and depositional landforms of different
magnitudes and dimensions. The description of the
mechanisms of these exogenetic processes and re­
sultant landforms would be attempted in the suc­
ceeding 14th and 16th chapters of this book.
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:
STRUCTURAL GEOMORPHOLOGY
170-184
G eo m o rp h ic ex p ressio n s o f uniclinal stru ctu re ; to p o g rap h ic ex p ressio n s
o f fau lt stru ctu re (fault g eo m o rp h o lo g y ) ; to p o g ra p h ic ex p ressio n s o f
folded structure (fold g eo m o rp h o lo g y ), in v ersio n or relief, fluvial cycle
o f erosion on folded structure ; to p o g rap h ic e x p re ssio n s o f dom ed
stru ctu re, fluvial cy cle o f erosion on d o m ed stru ctu re.
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CHAPTER 10
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10
STRUCTURAL GEOMORPHOLOGY
m ovem ents, are included in broader term o f tecton ­
ics.
T he in flu en ce o f tectonic m ovem ents and
resultant structural features on landform s is o f so
param ount im portance that several term inologies
sig n ify in g tecton ics-land form s, structure-landform s
relation sh ip s have been floated e.g. ‘geological
geomorphology’ (R.J. C horley, et. al, 1985), ‘struc­
tural geomorphology’ (J. Tricart, 1974), ‘tectonic
lan d form s’ ( A .B lo o m ,
1 9 7 8 ),
tectonic
geomorphology’ etc. The g eo lo g ica l controls o f
landform d evelopm en t have been d iscu ssed in chap­
ter 2 (con cep t 2) briefly but these w ill be elaborated
in this chapter.
E m phasising the sign ifican t role o f structural
features in the d ev elo p m en t o f erosion al landform s
A .L . B loom has m aintained that, ‘It co u ld b e argued
that no subaerial re lie f can occu r until crustal uplift
has raised land ab ove se a -le v e l and that therefore all
subaerial landscapes are “tecto n ic” u n less they are
c o n s tr u c te d b y d e p o s it io n a l ( v o lc a n i c or
sedim entational) p ro cesses. H o w ev er, it is co n v en ­
ient to restrict the term to th ose lan d form s that are
su fficien tly undissected by erosion so that the shape
o f the fractured or deform ed surface can be d is­
cerned. A ll d egrees o f transition are found betw een
purely tectonic and totally erosional landform s’ (A.L.
B lo o m , 1978). B ut here w e are not con cerned with
either pure tecton ic landform s (w h ich m ay not be
older than Quaternary as m ost o f the tecton ic fea­
tures o f the past have been greatly m od ified by
denudational p ro cesses) or pure denudational (ero­
sion al or d ep o sitio n a l) landform s rather w e are con­
cerned w ith geom orp h ic ex p ressio n s of tectonic
m ovem en ts and resultant structural features, say
disposition of rocks (su ch as tabular or horizontal,
B efore describ ing the association s betw een
tectonics and landform s and structure and landform s
it is necessary to explain a few term s related to this
aspect o f geom orp h ology. A ccord in g to C .D . O ilier
(1981) ‘tectonics is concerned w ith the form , pat­
tern and evolu tion o f the globe's major features such
as m ountain ranges, plateaus, fold belts and island
arcs. Structural geology concerns sm aller struc­
tures such as anticlines, faults and joints. Tectogenesis
m eans the study o f d eform ation .’ J. Tricart (1974)
divided tecton ics into tw o categories e.g . tectostatic
and dectodynamic types. ‘Tectostasy refers to the
actual d isp osition o f existin g strata (tabular, faulted
or folded) and tectodynamism to the deform ations
that the rocks underwent at the given tim e p eriod’ (J.
Tricart). Thus, both disp osition o f actual strata and
subsequent deform ation s by earth's en d o g en etic
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uniclinal, faulted, domal, folded etc. structures) in
response to denudational processes. Mode of gen­
esis, nature and ch aracteristics of pure tectonic fea­
tures resulting from diastrophic movements have
been described in the preceding chapter (9).
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171
STRUCTURAL GEOM ORPHOLOGY
J .T ric a rt( 1974) has rem a rk ed that, ‘the p ro c­
e s s e s of d issectio n , w h atev e r th e clim ate, are in flu ­
enced by the n atu re an d d isp o sitio n o f the rocks, and
by the general te cto n ic ev o lu tio n o f any given re­
gion. M o rp h o clim atic ero sio n is su b o rd in ate to re­
lief produced by stru ctu re , and this su b o rd in atio n is
partly a m atter o t s c a le .... In g en eral, it m ay be said
that structural in flu en ces p red o m in ate w hen an area
is viewed on a sm all scale, and m orphoclim atic
influences w hen it is seen on a larg er scale’ (J.
Tricart, 1974).
•»
10.1 G EO M O R PH IC
E X P R E S S IO N S
UNICLINAL STRUCTURE
stream s d evelop on the less re sista n t ro ck s. T h u s,
lines o f asym m etrical cu esta featu res h av in g steep e r
landw ard facing scarp slo p es and g e n tle r seaw ard
facing dip slopes are fo rm ed p arallel to the co ast
lines (fig. 2.10). T rib u taries jo in th e m a ster c o n se ­
quent or strike stream s alm o st at rig h t angle. T h e
stream s flow ing dow n th e d ip slo p e s are called d ip
s tre a m s w hile the stream s flo w in g in an ti-d ip d ire c ­
tion are called a n ti- d ip s tr e a m s (fig. 10.1). It m ay
be pointed out that dip stream s d rain on resistan t
rock beds w hile anti-dip stream s are d e v e lo p e d on
less resistant (soft) rocks. T h e rela tiv e le n g th s o f dip
and anti-dip stream s d ep en d on the an g le o f d ip p in g
strata. If the dip angles are rela tiv e ly g en tle, th e
slope lengths becom e lo n g er and h en ce stream s
draining on dip slope (dip stream s) are o f lo n g e r
lengths than the trib u taries d rain in g in o p p o site
direction (anti-dip stream s). T he d rain a g e d en sity on
dip slope and anti-dip slope is also v aria b le as a n ti­
dip side o f the ridges is ch aracterized by c lo sely
spaced stream s o f relatively sh o rter len g th s (h ig h e r
density) w hile relatively low d rain ag e d en sity due to
relatively o f longer length b u t w id ely sp aced stream s
OF
U niclinal or h u m o clin al stru ctu res are those
which rep resen t in clin ed rock strata (o f sedim en­
tary) at u niform dip an g le cau sed by general regional
tilt. ‘T hese stru ctu re s are form ed in tw o main w ays,
either by the u p lift o f a seq u en ce o f off-lapping
coastal plain sed im en ts or as part o f one lim b of a
large dom e or fo ld ’ (R .J. C horley et. al, 1985).
A ccording to R.J. Sm all (1970), ‘U niclinal struc­
tures (so m etim es referred to as ‘h o m o clin aF ) are
those in w hich a g en eral regional tilt has been given
by gentle earth m o v em en ts to the co n stituent ro ck s’.
Such stru ctu res in v o lv e both resistan t and soft rocks
and som e tim es th ere are altern ate bands o f soft and
resistant rocks and hence these are subjected to
d iffe re n tia l e ro s io n w h erein resistan t rocks are less
eroded than soft rocks.
S t r i k e Strtam
T he d iffere n tial ero sio n o f dipping strata o f
varying resistan c e gives birth to tre llis d ra in a g e
pattern and a few typical to p o g rap h ic features such
as s c a r p and v ale to p o g r a p h y , c u e s ta and h o g b a c k
ridges etc.
R ivers form th eir v alleys along soft rock beds
due to co m p arativ ely m ore erosion than the resistant
rock beds giving birth to the form ation o f s tr ik e
vales (fig. 2.10, ch ap ter 2) w hile resistant rock beds
are less eroded and hence bccom e lines o f asy m ­
metrical ridges or hills know n a s c u e s ta s having one
side of steeper scarp slopes w hile opposite side
represents gentle slope. H om oclinal structure form ed
due to general tilting o f sedim entary beds o f coastal
plains and retreat of sea w ater presents ideal co n d i­
tion for the developm ent o f trellis drainage pattern
having consequent and subsequent stream s, The
consequent stream s drain seaw ard across resislanl
and weak rock beds alike but the lateral consequent
Fig. 10.1 : D evelopm ent o f structurally controlled
streams on dipping strata, after C.D. Oilier,
1981
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on dip slope. It is, thus, ev id en t th at ‘the stru ctu ral
control ot tilted strata im poses a p o w erfu l asy m m e­
try on drainage n etw orks. E scarp m en t stream s are
steep, short and have h ig h grad ien ts. D ip slo p e
stream s are likely to have m ore g entle grad ien ts,
larger w atersheds, m ore trib u taries, and m o re su s­
tained flo w ’ ( A .B loom , 1978). It m ay be poin ted out
that due to d ifferen tial but co n tin u ed ero sio n the
m aster stream s d ev elo p ed betw een tw o cu esta s (fig..
10.2) m igrate laterally fo llo w in g the d irec tio n o f d ip
slope. ‘T he en tire ridge and valley sy stem m ig rates
laterally as w ell as d o w n w ard w ith tim e in a p ro cess
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172
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GEOMOR PHOLOOY
termed hom oclinal shifting (m onoclinal shifting
Cuesta is the m ost significant landform re­
sulting from continued erosion o f uniclinal/homiclinal
sedim entary structures alternated by resistant and
soft rock beds. ‘A s in the case o f m any summits in
fold ed rocks, cuesta landform s are a half-inverted
relief. In essen ce they d ev elo p in tabular, weakly
dipping beds under the action o f differential dissec­
tion w hich erodes the w eak beds on the high points
o f folds, the resistant beds at a low er structural level
persistin g’ (J.Tricart, 1974).
by G .K .G ilbert, 1877) (A .L . B lo o m , 1 9 7 8 )’.
Resistant
Rock
p re sen t
S u rf a ce
A s regards the m orphology o f cuestas, they
vary greatly spatially depending on local conditions,
‘but in their sim p lest form they com p rise a steep
scarp face, often exceed in g 30° in an gle and som e­
tim es displaying bare rock faces, and a lon g and
gen tle d ip -slo p e (o cca sio n a lly referred to as a
‘back slop e’ when the gradient o f the surface does
not exactly con cide with the angle o f d ip )’ (R.J.
Sm all, 1970).
F utur e
Surface
Homoclinal Shif t
Fig. 10.2 : D evelopment o f asymmetric drainage on
humoclinal strata and homoclinal shifting o f
ridge crests and valleys— after A.L. Bloom,
1978).
Fig. 10.3.
There is also variation in the d im en sion (scale)
and form (shape) o f cuestas. The continuity o f cuestas
is maintained w hen the anti-dip stream s are not
eroding actively but it is broken w hen these actively
Development o f double cuestas (escarpments)—after J. Tricart, 1974.
erode the cuestas (escarpm ents) resulting in the
developm en t o f num erous em baym ents. K aim urhill
ranges and m argins o f Bhander plateau (M .P .) hav­
ing sandstone capping and alternate bands o f vary­
in g com binations o f sh ales, sandstones and lim e­
stones present fin e exam p les o f continuous and
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d isc o n tin u o u s c u e sta s p u n ctu ated by frequent
em baym ents.
J. Tricart has described tw o d istin ctive types,
b esides a general sim p le type, o f cu estas e.g. twin
cuestas and double cuestas. ‘T w in cuestas appear
w hen, in order to reach the subjacent weaker sub-
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structural g e o m o r p h o l o g y
173
Stratum, the s tr e a m b e c o m e s fairly d eep ly incised
into the b ack slop e. T h is prod u ces tw o parallel asvm
metrical slo p es co m p o sed o f the sam e strata the
scarp face proper and the slo p e o f the d evelop in g
valley w hich fa ces u p s lo p e .... T w in cuestas m ust
not be co n fu sed w ith double cuestas w hich are
sim ply tw o superposed cuestas (fig . 1 0 .3 ), sin g le
slope being m ade up o f tw o pairs o f beds. The
existen ce o f a double cuesta im p lies differential
scarp retreat’ (J. Tricart, 1974). T he progressive
dissection o f twin cuestas results in the form ation o f
Fig. 10.4 : D evelopm ent o f butte due to dissection o f cuesta (scarp)—after J. Tricart, 1974.
isolated flat-top p ed (by resistant caprock) buttes
(fig. 10.4) T h e escarp m en ts or ridges having sym ­
m etrical slo p e s on both sid es are called hogback
ridges or sim p ly hogbacks.
developm ent o f ‘con cave profile o f a cuesta, w ith a
w ell marked escarpm ent crest in the resistant bed
and long regular slopes with a parabolic curvature in
the weak bed’ (fig. 10.4).
(2) Dip angle o f the resistant cap-rock con ­
trols the height o f cuestas. Gentle dip angles (less
than 5°) o f rock beds are associated w ith cuestas w ith
ing factors—
height w hile greater dip an gles produce lo w
(1)
Lithological factors— T w o aspects greater
of
cuestas.
It may be m entioned that the height o f
lithology viz. (a) relative thickness o f rock beds in
cuestas is determined by the m ode o f d issection and
general and o f caprock in particular and (2) varia­
dow nw asting w hich is controlled by dip angles.
tions in the relative resistance o f rock strata are
W hen the dip angles exceed 45°. the cuestas have
important. T he relative thickness o f caprock and
sym m etrical slopes on both sides and thus grade into
underlying beds determ ines the nature o f cuesta
hogbacks. ‘The dip o f cuesta form ation has also
profile, and relative altitude. Thicker beds o f caprock
been shown to influence the m orphom etry o f its dip
generally produce b old and high cuesta. It is not only
slope and on C linch m ountain, a cu esta o f quartzite,
the thickness o f the resistant caprock but also the
sandstones and shales in the fold ed A pplachians,
thickness o f underlying weak rock strata w hich also
stream lengths, basin areas and hypsom etric inte­
control the height o f cuesta because the thicker the
grals bear significantly negative relationships to the
underlying w eak rock strata, the greater the d issec­
dip
w hich varies from less than 20° to more than 60°’
tion at the foot o f the scarp and hence higher w ill be
(R.J. Chorley et. al, 1 9 8 5 ).’
entire cuesta. The resistance or durability o f caprock
determines the nature and m agnitude o f dissection.
(3) The am ount o f scarp retreat (recession)
The relative resistance o f rock strata (resistant caprock
determ ined by the nature and rate o f m assm ovem ent
and weak underlying beds) favours differential ero­
on the cuesta slope, spring sapping, d issection by
sion wherein underlying weak rock beds arc eroded
streams at the foot o f the scarps, w eathering at scarpmore than the overlying caprock resulting in the
foot etc. determ ines the developm en t o f scarp-vale
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T he h eigh t, d im en sio n , reliefs and cross-sec­
tional form s o f cu esta are controlled by the fo llo w ­
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T he tectonic expressions (reliefs) of faulting include different typ es of fault scarps e.g. (t)
original or active fault scarps, (2 ) residual fault
scarps and (3 ) co m p o site fault scarps. T he scarps
representing the fault plane o f upthrown block is
called original or active fault scarp. It m ay be
pointed out that the tecto n ic reliefs or tectonic ex­
p ression s o f faults are the direct result o f faulting
activity in v o lv in g relative d isp la cem en t of crustal
rocks. ‘B y d efin ition , all a ctiv e fault scarps are
original, so it is not necessary to add the adjective
‘a ctiv e’ (J. Tricart, 1974). On the other hand, a
residual fault scarp is that w h ich is form ed after the
form ation o f original or a ctiv e fault scarp and the
renew al o f faulting activity after a period o f no
tectonic activity (period o f q u iescen ce). S o m e scien ­
tists m aintain that residual scarps are denudational
as they are form ed after erosion during period o f
relative calm (q u iescen ce). ‘D uring the a ctiv e p e­
riod, the scarps m ay be the faults fu n ction as true
fault scarps, w h ile during the q u iet periods erosion
converts these into residual scarp s’ (J. Tricart, 1974).
If the tecton ic activity is reactivated, fresh scarp is
generated b elo w residual fault scarp due to upward
m ovem ent o f upthrown b lock , thus the resultant
entire scarp is ca lled composite fault scarp. ‘A
co m p o site fault scarp is thus a scarp due to a fault
that has been interm ittently a ctiv e, so that the forms
o f erosion have varied b etw een th ose associated
w ith active fault scarps and th ose o f residual fault
scarps’ (J. Tricart, 1974).
topography in a region characterized by uniclinal
structures. Besides, uniclinal shifting of streams in
down-dip direction results in the undercutting of
scarp base which accentuates cuesta profile.
(4) Long continued erosion results in the
b ev ellin g o f p reviously form ed cuestas in a scarpand-vale topography. R.J. S m all has observed that,
‘In an area o f h eterogen eou s gen tly dipping rocks
w h ich has recently been planed by erosion and then
a ffected by lim ited stream in cision , all the escarp­
m ents w ill d isp lay sum m it b ev els and, irrespective
o f rock th ick n ess, durability (resistan ce) or angle o f
dip, w ill reach approxim ately the sam e elevation s.
W ith the p assin g o f tim e, h ow ever, th ese latter
factors w ill reassert th em selv es, and diversification
in the form and h eigh t o f the individual cuesta w ill
gradually o ccu r’ (R.J. Sm all, 1970).
1 0.2 TOPOGRAPHIC EXPRESSIONS OF FAULT
STRUCTURE (FAULT GEOMORPHOLOGY)
A fault is a fracture in the crustal rocks wherein
the rocks are d isp laced along a plane called as ‘fault
p la n e’ (fig . 9 .9 ). In other w ords, w hen the crustal
rock s are d isp laced due to tensional forces caused by
the en d o g en etic m o v em en ts alon g a plane, the re­
sultant structure is ca lled a fault. In fact, ‘faulting
in v o lv e s d ifferential m ovem en t o f strata on either
sid e o f fau lt-p lan e (in v o lv in g a sin g le plane o f shear­
in g) or fau lt-zon e (in v o lv in g a num ber o f clo sely
spaced fau lt-p lan es) as a result o f either com p res­
sion al or tensional forces in the earth's crust. The
differential m ovem en t m ay be upwards, dow nw ards,
horizontal, ob liq u e or even rotatory’ (R.J. Sm all,
T he study o f fault geomorphology in v o lv es
3 a sp ects o f faulting e.g . (1) types o f d isp lacem ent o f
rock b lo ck s and thus reusltant fault types, (2) tec­
ton ic ex p ressio n s o f faulting and (3 ) geom orphic
e x p ressio n s o f faulting.
G eom orph ic ex p ressio n s resu ltin g from dif­
ferential erosion o f fault scarps and upthrown and
dow nthrow n fault b lo ck s in clu d e d ifferen t types o f
fault-line scarps e.g . (1 ) norm al or consequent
fault-line scarps, (2) reversed or o b seq u en t or oppo­
site fau lt-lin e scarps, (3 ) reseq u en t fau lt-lin e scarps,
(4) subdued fa u lt-lin e scarps, (5 ) ex h u m ed fault-line
scarps, (6 ) exaggerated fa u lt-lin e scarps etc.
B ased on d ifferen t typ es o f m o v em en ts, as
referred to a b ove, d ifferent typ es o f faults are cre­
ated in the crustal rocks viz. normal and reverse
faults, (fig . 9 .1 0 ), lateral or strike-slip faults (fig.
9 .1 1 , a lso know n as transverse, tear or transcurrent
fau lts) d ivid ed in to tw o su b typ es— right lateral or
dextral fault and left lateral or sinistral fault, step
fau lts (fig . 9 .1 2 ) etc., the characteristic features and
m o d e o f form ation o f w h ich have been d iscu ssed in
the p reced in g chapter (9).
(1)
Normal or original fault-line scar
know n as con seq u en t fault scarp is form ed due to
erosion o f w eak rocks o f d ow n th row n b lock s. Such
fau lt-lin e scarps are oriented tow ards the direction
o f original fault scarps (fig . 1 0 .5 (1 )). T h is type o f
fau lt-line scarps results due to p rolon ged erosion o f
less resistant beds o f dow n th row n b lock w hen the
p rocess o f the form ation o f faults has practically
cea sed and fault rem ains in a ctiv e for lon g period of
tim e.
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1 970).
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Normal Original
Fault Scarp
ime Stone
opposed
Fig 10 5 • D evelo p m en t o f different types o f fa u lt line s c a r p s -( 1) normal or original fa u lt scarp, actual fa u lt indicated
by A -B is concealed under scree cover derived through the erosion o f fa u lt scarp ; (2) dissection o f original
fault scarp due to prolonged erosion resulting in the segmentation o f scarp fa ces— s 1, s2, s3, s4 an d thinning
o f m arl cover on downthrown block; (3) opposed or reversedfault-line scarp developed on dow nthrow n block,
and separation o f original fa u lt scarps (b u ttes)-a fter J. Tricart (1974, slightly modified).
opm ent than a con seq u en t scarp, th ou gh th is is not
invariably the ca se . .. the reversal o f the fa u lt-lin e
scarp is p o ssib le on ly b eca u se a fall in b a s e -le v e l has
exp osed to denudation the w eak rocks on the upthrown
sid e o f the fau lt’ (R.J. S m a ll, 1 970). ‘S u ch o p p o se d
fault scarps are a lw a y s due to lith o lo g ic a l con trol o f
denudation and in the nature o f th in g s th ey are fau ltline scarp s’ (J. Tricart, 1 9 7 4 ). It m ay b e m en tio n ed
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(2) O p p o s e d f a u lt- lin e s c a r p s are also known
as reversed or ob seq u en t scarps w hich d ev elo p in
opposite direction to the original fau lt-line scarps
due to no further fau ltin g and erosion o f w eaker
strata o f upthrown b lo ck s o f the faults. Such faultline scarp s are fo rm e d at m uch later date at relatively
low er height (fig . 10.5 (3 )). ‘A n obsequent fault-line
scarp w ill norm ally re p re s e n t a later stage o f d ev el-
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that it is not necessary that before the form ation o f
ob seq uent fau lt-line scarps the original or normal
fault scarps are le v elled down due to continued
erosion. T he on ly condition is that the fault has
b ecom e in active and the w eaker rocks have been
su fficien tly exp osed due to su fficien t recession o f
original scarp so that stream s m ay excavate their
v a lley s on the exp osed w eaker rocks (as is seen in
fig. 10.5 (3 ) w here marl bed lying under lim eston e
cover has been su fficien tly exp osed as the original
scarps have receded too far and new stream has
eroded the marl outcrop at the ed ge o f original fault
scarp).
(3 ) R e s e q u e n t fa u lt-lin e sc a r p s are form ed
due to renew ed dow nw ard erosion caused by further
fall in b a se-lev el o f erosion. In fact, resequent scarps
result from the reversal o f obsequent scarps and are
oriented in the direction o f the original or normal
(con seq u en t) scarps but are m uch older than the
latter (fig. 2.8 (3).
(4) C o m p o site fa u lt-lin e s c a r p s are those
w hich o w e their origin partly due to faulting and
partly due to erosion. T h ese represent tw o situations
viz. (i) upper portion o f fault scarp due to faulting
and low er portion form ed by erosion , and (ii) upper
portion form ed due to erosion and low er portion o f
fault origin. A ccord in g to C .A . C ctton such faultlin e scraps are form ed w hen fault activity b ecom es
inactive and dow nthrow n b lock having greater thick­
n ess o f relatively w eaker form ation is eroded dow n
to con sid erab le depth, with the result original fault
scarp is exten d ed dow nw ard. T hus, the resultant
fau lt-line is characterized by upper faulted segm en t
and lo w er eroded segm en t. A ltern atively, fault scarp
is form ed due to faulting (fig . 10.6 (1 )). A fter pro­
lon ged erosion original fault scarp disappears and
the faulted region is le v elled (fig . 10.6 (2 )). Fall in
base level renew s vigorou s erosion o f d ow nthrow n
b lock and thus is form ed resequent fa u lt-lin e scarp
(fig. 10.6(3). Fault again b eco m es active and the
Fig. 10.6 , Stages o f the fo rm a tio n o f com posite faultline scarps—(1) fo rm a tio n o f original fa u lt
scarp, (2) obliteration o f fa u lt scarp due to
erosion, (3) form ation o f resequent fa u lt—
line scarp due to renew ed erosion, and {4)
form ation o f fa u lt scarp due to fu r th e r fault­
ing— based on C.A. Cotton).
downthrow n block is further thrown dow nw ard along
the original fault plane and thus the resultant faultlin e scarp b ecom es co m p o site the upper part o f
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w hich is erosional w h ile the lo w er part is faulted
(fig . 10.6(4).
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STRUCTURAL g e o m o r p h o l o g y
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177
<5)I, ReSUZ * Cted 0r
fault-line
g O j f S — It m ay be p oin ted out that in so m e situadons the fault scarp s, after the fault becomes inac­
tive, are eroded d ow n to su ch ex ten t that the low er
portion o f the fault scarp is buried under thick cover
o f eroded m aterials (fig. 10.7(2)). T he renew ed
erosion o f deposited materials uncovers the buried
fault scarp w hich is called as exhum ed or resur­
rected fault scarp (fig. 10.7(3)). ‘E xhum ed fault
scarps w hich are but a variety o f the faultline scarp,
are usually subdued features o f m uch sm aller d im en­
sion than the throw o f the fault’ (J. Tricart, 1974).
J. Tricart has opined that, ‘O ne important
factor controls the evolution o f all faultline scarps,
and that is the relation betw een the throw o f the fault
and the thickness o f the hard and so ft strata...
faultline scarps present one other d ifferen ce from
original fault scarps. S in ce they are product o f d if­
ferential erosion, they can on ly occu r w here the
rocks offer sharp contrasts in resistan ce, as on the
continental platform s’ (J. Tricart, 1974). P rolonged
erosion o f graben results in inversion o f relief w herein
Fig. 1 0 .8 :
Stages o f inversion o f r e lie f in a graben.
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Fig. 10.7: Stages o f the form ation o f resurrected faultline scarp : I. form ation o f original fa u lt
scarp, 2. fa u lt scarp covered under eroded
materials, 3. reappearance o f fa u lt scarp due
;
to removal o f deposited materials through ’
renewed erosion.
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GEOMORPHOLOGY H
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178
' ~ 'r'
primary horsts are eroded dow n w hile original rift
va lley is less eroded and hence the valley rises above
the eroded horsts thus inversion o f relief is the result
fold ed structure is the d e v e lo p m e n t o f in v ersio n o f
r e lie fs i.e. in v e r te d r e lie f characterized by anticli.
n al v a lle y s and s y n c lin a l rid g e s.
(fig. 10.8).
Inversion of Relief
10.3 TOPOGRAPHIC EXPRESSIONS OF FOLDED
STRUCTURE (FOLD GEOMORPHOLOGY)
I n v e r s io n o f r e lie f in fold ed structure is an
im portant but unique p h en om en on w hich causes
reverse seq u en ce o f top ographic features. Inversion
o f re lie f occu rs in the fo ld ed structure having sym ­
m etrical fold s h avin g alternate seq u en ce o f anti­
clin es and sy n clin es and sim p le form ation (fig.
10.9). W ith the initiation o f flu v ia l ero sion under the
p rocess o f c y c le o f ero sio n after the folding of
sedim entary rocks lo n g itu d in a l m a s te r co n seq u en t
s tr e a m s (s tr ik e s tr e a m s ) and tributary consequent
stream s fo llo w in g slo p e d irection are originated in
the sy n clin es and dip slo p e s o f the a n ticlin es respec­
tively. T he m aster co n seq u en t flo w s in the syncline
from higher slo p e tow ards le sser gradient. The
stream s origin atin g on the flanks o f the anticlines
(dip slo p es) jo in the m aster c o n seq u en ts as tributary
stream s. T h ese tributaries are ca lled as tra n sv erse
c o n s e q u e n ts or la te r a l c o n s e q u e n ts w h ich develop
their v a lley s through headw ard ero sio n o f the anti­
clin es. W ith m arch o f tim e the crests o f anticlines
are breached and s u b s e q u e n t s tr e a m s d ev elo p along
the axes o f a n ticlin es. T h e se su b seq u en t streams
con tin u e to d eep en their v a lle y s d ue to m axim um
vertical erosion o f an ticlin al crests b eca u se o f m axi­
m um tension on crests w ith the resu lt synclinal
m aster con seq u en t stream s are e lim in a ted and anti­
clin al stream s b e co m e m aster stream s. T h is process
results in the form ation o f v a lle y s in the place of
a n ticlin es and rid ges in the p la ce o f s y n c lin e s. Thus,
Sedim entary rock beds are squeezed and buck­
led and fold ed into an ticlin es and syn clin es due to
lateral com p ressive forces. T he folded structure
ranges from sim p le fold s (figs. 9.2 and 9 .3 ) to
co m p lex fo ld s (i.e. recum bent fold s) depending on
intensity o f co m p ressiv e forces. Sim ple folded struc­
ture is characterized by sequ en ce o f anticlines and
sy n clin es (fig. 9 .2 ).
The g e o m e tr y o f folded structure includes
an ticlin e, syn clin e, lim bs, axis o f fold or axial plane,
ax is o f syn clin es, dip, strike etc. T he upfolded rock
strata in arch-like form are called a n tic lin e s w hile
the d ow n folded structure form ing trough-like fea ­
ture is called s y n c lin e (fig. 9 .3 ). The tw o sid es o f the
fold are called lim b s o f the fold. The plane w hich
bisects the angle b etw een the tw o lim bs o f the
anticline or the m id d le lim b o f the sy n clin e is called
the a x is o f fo ld or a x ia l p la n e (fig . 9 .2 ). On the basis
o f anticline and sy n clin e these axial planes are called
as a x is o f a n tic lin e and a x is o f s y n c lin e resp ec­
tively. The inclination o f rock beds with respect to
horizontal plane is term ed as d ip (fig . 9 .4 ) w h ile ‘the
s tr ik e o f an in clin ed bed is the direction o f any
horizontal line along a bedding p la n e’ (A . H olm es
and D .L . H olm es). A n tic lin o r iu m refers to those
fo ld ed structures in the regions o f folded m ountains
w here there are a series o f m inor an ticlin es and
sy n c lin e s w ithin on e ex ten siv e anticline (fig . 9 .5 )
w h ile s y n c lin o r iu m represents such a fold ed struc­
ture w h ich in clu d es an ex ten siv e sy n clin e havin g
num erous m inor an ticlin es and sy n clin es. F old s are
o f d ifferent typ es viz. sym m etrical fo ld s, a sy m ­
m etrical fo ld s, m on o clin a l fo ld s, iso clin a l fo ld s,
recum bent fo ld s, overturned fo ld s, plu n ge fo ld s, fan
fo ld s, open fo ld s, c lo se d fo ld s etc. w h ich h ave al­
ready been d iscu ssed in the precedin g chapter (for
d etails se e chapter 9, fig s. 9 .6 , 9 .7 and 9 .8 ).
the p reviou s top ograp h ic featu re (fig . 1 0.9 vl and 2))
o f origin al a n ticlin es and s y n c lin e s are reversed by
the form ation o f s y n c lin a l r id g e s (in place of
original a n ticlin es) and a n t ic lin a l v a lle y s (in the
p lace o f origin al a n tic lin e s, fig . 1 0 .9 (5 )) due to
p ro lo n g ed d en u dation and th e p r o c e ss o f inversion
o f r e lie f is co m p leted .
Fluvial Cycle of Erosion on Folded Structure
I n itia l S ta g e — T h e fo ld e d structure, here,
im p lies norm al structure ch aracterized by regular
arrangem ent o f alternate a n ticlin es and sy n clin es. In
other w o rd s, fo ld e d m ou n tain is co n sid ered to have
b een form ed d ue to fo ld in g o f sed im en tary rocks by
c o m p r e ssiv e fo rce. S u ch structure is sim p le and is
F o ld g e o m o r p h o lo g y in clu d es the d e v e lo p ­
m en t o f drainage pattern and topographic features
d u e to d enudational p ro cesses on fo ld ed structure.
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O n e o f th e resultant features o f p rolon ged ero sio n o f
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s
I
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STRU CTU RA L G E O M O R P H O L O G Y
179
Fig. 10.9 : Stages o f inversion o f relief.
streams begins w ith the upliftm ent and folding o f
rocks. It is hypothesised that the region after folding
remains stable for long g eo lo g ica l period and thus
the cy cle o f erosion passes through su ccessiv e stages
o f youth, mature and old resulting in the sequential
changes in landform s through tim e.
ch aracterized b y o p en fo ld s an d a b se n c e o f recu m
bent fo ld in g , o v e rth ru s t fo ld s, n ap p es and th ru sts
T here is re g u la r a rra n g e m e n t o f a n tic lin e s an d
synclines which are devoid o f com p lexity.
e ° e
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strata in c lu d e b e d s o f re sis ta n t an d w eak ro ck s
Fluvial e ro sio n w ith th e in itia tio n o f co n seq u en t
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Limestone
sy n clin e
Fig. 10.10: Inversion o f relief— after J. Tricart, 1974
o f the a n ticlin es (d ip s lo p e s ) . L a teral con seq u en ts
ex ten d their c o u r se s u p s lo p e th ro u g h h ead w ard ero­
sio n and e sta b lish th eir v a lle y s o n a n tic lin a l a x es and
form g o r g e s. L ater o n , strea m s a ls o d e v e lo p on the
anticlinal a x es, w h ich are c a lle d as su b seq u en t streams.
T h e head w ard e r o sio n at th e a n tic lin a l cre sts results
in river capture w ith the r e su lt s e v e r a l s m a ll stream s
d e v e lo p e d on a n ticlin a l cr e sts are in tegrated and
an ticlin al a x ia l strea m s f o llo w in g th e strik e d irec­
tion d e v e lo p at the a n tic lin a l c r e sts ( A stream on fold
3 in fig . 1 0 .1 1 ). T h e p r o c e s s o f r iv er cap tu re co n tin ­
u es and all the tra n sv erse (la ter a l) strea m s are cap­
tured and the se c o n d m a ster strea m s d e v e lo p at the
anticlinal a x es and flo w p arallel to th e origin al m aster
syn clin al stream s. T h e se stream s are c a lle d as subse­
quent stream s (S stream on fo ld 4 in fig . 1 0 .1 1 ), w hich
d eepen their v a lle y s at the a n ticlin a l crests and try to
adjust them w ith the u n d erly in g fo rm ation s.
Y o u th fu l S ta g e — C onseq u en t stream s o rig i­
nate on the fo ld s in clu d in g both a n ticlin es and
sy n clin es. M aster con seq u en t stream s o r i g in a t e d
the syn clin al troughs. T h ese are ca lled s y n c lin a l or
lo n g itu d in a l c o n s e q u e n ts , the channel gradient o f
w h ich is determ ined by the slo p e o f sy n clin es. C o n ­
sequent stream s also originate on the dip slo p e s o f
the anticlines and jo in the m aster consequent synclinal
stream s as tributaries, w h ich are a lso ca lled as t r a n s ­
v e r se or la te r a l c o n s e q u e n t s tr e a m s . A l these
streams flo w d ow n the slo p e o f the structure and thus
fall under the category o f s e q u e n t s tr e a m s . In fig.
10.11 A stream d en otes m aster co n seq u en t w h ile B
and C represent lateral or transeverse co n seq u en t
tributary stream s. T he n ew ly esta b lish ed stream s
start to erode their valleys. Lateral consequent stream s
(B and C ) erode at faster rate than the m aster c o n s e ­
quent (A ) b ecau se o f the steeper slo p es o f the flanks
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Fig. 10.11: D evelopment o f flu via l cycle o f erosion on fo ld e d structure (after Von Engeln).
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STRUCTURAL GEOMORPHOLOGY
1ii
Mature Stage— The o n s e t o f m a tu re stag e is
heralded by a c c e le r a te d ra te o f v alley d e e p e n in f b v
stream s d e v e lo p e d o n a n tic lin a l crests. T h e su b se
qUent s tre a m s (S o n fo ld 5 in fig. 10 .1 l)or a n ticlin ai
streams e ro d e th e ir v a lle y s m o re th an m a ste r synclinal
consequent s tre a m s (A in f i g . 1 0 .H ) b e c a u se (i) the
anticlinal s tre a m s a re re la tiv e ly at h ig h e r h e ig h t and
have ste e p e r c h a n n e l g ra d ie n t th a n sy n c lin a l stream s
and (n ) s o ft r o c k b e d s u n d e r re s is ta n t c a p -ro c k o f
anticlines a re r e la tiv e ly at h ig h e r h e ig h t th an in the
syncline. T h u s , th e s o f t ro c k b e d s o f th e an ticlin es
are ero d ed m u c h b e fo re th e s o ft ro c k b ed s o f the
synclines. C o n s e q u e n tly , a n tic lin a l stream s d eep en
the a n tic lin e s d u e to v ig o ro u s d o w n c u ttin g and thus
the v alley s d e v e lo p e d o n a n tic lin e s b eco m e d eep e r
than th e v a lle y s d e v e lo p e d in th e sy n clin es. F u rth er,
the a n tic lin a l s tr e a m s a lso c a p tu re th e sy n clin al
m aster c o n s e q u e n t s tre a m s (A ) an d h e n c e the p re v i­
ous m a s te r s tre a m o f th e fo ld e d s tru c tu re is d ism e m ­
bered (fig . 1 0 .1 1 ). T h is re s u lts in th e rev ersal o f
previous to p o g ra p h ic fe a tu re s as an ticlin es are eroded
dow n to fo rm a n tic lin a l v a lle y s an d sy n clin al v al­
leys, b e in g h ig h e r in e le v a tio n th an th e an ticlin al
valleys, b e c o m e s y n c lin a l rid g e s (5 and 6 in fig.
10.11). T h is is c a lle d as in v e r s io n o f relief. It is
ev id en t th a t in v e rs io n o f r e lie f is th e re su lt o f d iffe r­
ential e ro s io n c a u s e d b y a v a rie ty o f facto rs viz. (a)
elev atio n d if f e r e n c e , (b ) re la tiv e re sista n c e o f ro ck
beds, an d (c ) g r a d ie n t/s lo p e d iffe re n c e b etw een an ­
ticlin al a n d s y n c lin a l c o n s e q u e n t stream s and (d)
m a x im u m te n s io n a l fo rc e at th e a n tic lin a l crests
w hich c a u s e s a n d a c c e n tu a te s c ra c k s an d th u s a u g ­
m ents w e a th e r in g a n d e ro s io n a l p ro c e sse s.
nal stream (A ) d ev elo p ed in th e o rig in al sy n clin e b u t
it flo w s at m u ch lo w e r elev atio n an d is o ld er than the
o rig in al co n seq u en t stream . T his stream is called
reseq u en t stream (R in fig. 10.11). R eseq u en t sim ­
p ly m ean s new co n seq u en t.
O ld S ta g e is h erald ed by th e cessatio n o f
activ e ero sio n and reliefs are su b d u ed and m o st o f
them are o b literated d u e to p ro lo n g ed d en u d atio n .
T he en tire fo ld ed m o u n ta in o u s reg io n b eco m es fe a ­
tu reless p la in -p en ep lain . S tream s are n o t a d ju sted to
stru ctu re as th e o rig in al stru ctu ra l featu res are c o v ­
ered u n d er th ick d ep o sits o f allu v ia.
I f the p en ep lain e d fo ld ed m o u n ta in o u s reg io n
is again up lifted th e seco n d cy cle o f flu v ia l ero sio n
m ay be in itiated w ith re ju v e n a te d stre a m s a n d p a ra l­
lel ridges and v alleys are fo rm ed .
T h ere is co n tro v ersy re g a rd in g th e o rig in o f
reseq u en t stream s in term s o f flu v ial c y cle o f ero sio n
o v er folded stru ctu re. S o m e g e o m o rp h o lo g ists in ­
clu d in g S.W . W o o ld rid g e and R .S . M o rg an (1 9 6 0 )
are o f the op in io n th at rese q u e n t stream s d ev elo p
du rin g second cycle o f ero sio n w h ile o th e rs in c lu d ­
ing A .K . L o b e c k (1939) b eliev e th a t th e se o rig in ate
d uring the 1st cy cle o f ero sio n , ev en d u rin g m atu re
stage. It m ay be su g g ested th a t w h e th e r th e rese q u e n t
stream s w ill o rig in ate d u rin g first o r seco n d c y cle o f
erosion d epends on rela tiv e resistan c e o f ro c k beds
and local co n d itio n s.
T o p o g ra p h ic ex p ressio n s o f c y c le o f e r o ­
sio n o v er fo ld ed stru ctu res in c lu d e in v e rte d re­
liefs, an ticlin al rid g es, sy n clin al rid g es, h o m o c lin al
ridges, synclinal valleys, anticlinal valleys, hom oclinal
valley s etc. (fig. 10.12).
T h e v e rtic a l e r o s io n a n d v a lle y d ee p e n in g by
s u b se q u e n t s tr e a m s (a n tic lin a l s tre a m s) b eco m e less
sig n ific a n t w h e n th e u n d e rly in g re s is ta n t ro c k beds
are e x p o se d d u e to r e m o v a l o f o v e rly in g b ed s th ro u g h
p ro lo n g ed e ro s io n . T h u s , th e s u b s e q u e n t stream s are
d ev elo p ed a n d e s ta b lis h e d o v e r a rid g e o resistan
rocks. N o w , th e r iv e r s in s te a d o f e ro d in g t e re sjs
ant beds, are s u b je c te d to u n ic lin a l/h o m o c lm a l shift­
ing along th e d ip s lo p e o f re la tiv e ly re s is ta n t rid g e s
(1) A n tic lin a l rid g e s are, in fact, stru ctu ra l in
c h a ra c te r and re p re se n t u p fo ld ed ro c k beds. T hese
are fu rth er acc en tu ated b eca u se o f m o re ero sio n o f
ad jacen t ro ck b ed s. T h e a n ticlin al rid g e s o f ero sio n al
o rig in are d ev elo p ed at th e end o f flu v ial cy cle o f
ero sio n w h en re sista n t b ed s a re e x p o se d to atm o s­
p h eric p ro cesses (7 in fig. 10.11 re p re se n ts anticlinal
rid g e o f e ro sio n a l o rig in w h ile 1 d e n o te s structural
an ticlin al rid g e).
Thus, the su b se q u e n t stre a m s easily ero d e e :sy
ridges b e c a u s e th e y a re o f w e a k lith o lo g y (so ft ro ck
beds). G ra d u a lly , th e s u b s e q u e n t stre a m s reac h he
(3) Homoclinal ridges are fo rm ed on th e
u n iclin al b ed s (u n ifo rm a lly in c lin e d ) o f re sista n t
ro ck s h av in g u n ifo rm slo p es on b o th sides.
.
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synclinal r id g e s th ro u g h u n ic lm a l sh iftin g
niately fo rm th e ir v a lle y s in th e s y n c 1
(originally s y n c lin a l v a lle y s ). N o w ,
is
sim ilar to th e o rig in a l m a s te r c o n s e q u e n t lo n g itu
(2) S y n c lin a l rid g es are o f ero sio n a l origin
and are fo rm ed d u e to m o re ero sio n o f an ticlin al
rid g es (6 in fig. 10.11 re p re se n ts sy n c lin a l ridges).
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GEOMORPHOLOGY
182
s u p e rin c u m b e n t m a te ria l is re m o v e d d u e to pro­
lo n g e d e ro sio n an d th e u n d e rly in g s tru c tu re is ex­
p o se d to th e s u rfa c e an d few d is in c tiv e fe a tu re s like
c u e s ta , h o g b a c k a n d rid g e s a re fo rm e d . D om es
fo rm ed d u e to u p w a rp in g a re c h a ra c te riz e d by the
d e v e lo p m e n t o f r a d ia l o r c e n tr ifu g a l d r a in a g e p at­
te r n h a v in g a se t o f s e q u e n t s tre a m s w h ic h follow
s l o p e g rad ie n t e.g. c o n se q u e n t, s u b se q u e n t, obsequent
%
an d re s e q u e n t s tre a m s (fig . 2 .9 ).
Fluvial Cycle of Erosion on Domed Structure
M o st o f th e p re s e n t d a y d o m e s h a v e p assed
th ro u g h sev eral p h a s e s o f flu v ia l c y c le o f e ro sio n
an d h e n c e th e r e lie f fe a tu re s d e v e lo p e d on d o m e s
d u e to d e n u d a tio n a l p ro c e s s e s a re p o ly c y c lic re lie fs .
It, th u s, b e c o m e s d iffic u lt to sp e ll o u t th e in itia l fo rm
o f d o m ed stru c tu re fo r th e in itia tio n o f f lu v ia l c y c le
o f ero sio n . It is a ssu m e d th a t firs t a d o m e is fo rm e d
du e to u p w a rp in g an d it is c o m p o s e d o f a lte rn a te
seq u en ce o f re sis ta n t an d so ft ro c k b e d s w h e re a s th e
co re o f th e d o m e c o n s is ts o f c ry s ta llin e ig n e o u s r o c k
o f re la tiv e ly h ig h d e g re e o f r e s is ta n c e in re la tio n to
ero sio n . T h e ro ck b e d s a re n o rm a lly d is p o s e d w ith ­
o u t any fa u lt o r re c u m b e n t fo ld .
Fig. 10.12 : Development o f morphological features on
anticlines and synclines o f folded structure
due to fluvial erosion.
(1)
Y o u th f u l s ta g e is c h a ra c te riz e d b y e m e
g en ce o f stre a m s w ith th e fo rm a tio n (d o m in g o f
o v e rly in g ro c k s d u e to e n d o g e n e tic fo rc e ) o f d o m e .
S tream s d e v e lo p on th e s lo p e s o f th e d o m e a n d d ra in
d o w n th e slo p e a n d th u s th e s e a re c o n s e q u e n t
stream s. B e c a u se o f r o u n d e d s h a p e o f d o m e c re st,
stre a m s ra d ia te in all d ire c tio n s . In o th e r w o rd s,
c o n s e q u e n t s tre a m s a fte r o rig in a tin g o n d o m e c re s t
ra d ia te in all d ire c tio n s a n d flo w d o w n s lo p e . T h e
re s u lta n t d ra in a g e p a tte rn b e c o m e s r a d i a l o r c e n ­
tr i f u g a l d r a i n a g e p a t t e r n w h ic h is in d ic a tiv e o f
y o u n g d o m e s. Y o u n g c o n s e q u e n t s tre a m s d ra in
d o w n slo p e o n th e fla n k s o f th e d o m e s fo llo w in g dip
a n g le o f ro c k b e d s (fig . 1 0 .1 3 A ). V e ry fe w trib u ta ry
stre a m s are d e v e lo p e d . C o n s e q u e n t s tre a m s a re ac­
tiv ely e n g a g e d in v a lle y d e e p e n in g th ro u g h v ertical
e ro s io n . Y h ey e x te n d (le n g th e n ) th e ir c o u rs e s th ro u g h
h e a d w a rd e ro s io n a n d try to r e a c h j h e c re s t o f the
d o m e . H e a d w a rd e ro s io n is a s s is te d b y w e a th e rin g ,
slu m p in g a n d m a s s m o v e m e n t. G ra d u a lly a n d g ra d u ­
ally c o n s e q u e n t stre a m s re a c h th e d o m e c re sts, breach
th e m an d fo rm d e p re s s io n s (fig . 1 0 .1 3 B ) a n d b asin s
w h ic h a re o f s m a lle r d im e n s io n in th e b e g in n in g b ut
c o n tin u o u s ly th e y g ro w in s iz e d u e to c o n tin u e d
e ro s io n a n d w e a th e rin g . I t m a y b e p o in te d o u t th a t
(4) S y n clin a l v a lley s are o f stru ctu ral o rig in
and rep resen t stru ctu ral valley s fo rm ed d u e to d o w n
fo lding o f rock beds. T h e ero sio n al sy n clin al v alleys
also called as re se q u e n t v alley s are fo rm ed d u e to
e ro sio n o f sy n clin al rid g es at th e en d o f c y cle o f
erosion o r d u rin g late m atu re stag e (v alley o f R in
fig. 10.11).
(5) A n ticlin a l v a lley s are o f ero sio n al o rig in
as they are fo rm ed d u e to activ e d o w n c u ttin g o f
a n ticlin al crests by su b se q u e n t stream s. T h e se in d i­
cate in v e rsio n o f reliefs.
(6) H o m o c lin a l v a lley s are o f ero sio n a l o ri­
g in and d e v e lo p b etw een h o m o c lin a l rid g e s and
re sista n t beds o f a n ticlin es. In fact, th e situ a tio n o f
re la tiv e ly so ft ro ck b ed s b etw ee n tw o b ed s o f re s is t­
an t ro ck s le ad s to ero sio n o f so ft b ed s an d h e n c e th e
d e v e lo p m e n t o f such valley s.
10.4 TOPOGRAPHIC EXPRESSIONS OF DOMED
STRUCTURE
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D o m ed structure results eith er due to upw arping
o f cru sta l s u rfa c e e ffe c te d by d ia stro p h ic fo rce o r
d u e to in tru sio n o f m a g m a into su rfic ia l ro ck s. T h e
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K -V
SfHOCTURAL GEOMORPHOLOGY
183
ex posing u nderlying so ft ro ck b ed s (fig. 10.13 B).
T he eroded and ex p o sed p arts o f u p p er resistan t rock
beds, o v erlooking the b asin s d ev elo p ed at th e d om e
crest, form sca rp s w hich are su b jected to gradual
recession tow ards b ack slo p e b ecau se o f continued
b ack w astin g through w eath erin g and erosion. T his
results in gradual in crease in the size o f th e basin.
A ctive dow n cu ttin g by the riv ers resu lts in the d eep ­
ening o f the basin. T h e steep en ess o f scarp s d ep en d s
on relative resistan ce o f th e ro ck b ed s as steep scarps
are associated w ith resistan t beds w h ile soft beds
give birth to scarps o f g en tle g rad ien t. D o w n w ard
erosion o f the basin d ev elo p ed at th e d o m e c rest
continues till all the so ft ro ck beds are n o t e ro d e d and
resistan t cry stallin e co re is n o t ex p o sed .
A
(2)
M atu re S ta g e— V alley d e e p e n in g sto
with the b eginning o f m atu re stag e as by th is s ta g e all
the soft rock beds o v erly in g re sis ta n t c o re o f the
dom e have been ero d ed and rem o v ed . R iv e rs e x te n d
their co u rses on cry stallin e core. T h e re is m a x im u m
relief in the early m atu re stag e. N u m ero u s trib u ta rie s
as su b seq u en t stream s d ev elo p an d jo in c o n se q u e n t
stream s alm o st at rig h t angle. H ead w ard e ro s io n by
these tributaries resu lts in sev eral c ases o f riv e r
capture. C o n seq u en tly , a n n u la r d r a in a g e p a ttern
develops on the b reach ed d o m e crest. D iffe re n tia l
erosion o f altern ate b ed s o f re sista n t and so ft ro ck s
results in the fo rm atio n o f rid g es o f v a ry in g sizes and
shapes. T h e ridges, h av in g steep slo p es an d u n ifo rm
g rad ien t on both sides, are called h o g b a c k s w h ile
asy m m etrical rid g es w ith g en tle slo p e are k n o w n as
cu esta s. S trik e v a lley s are d ev e lo p e d o v e r so ft ro ck
beds betw een h o m o clin a l rid g e s a n d h o g b a c k s. A
n etw o rk o f su b se q u en t, o b se q u e n t and reseq u en t
stream s d ev elo p d u rin g late m a tu rity . A fte r the d is ­
sectio n and rem o v al o f all the o v e rly in g so ft rock
C
beds the w ell d ev elo p ed stre a m s ero d e the re sista n t
cry stallin e ro ck s o f th e co re o f th e d o m e. T h e fe a ­
tures o f ero d ed co re d ep en d on its size and lith o lo g ical
ch a ra c te ristic s. T h e co re h av in g la rg e r n u m b e r o f
re sista n t b ed s is less e ro d e d an d h e n c e upper surface
%
10.13 : Stages o f developm ent o f flu via l cycle o f ero­
sion on dom ed structure, A-initial, B-youth.
C-maturity and D -old stages.
b eco m es u n d u la tin g a n d th e ero d e d d o m e appears as
a d isse c te d p lateau . O n th e o th e r h an d , th e ce n tra l
part o f the d o m e s b e c o m e s a b ro ad basin ifth e d o m e
is o f sm all size and is co m p o sed o f le ss re s is ta n t
beds.
the top rock co v er o f the d om e h as been sh o w n to be
o f resistant rock w h ich is d eep ly cut by the stream s
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f
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184
GEOMORPHOLOGY
3.
h o­ m o c lin a l rid g e s. T h e e n tire d o m e is e ro d e d down
Old Stage is c h a ra c te riz e d by m a rk e d re
to fe a tu re le s s p la in a n d u ltim a te ly th e d o m e is
c o n v e rte d in to a p e n e p la in a n d th u s o n e phase
o f flu v ial c y c le o f e ro s io n is c o m p le te d provided
th a t th e re g io n re m a in s s ta b le fo r th e d esired
le n g th o f tim e (fo r th e c o m p le tio n o f c y c le o f ero­
sio n ).
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d u c tio n in v e rtic a l e ro s io n b u t p h e n o m e n a l in c re ase
in la te ra l e ro s io n w ith th e re su lt th e re is g rad u al
d e c re a s e in th e re lie fs d e v e lo p e d d u rin g m a tu re
s ta g e (fig . 10.13 C ). T h e c e n tra l p art o f cry sta llin e
ro c k s is a lso e ro d e d . C o n tin u o u s lateral ero sio n
c a u s e s d is a p p e a r a n c e o f c u e s ta s, h o g b a c k s an d
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:
PLATE TECTONICS
M e a n in g a n d c o n c e p t ; p la te m a r g in s ; p a l a e o m a g n e t i s m - s o u r c e o f
g e o m a g n e tic fie ld , re m a n e n t m a g n e tis m , r e c o n s t r u c t i o n o f
p a la e o m a g n e tis m , re v e rs a l o f p o la rity ; s e a - f lo o r s p r e a d i n g ; p l a t e m o ­
tio n ; c a u s e s o f p la te m o tio n ; p la te te c to n ic s a n d c o n t i n e n t a l d r i f t ; p l a t e
te c to n ic s a n d m o u n ta in b u ild in g ; p la te te c to n ic s a n d v u l c a n i c i t y ; p l a t e
te c to n ic s a n d e a r th q u a k e s .
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CHAPTER 11
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11
PLATE TECTO N ICS
N e w c o n c e p ts a n d th e o rie s b a s e d o n e v i­
P la te te c to n ic th e o ry , a s ig n if ic a n t s c ie n tif ic
d e n c e s a n d in te r p r e ta tio n o f s e a -flo o r s p re a d in g and
p a la e o m a g n e tic f ie ld h a v e b e e n a d v a n c e d a fte r 1960
in t h e f i e l d o f g e o l o g y , g e o p h y s i c s a n d
g e o m o rp h o lo g y , o f w h ic h th e o ry o f p la te te c to n ic s
a d v a n c e m e n t o f th e d e c a d e 1 9 6 0 's, is b a s e d o n tw o
m a jo r s c ie n tific c o n c e p ts e .g . (1 ) th e c o n tin e n ta l
d rift an d (2) th e c o n c e p t o f s e a - f lo o r s p re a d in g .
L ith o sp h e re is in te rn a lly m a d e o f r ig id p la te s (fig .
6 .7 ). S ix m a jo r a n d 2 0 m in o r p la te s h a v e b e e n
id e n tifie d so fa r (E u ra s ia n p la te , I n d ia n - A u s tr a lia n
in m o s t s ig n if ic a n t. T h e p r e s e n t c h a p te r d eals w ith
v a rio u s a s p e c ts o f p la te te c to n ic s v iz. m e a n in g an d
c o n c e p t, p a la e o m a g n e tis m , s e a -flo o r sp read in g , p late
p la te, A m e ric a n p la te , P a c ific p la te , A f ric a n p la te ,
e x p re s s io n s u c h a s c o n tin e n ta l d rift, v u lc a n ic ity ,
an d A n ta rc tic p la te ) (fig . 1 1.1). I t m a y b e m e n tio n e d
th a t th e te rm ‘plate’ w a s f ir s t u s e d b y C a n a d ia n
s e is m ic a c tiv ity , m o u n ta in b u ild in g etc.
g e o p h y s ic is t J. T u z o W ils o n in 1 9 6 5 . Mackenzie
m a rg in s , p a te m o v e m e n ts a n d re s u lta n t g eo lo g ic
an d Parkar d is c u s s e d in d e ta il th e m e c h a n is m of
p la te m o tio n s o n th e b a s is o f Euler's geometrical
theorem in 1967. T h e y p o s tu la te d a ‘paving stone
hypothesis’ w h e re in th e o c e a n ic c r u s t w a s c o n s id ­
e re d to b e n e w ly fo rm e d at m id -o c e a n ic rid g e s a n d
d is tro y e d a t th e tre n c h e s . Isacks a n d sykes c o n ­
firm e d th e ‘p a v in g s to n e h y p o th e s is ’ in 1 967. W J .
Morgan a n d Le Pichon e la b o r a te d the v a rio u s
asp e c ts o f p la te te c to n ic s in 1 9 6 8 . I t m a y , th u s ,
b e p o in te d o u t th a t th e th e o ry o f p la te te c to n ic s
is n o t re la te d to an y in d iv id u a l s c ie n tis t ra th e r
a h o s t o f s c ie n t is ts o f v a r io u s s c ie n t if ic d i s ­
c ip lin e s a n d r e s e a r c h g r o u p s a n d e x p e d i tio n s
h a v e c o n trib u te d in th e d e v e lo p m e n t of th is v a lu a ­
11.1 MEANING AND CONCEPT
T h e r ig i d lith o s p h e r ic s la b s o r rig id a n d so lid
c ru s ta l la y e r s a r e te c h n ic a lly c a lle d p la te s (fig . 6 .7 ).
T h e s tu d y o f w h o le m e c h a n is m o f e v o lu tio n , n atu re
a n d m o tio n s o f p la te s , d e fo rm a tio n w ith in p la te s a n d
in te rra c tio n s o f p la te m a rg in s w ith e a c h o th e r is
c o lle c tiv e ly c a lle d a s plate tectonics. In o th e r w o rd s,
th e w h o le p ro c e s s o f p la te m o tio n s a n d re s u lta n t
d e fo rm a tio n s is re fe r r e d to as p la te te c to n ic s. ‘M o v ­
in g o v e r t h e W e a k a s th e n o s p h e r e , in d iv id u a l
lith o s p h e ric p la te s g lid e s lo w ly o v e r th e s u rfa c e o f
th e g lo b e ; m u c h as a p a c k o f ic e o f th e A rc tic O c e a n
d rifts u n d e r th e d ra g g in g fo rc e o f c u rre n ts an d w in d s ’
(A .N . S tra h le r a n d A .H . S tra h le r, 1978).
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b le c o n c e p t of th e s e c o n d h a l f o f the
20th
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GEOMORPHOLOGY
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186
P lates a re c la ssifie d in to 3 ty p e s viz. oceanic
p lates (h av in g o cean ic cru st), co n tin en tal p lates (hav­
ing co n tin en tal cru st) an d co n tin en ta l-o cea n ic plates.
c e n tu ry . N o w th e co n tin en tal d rift and d isp la c e ­
m e n t is c o n sid e re d a reality on th e basis o f plate
te cto n ic s.
Euraision
iP ° ! c !
Amerio
^ ia te
African
,3 l.p o cific
X P ja U .
/ : plate •
Antarctic plate
'
Destructive margin
s '
s '
s '. . .
Constructive margin
Fig. 11.1: Distribution o f plates. The names o f 6 major plates have been shown on the map and 5 m inor plates have been
indicated by numbers viz. 1- Nasca plate, 2. Scotia plate, 3. Phillippine plate, 4. Caribbean plate, and 5.
Arabian plate.
o p p o site d ire c tio n s (fig . 6 .8 ). D iv e rg e n t p la te m ar­
g in s are c o n s tru c tiv e in th e s e n s e th a t th e re is co n ­
tin u o u s fo rm a tio n o f n e w c ru s t a lo n g th e s e m arg in s
b e c a u se o f c o o lin g a n d s o lid ific a tio n o f b a s a ltic lava
w h ich c o m e s u p as m a g m a d u e to riftin g o f plates
alo n g th e m id -o c e a n ic rid g e s . D iv e rg e n t m o v e m en t
o f p la te s (i.e. m o v e m e n t o f tw o p la te s in o pposite
d ire c tio n s ) re s u lts in (i) v o lc a n ic a c tiv ity o f fissu re
flo w o f b a sa ltic m a g m a , (ii) c re a tio n o f n e w oceanic
c
ru sts, (iii) fo rm a tio n o f s u b m a rin e m o u n ta in ridges
(1 )
C o n s tr u c tiv e p la te m a r g in s are also
a n d rise s, (iv ) c re a tio n o f tra n s fo rm fa u lts , (v ) o ccu r­
c a lle d a s ‘d iv e r g e n t p la te b o u n d a r ie s ’ o r ‘a ccr etin g
re n c e o f s h a llo w fo c u s e a rth q u a k e s , (v i) d riftin g o f
p la te b o u n d a r ie s ’. I t m ay b e m e n tio n e d th a t a
o
c e a n ic p la te s etc.
d is tin c tio n m a y be d ra w n b etw ee n p la te m a rg in s an d
p la te b o u n d a rie s e .g . p la te m a rg in re p re s e n ts m a r­
(2 )
D e s tr u c tiv e p la te m a r g in s
a re a
g in a l p a r t o f th e p la te w h e re a s p la te b o u n d a ry re p re ­
te rm e d as ‘c o n v e r g e n t p la te b o u n d a r ie s ’ o r ‘con­
s e n ts ‘su rfa c e tra c e o f th e zo n e o f m o tio n b e tw e e n
s u m in g p la te m a r g in s ’ b e c a u s e tw o p la te s m ove
tw o p la te s .’ C o n s tru c tiv e p la te b o u n d a rie s re p re s e n t
to w a rd s e a c h o th e r (fa c e to fa c e ) o r tw o plates
z o n e s o f d iv e rg e n c e w h e re th e re is c o n tin u o u s
c o n v e rg e a lo n g a lin e a n d c o llid e w h e re in leading
u p w e llin g o f m o lte n m a te ria l (la v a ) an d th u s new
e d g e o f o n e p la te ( o f re la tiv e ly lig h te r m aterial)
o c e a n ic crust is c o n tin u o u s ly fo rm e d . O c e a n ic p la te s
o v e rrid e s th e o th e r p la te ( o f re la tiv e ly d e n s e r m ate­
sp lit a p a rt a lo n g th e m id -o c e a n ic rid g e s an d m o v e in
ria l) a n d th e o v e rrid d e n p la te is s u d u c te d o r th ru st
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1 1 .2 PLATE MARGINS
It m a y b e h ig h lig h te d th a t te cto n ic ally p late
b o u n d a rie s o r p la te m a rg in s are m o st s ig n ific a n t
b e c a u se all te c to n ic a c tiv itie s o c c u r alo n g th e p la te
m a rg in s e.g . s e is m ic e v en ts, v u lc a n ic ity , m o u n ta in
b u ild in g , fa u ltin g etc. T h u s, th e d e taile d stu d y o f
p la te b o u n d a rie s is n o t o n ly d e s ira b le b u t is also
n e c e ss a ry . P la te m a rg in s are g e n e ra lly d iv id e d into
th re e c a te g o rie s as fo llo w s (fig . 6 .7 ).
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PLATE TECTONICS
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187
into upper m a n tle a n d th u s a p a rt o f th e cru st (p late)
is lost in th e m a n tle (fig . 6 .8 ), th is is w hy co n v erg e n t
plate m arg in s a re c a lle d d e s tru c tiv e m arg in s. T he
zone o f co llisio n o f c o n v e rg e n t p la tes is also called
as ‘collision z o n e ’, ‘s u b d u c tio n z o n e ’ and ‘B e n io ff
zone’ (afte r th e s c ie n tis t H u g o B en io ff). C o n v e r­
gence, c o llisio n a n d re s u lta n t s u b d u c tio n o f h eav ier
plate m arg in u n d e r lig h te r p la te m a rg in resu lts in (i)
occurrence o f ex p lo siv e ty p e o f volcanic eruptions, (ii)
deep focii earthquakes, (iii) form ation o f folded m oun­
tains, island arcs an d festo o n s, o cean ic trenches etc.
(3)
C o n serv a tiv e p la te m a r g in s are a
called as sh ea r p la te m a rg in s and p a r a lle l/tr a n s ­
fo rm fa u lt b o u n d a ries w h ere tw o p la tes p ass or
slid e past each o th e r alo n g tran sfo rm fau lts. T h ese
are called c o n serv ativ e b ecau se cru st is n e ith e r c re ­
ated nor d estro y ed . T h e sig n ifican t te cto n ic e x p re s ­
sion o f such situ a tio n is th e creatio n o f tran sfo rm
faults w hich m o v e, on an av erag e, p arallel to the
d irectio n o f plate m o tion. T ran sfo rm fau lts o ffse t
m id-oceanic ridges. B esides ocean ic transform faults,
there are also co n tin en tal tran sfo rm fau lts e.g. S an
A n d reas fault (C alifo rn ia, U S A ), A lp in e fa u lt (A f­
rica) etc. It m ay be m en tio n ed th at S an A n d re a s fau lt
‘is rid g e to rid g e tran sfo rm fa u lt.’ T h e o th e r m a n i­
festatio n s o f c o n serv ativ e p late m a rg in s in c lu d e no
v olcanic activ ity , seism ic ev en ts, c re a tio n o f rid g e
and valley, fractu re zone etc.
P la te c o llis io n s a re o f th ree ty p es viz. (i)
ocean— o c e a n c o llis s io n (c o llis io n o f tw o ocean ic
plates), (ii) c o n tin e n t-c o n tin e n t co llisio n (co llisio n
of tw o c o n tin e n ta l p la te s ) an d (iii) o c e a n -co n tin e n t
collision (c o llis io n o f o cea n ic a n d co n tin en tal plates).
O c ea n -o c ea n c o llis io n in v o lv e s c o llisio n o f tw o
co n v erg e n t p la te s h a v in g o c e a n ic cru sts w here one
oceanic c ru s t h a v in g re la tiv e ly d e n se r m aterial is
su b d u cted in to u p p e r m a n tle . S u ch co llisio n and
su b d u ctio n o c c u rs a lo n g e a s t A sia and th e resu ltan t
tectonic e x p re s s io n o f p la te c o llisio n and su b d u ctio n
includes d e fo rm a tio n in cru sta l area, v u lcan ism ,
m e ta m o rp h ism , fo rm a tio n o f o cea n ic trench es, is­
land arcs a n d fe s to o n s etc., an d o c c u rre n c e o f e a rth ­
q u ak es. O c e a n -c o n tin e n t c o llis io n in v o lv es co lli­
sion o f o n e o c e a n ic p la te h a v in g o cea n ic cru st and
o th er o n e o f c o n tin e n ta l p la te h av in g co n tin en tal
cru st a lo n g B e n io f f z o n e (s u b d u c tio n zone) and the
re su lta n t te c to n ic e x p re s s io n s are d efo rm a tio n o f
cru stal ro c k s , m e ta m o rp h ism , v o lcan ic eru p tio n s,
fo rm a tio n o f fo ld e d m o u n ta in s an d o ccu rren c e o f
d e e p -fo c u s e a rth q u a k e s . C o llis io n o f A m erican and
P acific p la te s is a ty p ic a l e x a m p le o f this categ o ry
and fo rm a tio n o f m a je stic w estern co rd ille ra o f N.
A m e ric a a n d A n d e s o f S. A m e ric a is sig n ifican t
re s u lta n t te c to n ic e x p re s s io n o f su ch situ atio n . It
m ay b e m e n tio n e d th a t o n e o l the m a n ifestio n s o f
c o n tin e n t-o c e a n ic p la te c o llisio n is th e ex p o su re o f
d eep o c e a n ro c k s th ro u g h th e ir th ru stin g in resu ltan t
m ountain m asses. T h is process is called obduction
w hich is o p p o site to su b d u ctio n as the fo rm er im plies
thrusting up w h ile th e latter m ean s thrusting dow n.
11.2 PALAEOMAGNETISM
P alaeo m ag n etism refers to th e p re s e rv a tio n
o f m agnetic p ro p erties in th e o ld e r ro ck s o f th e e arth .
It m ay be m en tio n ed that w hen an y ro ck , w h e th e r
sed im en tary o r igneous, is fo rm ed it g ets m a g n e tis e d
d ep en d in g on the p resen ce o f iron c o n te n t in th e ro c k
and is p reserv ed (frozen at te m p e ra tu re b e lo w C u r i e
p o in t, w hich is g en erally 600°C ). It w as th e y e a r
1600 A .D . w hen W illiam G ilb ert, th e p h y s ic ia n o f
Q ueen E lizab eth , p o stu la ted th a t th e e arth b e h a v e d
like a g ia n t m a g n e t and m a g n etism o f th e e a rth w as
p ro d u ced in the in n er p art o f th e earth . T h e m a g n e tic
field o f the earth is like a g ia n t b a r m a g n e t o f d ip o le s,
located in th e cen tre (co re) o f th e e a rth an d is a lig n e d
ap p ro x im ately alo n g th e axis o f ro ta tio n o f th e e a rth .
W hen the long axis o f d ip o le b ar m a g n e t is e x te n d e d
it intersects the e a rth ’s su rface at tw o c e n tre s w h ic h
are called no rth and so u th m a g n etic p o le s. It m a y be
p o in ted o u t th at m ag n etic so u th p o le o f th e e a rth is
near its (earth 's) g eo g rap h ical n o rth p o le an d v ic ev ersa (i.e. m ag n etic n o rth po le is lo c a te d n e a r g e o ­
g rap h ical so u th p o le). I f an o rd in a ry sm all m a g n e t is
freely su sp en d ed at th e ea rth 's su rfa c e th e n th e
earth 's so u th m a g n etic p o le attra c ts n o rth p o le o f
sm all m ag n et and ea rth 's n o rth m a g n e tic p o le a t­
tracts so u th po le o f sm all m ag n et. It m ay be cla rifie d
that as p er general ru le w h en tw o m ag n ets are b ro u g h t
to g eth er, th en th e ir sim ila r p o les rep el e a c h o th e r but
o p p o site p o les attra c t e a c h o th er.
C o n tin e n t-c o n tin e n t c o llis io n in v o lv es c o l­
lisio n o f tw o c o n tin e n ta l p la te s alo n g B e n io fl zone
and is re sp o n sib le fo r c re a tio n o f fo ld ed m o u n tain s
and o c c u rre n c e s o f e a rth q u a k e s o f
v a r y in g
m a g n itu d es. T h e co llisio n o f A sia tic -In d ia n plates,
and E u ro p ea n -A fric an p la tes is ty p ical ex am p le o f
su ch situ a tio n and th e fo rm a tio n s o f A lp in e and
H im a la y a n m o u n t a in o u s c h a i n s a r e m a jo r
m a n ife stio n s
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A freely su sp e n d ed m ag n et on th e e a rth 's
su rface d o es not in d ic a te g eo g ra p h ic a l n o rth an d
so u th p erfectly b eca u se th e ax is o f m a g n e tic n o rth
and so u th p o les is not p erfectly a llig n ed alo n g th e
axis o f g eo g ra p h ic a l n o rth an d s o u th p o le s. T h is
c au ses an g u lar in c lin a tio n b etw ee n th e m a g n e tic an d
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188
T h e re q u ire d en e rg y to m a in ta in geo m ag n etic
field is b e lie v e d to c o m e fro m th re e p o ssib le sources:
(1) h e a t en erg y re le a s e d fro m th e d isin te g ra tio n o f
ra d io a ctiv e e le m e n ts o f th e c o re o f th e earth . It is
a rg u ed th a t th is so u rc e o f en erg y fo r th e generation
o f c o n v e c tiv e c u rre n ts (e le c tric a l c u rre n ts) m ay not
be p o s sib le b e c a u se if w e a c c e p t th is prop o sitio n
th en d iffic u lty a rise s in th e p ro c e s s o f co o lin g o f the
c ru st o f th e e a rth b e c a u se su c h situ a tio n (generation
o f h e a t en erg y fro m ra d io a c tiv e e le m e n ts) w ould
also p rev ail in th e m a n tle a n d h e n c e th e c ru st cannot
co o l b e c a u se th e re w o u ld b e c o n s ta n t su p p ly o f heat
en erg y fro m b e lo w (fro m th e m a n tle ). (2 ) T h e d o w n ­
w ard tran sfer o f ferro m a g n esian m aterials from m antle
in to co re re su lts in th e re le a s e o f g ra v ity fo rc e in the
c o re w h ich in tu rn p ro d u c e s e n e rg y . (3 ) T h e m o v e­
m en t o f m a te ria ls fro m in n e r c o re to th e o u te r core
resu lts in th e h e a tin g o f o u te r c o re th ro u g h heat
en erg y re le a se d fro m in n e r c o re (fo r d eta ils see
c h a p te r 5, g e n e r a tio n a n d t r a n s f e r o f h e a t in sid e
th e e a r th ) .
g e o g ra p h ic a l axes. T h is a n g u lar in c lin atio n is calle d
m a g n e tic d eclin a tio n w hich, in fact, d en o tes an g u ­
la r in c lin a tio n b etw ee n th e d irec tio n o f freely su s­
p e n d e d m a g n e t at an y p a rt o f th e earth 's su rface and
th e d ire c tio n o f earth 's g eo g rap h ical n o rth -so u th
p o le axis. O n th e o th e r h and, an g u la r in c lin atio n
b etw ee n freely su sp e n d ed m ag n etic n eed le and h o ri­
zo n tal p la n e o f th e earth 's su rface is called m a g n etic
in c lin a tio n o r m a g n e tic d ip. I f a m ag n etic n eed le is
freely su sp e n d ed at th e n o rth p o le o f th e earth, th e
n o rth p o le o f the m a g n e t bein g c lo se r to the so u th
m a g n etic p o le o f th e earth (w hich is, in fact, near
g e o g ra p h ic a l n o rth p o le) w ould be attracted m ore
a n d m a g n e tic n eed le b eco m es p erp en d icu lar. C o n ­
se q u e n tly , n o rth p o le o f th e su sp en d ed m ag n etic
n eed le d ip s d o w n w a rd vertically . T h e situ atio n is
re v e rse d in th e so u th ern h em isp h ere. T h u s, m a g ­
n etic d ip beco m es 90° on g eo g rap h ical north and
so u th p o les o f the earth. M ag n etic dip b eco m es zero
w h erev er freely susp en d ed m ag n etic needle becom es
h o rizo n tal at th e earth 's su rface. T h e im ag in ary line
jo in in g p la ces o f zero m ag n etic dip angle is called
m a g n etic eq u ator. T h e m ag n etic dip angle increases
p o lew ard . It m ay be p o in ted o u t th at th ere m ay be
spatial and tem p o ral variatio n in the in ten sity o f
sim p le d ip o le m a g n etic field.
R em a n en t M a g n etism
T h e g e o c e n tric a x ial d ip o le m a g n e tic field
re p re se n ts 95 p e r c e n t o f th e e a rth ’s to tal m ag n etism .
T h e re m a in in g p o rtio n is re p re s e n te d by irregular,
scattered an d w eak m a g n e tic field s. It m ay be pointed
o u t th a t th e re is n o su ch g ia n t b a r m a g n e t in sid e the
earth b u t th e re is m o re c o n c e n tra tio n o f m ag n etism
in th e ro ck s o f th e c o re o f th e e a rth in th e sh a p e o f a
b ar m ag n et. T h e h o t a n d liq u id la v a a n d m a g m a w ith
h ig h fe rro m a g n e sia n c o n te n ts , w h e n c o o le d and so­
lid ifie d to fo rm ig n e o u s ro c k s , g e t m a g n e tis e d , the
reco rd s o f w h ic h are p re s e rv e d in th e ro c k s. Such
m a g n e tism p re s e rv e d (fro z e n ) in th e ro c k s a re called
r e m a n e n t o r p a l a e o m a g n e tis m . It is to b e rem em ­
b ered th a t th e n e w ly fo rm e d ro c k s a re m a g n e tise d in
th e d ire c tio n o f e x istin g g e o m a g n e tic fie ld , a n d thus
th e m a g n e tic in c lin a tio n /d ip o f n e w ly fo rm e d rocks
is th e s a m e as th a t o f th e g e o m a g n e tic fie ld at the
tim e o f th e fo rm a tio n o f s a id ig n e o u s ro c k s. T hus, it
is e v id e n t th a t th e o rie n ta tio n an d m a g n e tic inclina­
tio n o f p a la e o m a g n e tis m p re s e rv e d in th e rocks is
alw ay s in a c c o rd a n c e w ith th e p re v a ilin g m agnetic
in c lin a tio n o f g e o m a g n e tic fie ld . T h e in ten sity o f
su ch p a la e o m a g n e tism /re m a n e n t m a g n e tism depends
on th e c o m p o sitio n o f m in e ra ls o f la v a an d m a g m a at
th e tim e o f c o o lin g an d s o lid ific a tio n an d on the
in ten sity o f g e o m a g n e tic fie ld o f th a t p e r i o d (w hen
th e c o n c e rn e d ig n e o u s ro c k s w e re fo rm e d ). Simi"
larly , s e d im e n ta ry ro c k s , a t th e tim e o f th e ir fo rm a­
tio n , are a lso m a g n e tis e d , th e in te n s ity o f w hich
d e p e n d s o n th e a m o u n t o f fe rro m a g n e sia n m inerals
p re s e n t th e re in . S o m e tim e s, th e m a g n e tis m (w ealf^ j
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S o u r c e o f G e o m a g n etic Field
T h e o rig in o f g eo m ag n etic field is in no case
re la te d to m an tle ra th e r it is related to th e o u ter co re
o f th e earth b eca u se o f the fact th at th ere is gradual
w e stw a rd m ig ratio n o f g eo m ag n etic field at the rate
o f 0.18° p e r y e a r w h ich p ro v es th a t the ro tatio n o f
g e o m a g n e tic field is slo w er than the ro tatio n o f the
earth . T h is in d ire c tly p ro v es th at th e co re o f th e earth
ro ta te s at slo w e r rate than th e o v erly in g m an tle. It
m a y b e s ta te d th a t ‘th e m ag n etic field can n o t be a
p e rm a n e n t p ro p e rty o f th e m aterial o f the c o r e ........
m u s t th e re fo re b e c o n tin u o u sly p ro d u ced and m a in ­
ta in e d ’ (A . and D o ris L. H o lm es, 1978). I f p e rm a ­
n e n t g e o m a g n e tic fie ld is n o t p o ssib le th en th e c o n ­
tin u o u s p ro d u c tio n an d m a in te n a n c e o f g eo m ag n etic
field m ay be p o s sib le o n ly w h en th e re w o u ld be
p re s e n c e o f m a te ria ls o f h ig h e le c tric a l c o n d u c tiv ity
in th e c o re so th a t e le c tric a l cu rre n ts m ay be g e n e r­
ated . It is fu rth e r p o in te d o u t th a t th e g en e ra tio n o f
e le c tric a l c u rre n ts is p o ssib le o n ly in m e tallic liq u id
m a te ria ls and su c h situ a tio n is fo u n d in th e o u te r co re
o f th e e a rth w h ic h fu n c tio n s as s e lf ex c itin g d y ­
n a m o . T h u s, th e e n e rg y c o m in g o u t o f th e co re is
tra n s fo rm e d in to e le c tric a l c u rre n ts w h ich in a sso ­
ciation w ith m e ta llic liq u id s u b sta n c e s p ro d u c e g e o ­
c e n tr ic d ip o le m a g n e tic fie ld .
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pi^ tb t e c t o n ic s
189
o f sedimentary rocks is destroyed due to chem ical
c h a n g e Rem anent m agnetism preserved in the rocks
is recorded w ith the help o f g a lv a n o m eter .
C en o zo ic lavas. B lack ett an d his asso ciates d e te r­
m ined the position o f p oles b efo re 2 0 0 m illio n y ears
in B ritish Isles on the basis o f p alaeo m a g n etic re c o n ­
stru ctio n o f san d sto n es. T h e study rev ea led c o n s id ­
erab le ch an g es in the p o sitio n s o f p o le s in th e p ast.
T h is study, thus, rev ealed th e fact, ‘th a t m agnetic
poles have changed their position s a n d there has
been considerable w andering in the p o sitio n o f
poles. ’ O n the basis o f this rev elatio n tw o in fere n ces
may be d raw n —
Reconstruction of Palaeomagnetism
T h e re c o n s tru c tio n o t p alaeo m a g n etism in ­
volves th e c o lle c tio n o f ro c k sa m p le s o f th e sam e age
from d iffe re n t p la c e s an d d e te rm in a tio n and reco rd ­
ing o f th e ir o rie n ta tio n . It m ay be p o in ted o u t that
som e c h a n g e s m a y ta k e p la ce in th e o rig in al o rien ­
tation o f m a g n e tis m d u e to te cto n ic ev en ts. Any
w ay, a f te r th e d e te r m in a tio n o f o rie n ta tio n o f
p a laeo m a g n etism , th e m a g n itu d e , d eclin atio n and
inclination o f lo c a l fo rc e are m e asu re d w ith the help
o f m a g n e to m e te r . It is a ssu m ed th a t g en erally at the
tim e o f m a g n e tis a tio n o f ro c k s (p alaeo m ag n etism )
the g e o m a g n e tic fie ld is d ip o la r in sh ap e and th ere is
a p p r o x im a te c o i n c i d e n c e b e tw e e n a v e r a g e
g eo m ag n etic fie ld (a v e ra g e , b eca u se it varies tem ­
porally) an d c o n te m p o ra ry g eo g rap h ical poles. B ased
on this a s s u m p tio n a v e ra g e p a laeo m a g n etic in clin a­
tion/dip o f ro c k s o f a c e rta in p la ce and o f a certain
tim e is d e te rm in e d , o n th e b asis o f w h ich the latitude
o f th a t p la c e e x is tin g at th a t tim e is d eterm in ed on the
basis o f th e fo llo w in g e q u a tio n —
w hen
tan I =
2 tan X
I
=
m a g n etic inclination
A,
=
latitu d e
(1) T he p o les m u st h av e c h a n g e d th e ir p o s i­
tions and the co n tin en ts and o cean b asin s m ig h t h av e
rem ained statio n ary at th e ir p laces th ro u g h o u t g e o ­
logical tim e.
(2) P o lar w an d erin g has o c c u rre d d u e to c o n ­
tinental drift i.e. co n tin en ts c h an g e d th e ir re la tiv e
positions w hile m agnetic p o les re m a in e d sta tio n a ry .
P o lar w an d erin g cu rv es are p re p a re d fo r d if­
ferent co n tin en ts on the basis o f d ata d e riv e d th ro u g h
p alaeo m ag n etic reco n stru ctio n . A s p e r ru le i f th ere
has not been continental drift, then the p o la r w an ­
dering curves o f different continents a t a certain
time p eriod (sam e tim e f o r a ll the co n tin en ts) sh a ll
be the same, but i f the contin en tal d rift has o c­
curred then these p o la r w andering curves w o u ld be
different fo r each continent. T h e m a g n e tic p o la r
w andering curves, w hen p lo tted fo r d iffe re n t c o n ti­
nents for sam e perio d , d iffe r fro m e a c h o th er. T h is
clearly show s that p o les h av e n o t c h a n g e d th e ir
p ositions rath er c o n tin en ts h av e c h a n g e d th e ir p o s i­
tions. T hus, it is co n clu d ed th a t ‘the con cepts o f
perm anency o f continents an d ocean basins a n d
p o la r w andering stand autom atically rejected a n d
continental displacem ent an d d rift becom es a rea l­
ity. ’ It is, thus, valid ated th a t if the re la tiv e p o sitio n s
o f co n tin en ts have ch an g ed , th e p o sitio n o f m a g n etic
po le d eterm in ed on the b asis o f c o n te m p o ra ry ro ck s
o f a co n tin en t w ould d iffe r fro m th e p o sitio n o f
m ag n etic p o le (o f sam e p erio d ) o f th e o th e r c o n ti­
nents. It m ay be fu rth er elab arate d . S o lo n g as tw o
co n tin en ts are jo in e d to g e th e r o r are n o t d riftin g in
relatio n to o n e an o th er, th e m a g n etic p o la r w an d er­
ing cu rv es fo r sam e p e rio d w o u ld b e the sa m e fo r
both th e c o n tin en ts. A cc o rd in g to A .G . W e g e n e r all
th e co n tin en ts w ere jo in e d to g e th e r in th e fo rm o f
P an g aea till late P erm ian p erio d . I f th is w as so, th en
th ere sh o u ld be on ly o n e p a laeo m a g n etic p o le fo r all
th e c o n tin en ts d u rin g P ala eo zo ic era. T h is in fere n ce
b ecam e tru e w h en th e p alaeo m a g n etic p o la r w a n ­
d erin g cu rv e w as p rep ared fo r P a la eo zo ic P a n g a e a
by jo in in g all th e p re se n t d ay co n tin e n ts to g e th e r so
as to co n ce iv e th e situ a tio n in P a la e o z o ic era.
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T h u s , th e la titu d e , so d eterm in e d helps in
d e te rm in in g th e d is ta n c e o f p o les and the d irection
o f p o le s is d e te rm in e d on th e basis o f palaeo m ag n etic
d ec lin a tio n (D ). O n th e b asis o f d ista n ce and d irec­
tion o f g e o g ra p h ic a l p o le s fro m th e selected place
(from w h e re th e ro c k sam p les are co llected ) the
p o sitio n o f p o le s o f th e g lo b e , at th e tim e o f the
fo rm atio n o f th e s a m p le ro ck s, is d eterm in e d . T here
m ay b e s o m e e rro rs in th e afo resaid p ro cess o f
d e te rm in a tio n o f th e p o sitio n o f th e g lo b e, viz. (i) at
the tim e o f p a la e o m a g n e tic rec o n stru c tio n the im ­
p act o f o n ly g e o m a g n e tic field is co n sid ere d w hile
m in o r m a g n e tic fie ld s are ig n o red ; (ii) sam p led
rocks m ig h t h a v e e x p e rie n c e d m a g n etic ch an g es,
(iii) so m e e rro rs m a y c ro p u p at th e tim e o f o rie n ta ­
tion e tc. In o rd e r to re m o v e th e se e rro rs sev eral ro ck
sam p les o f s a m e ag e are c o lle c te d and th e p o sitio n o f
p o le s is d e t e r m in e d a f te r th e s tu d y o f th e ir
p a la e o m a g n e tism an d c a lc u la tio n o f av era g e value
on th e b a s is o f sta tistic a l m eth o d s.
B a s e d on th e a b o v e m e th o d th e p o sitio n s o f
p o le s w e re d e te rm in e d in Ja p a n , Italy , F ran ce etc. on
th e b a s is o f p a la e o m a g n e tic r e c o n s tru c tio n o f
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C SOMORPHOLOGY
190
th e ro ck s o f all th e c o n tin e n ts at th a t tim e (d u rin g
rev ersed d ire c tio n o f g e o m a g n e tic fie ld ) are m a g n et­
ized ag ain in th e d ire c tio n o f g e o m a g n e tic fie ld but
th is tim e th e d ire c tio n o f m a g n e tism o f ro ck s is
o p p o site to th e d ire c tio n o f p re v io u s ly fo rm e d and
m a g n e tiz e d ro c k s b e c a u se n o w th e d ire c tio n o f
g e o m ag n etic field h as g o t re v e rs e d itse lf. It is g e n e r­
ally b e lie v e d th a t fie ld re v e rs a l o c c u rs a t reg u la r
in terv al o f tim e.
S c ie n tists h a v e m e a su re d m a g n e tic p o la rity
o f ro ck s u p to 4 .5 m illio n y e a rs w h ic h d e n o te s d e fi­
n ite and p e rfe c t tim e se q u e n c e . T h e ro c k s fo rm e d at
th e sam e tim e p e rio d in all th e c o n tin e n ts d en o te
sam e p o la rity . F ig . 11.2 s h o w s tim e s e q u e n c e o f
rev ersal o f g e o m a g n e tic fie ld o r p o la rity re v ersal
u p to 4 .5 m illio n y e a rs. It is e v id e n t fro m fig . 11.2
th a t th e re are fo u r p o la rity e p o c h s w h e re in tw o
ep o ch s (e.g. G a u ss a n d B ru h n e s ) a re o f n o rm a l
p o la r it y w h ile tw o e p o c h s ( e .g . G ilb e r t an d
M a tu y a m a ) are o f r e v e r s e p o la r ity . P o la rity e v en ts
w ith in d iffe re n t g e o m a g n e tic p o la rity e p o c h s h ave
b een n am ed a fte r th e p la c e w h e re re m a n e n t m a g n e t­
ism (p a la e o m a g n e tis m ) w a s s tu d ie d first.
It is, th u s, fin ally p ro v ed th at based on p o la r
w anderin g curves o f different p eriods f o r different
co n tin en ts on th e basis o f data d eriv e d fr o m
p alaeom agn etic reconstruction not only the con­
c ep t o f con tin en tal d rift is validated but the m ech a­
nism o f disru ption o f W egener's Pangaea, separa­
tion o f d ifferen t contin en ts an d th eir displacem ent
is also validated.
R eversal o f Polarity
T h e stu d y o f p alaeo m a g n etism also rev ealed
th a t m a g n e tiz a tio n o f so m e ro ck s w as not co n fo rm al
to the g e o m a g n e tic field i.e. the ro ck s w ere m a g n et­
ized in o p p o s ite d ire c tio n o f m ain g eo m ag n etic field.
It w as fu rth e r su b sta n tia te d d u rin g th e d ecad e 19506 0 th a t th e o c c u rre n c e o f rev ersely m ag n etized ro ck s
w as not ra re p h en o m en o n rath e r it w as u n iv ersal
p h en o m en o n . T h e av ailab le data o f p alaeo m ag n etism
re v e a ls th e fa c t th a t ab o u t 50 p er c en t o f th e ro ck s o f
th e cru st h av e g o t m ag n etized in o p p o site d irec tio n
to th e g e o m a g n e tic field. T h ere m ay be tw o p o s si­
b ilitie s in this re g a rd —
(1) A t the tim e o f m a g n etiz atio n o f ro ck s at
g iv e n tim e p e rio d so m e ro ck s m ig h t h av e been
m a g n etiz ed in o p p o site d irec tio n to th e g eo m ag n etic
field o r in itia lly all th e ro ck s w ere m a g n etiz ed in th e
d ire c tio n o f g e o m a g n e tic field b u t at a later d ate th e
d ire c tio n o f so m e ro ck s m ig h t h av e c h an g e d and
h e n c e o p p o site d irec tio n o f p a la e o m a g n e tism o f
ro ck s m ig h t h av e b eco m e p o ssib le. T h is m e ch an ism
o f re v e rsa l o f p o la rity is called s e lf r e v e r s a l.
1 1 .3 SEA-FLO O R SPR E A D IN G
T h e c o n c e p t o f s e a -flo o r sp re a d in g w as first
p ro p o u n d e d by P ro f. H ary H ess o f th e P rin c e to n
u n iv e rsity in th e y e a r 1960. H is c o n c e p t w as b a s e d
on th e re s e a rc h fin d in g s o f a la rg e n u m b e r o f m a rin e
g e o lo g ists, g e o c h e m is ts , g e o p h y s ic is ts etc. M a sso n
o f th e S c rip p s I n s titu te o f O c e a n o g ra p h y o b ta in e d
sig n ific a n t in fo rm a tio n a b o u t th e m a g n e tis m o f the
ro ck s o f s e a -flo o r o f th e P a c ific O c e a n w ith th e h elp
o f m a g n e to m e te r. L a te r o n h e s u rv e y e d a lo n g stretch
o f th e s e a -flo o r o f th e P a c ific O c e a n fro m M e x ic o to
B ritish C o lu m b ia a lo n g th e w e s te rn c o a s t o f N o rth
A m e ric a . W h e n th e d a ta o f m a g n e tic a n o m alie s
o b ta in e d d u rin g th e a fo re s a id s u rv e y w e re d isp la y e d
on a c h a rt, th e re e m e rg e d w e ll d e fin e d p a tte rn s o f
strip e s (fig . 6 .9 ). B a s e d o n th e se in fo rm a tio n H ary
H ess p ro p o u n d e d th a t th e m id -o c e a n ic rid g e s w ere
situ a te d o n th e ris in g th e rm a l c o n v e c tio n cu rren ts
c o m in g up fro m th e m a n tle (fig . 6 .1 0 ). T h e o cean ic
c ru s t m o v e s in o p p o s ite d ire c tio n s fro m m id -o cean ic
rid g e s an d th u s th e re is c o n tin u o u s u p w e llin g o f new
m olten m a teria ls (la v a s) alo n g th e m id -o c e a n ic ridges.
T h e se m o lte n la v a s c o o l d o w n a n d s o lid ify to fo rm
new c ru s t a lo n g th e tra ilin g e n d s o f d iv e rg e n t plates
(o c e a n ic c ru s t). T h u s , th e re is c o n tin u o u s c re a tio n o f
n ew c ru s t a lo n g th e m id -o c e a n ic rid g e s . T h is, ac ­
c o rd in g to H ess, p ro v e s th e fa c t th a t s e a -flo o r sp read s
a lo n g th e m id -o c e a n ic rid g e s a n d th e e x p a n d in g
cru sts (p lates) are d estro y ed alo n g th e o cean ic trenches.
(2 ) A lte rn a tiv e ly , o rig in ally th e m a g n e tiz a ­
tio n o f re v e rs e ly m a g n etiz ed ro ck s m ig h t h av e tak en
p la c e in th e d ire c tio n o f g eo m a g n e tic field b u t at a
la te r d a te th e re m ig h t h av e b een rev ersal in th e
d ire c tio n o f g e o m a g n e tic field itself. T h is m e c h a ­
n ism o f re v e rs a l o f p o la rity is calle d g e o m a g n e tic
fie ld r e v e r sa l.
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T h e firs t p o s s ib ility o f rev ersal o f p o la rity i.e.
s e lf r e v e r s a l o f p o la r ity , as re fe rre d to a b o v e, co u ld
n o t be s u b sta n tia te d on th e b asis o f av a ila b le field
d a ta th o u g h N eel s u g g e s te d a few th e o re tic a l p o s si­
b ilitie s to v a lid a te s e lf rev ersal. M o st o f th e s c ie n ­
tists a re o f th e o p in io n th at te rre stria l ro ck s are
m a g n e tiz e d a lw a y s in th e d ire c tio n o f g e o m a g n e tic
f ie ld , b u t th e re is re v e rs a l in th e d ir e c tio n o f
g e o m a g n e tic fie ld , i.e. n o rth -s o u th d ire c tio n o f
g e o m a g n e tic field a fte r c e rta in tim e b e c o m e s so u th n o rth . F o r e x a m p le , if th e g e o m a g n e tic field is in
n o rm a l d ire c tio n (n o rth -s o u th ), all th e ro ck s o f all
th e c o n tin e n ts fo rm e d a t th at tim e are m a g n e tiz e d in
n o rm a l d ire c tio n b u t w h en th e n o rm al d ire c tio n o f
g e o m a g n e tic fie ld g e ts re v e rs e d (s o u th -n o rth ), all
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P L A T E T E C T O N IC S
T hese facts p ro v e th a t the co n tin en ts and ocean
basins are in c o n sta n t m otion. *
W . G . V in e and M atth eu s co n d u cted the m ag ­
netic su rv ey o f th e cen tral part o f C arlsb erg R idge in
Indian O cean in 1963 and co m p u ted th e m ag n etic
p ro files on the b asis o f g en eral m ag n etism . W h en
they com pared the co m p u ted m ag n etic p ro files w ith
the pro files o f m agnetic an o m alies p lo tte d on th e
basis o f actual d ata o b tain ed d u rin g th e su rv ey , they
found sizeab le d ifferen ce b etw een th e tw o p ro files.
W hen they plotted the m ag n etic p ro files on th e b asis
o f altern ate bands o f norm al and rev erse m a g n etism
in separate stripes o f 20 km w idth on e ith e r sid e o f
the ridge, they found co m p lete p arallelism b etw een
the com puted p ro files and o b serv ed p ro files.
V ine and M attheus have op in ed on the basis
o f th e e v id e n c e s o f te m p o ra l re v e rs a l in th e
geom agnetic field and the concept o f sea-flo o r spreading as propounded by D eitz and H ess th a t w h en
m olten hot lavas co m e up w ith th e risin g th e rm al
convection currents along th e m id -o cean ic rid g es
and get cooled and solidified, th e se also g et m a g n e t­
ized at the sam e tim e, in acco rd an ce w ith th e th e n
g eom agnetic field and thus altern ate b an d s o r strip e s
o f m agnetic anom alies are fo rm ed on e ith e r sid e o f
the m id-oceanic ridge. In o th er w ords, w hen m o lten
lavas are upw elled along the m id -o cean ic rid g es,
these divide the earlier basaltic lay er in to tw o eq u al
halves and these basaltic layers slide h o riz o n ta lly on
either side o f the m id-oceanic ridges. T h e fin d in g s o f
C ox, D oell and D alrym pal (1964), O p d y k e et. al
(1966) and H eritzler (1966) h av e v alid ated th e fo l­
low ing facts— (i) th ere is rev ersal in th e m ain
geom agnetic field o f the earth (know n as g eo cen tric
dipole m agnetic field), (ii) norm al and rev erse m a g ­
netic am om alies are found in altern ate m a n n er on
eith er side o f the m id -o cean ic ridges, (iii) th e re is
com plete p arallelism in the m agnetic an o m alies on
either side o f the m id-oceanic ridges and (iv) th ere is
parallelism in the tim e seq u en ce o f p alaeo m a g n etic
epochs and ev en ts calcu lated for 4.5 m illio n y ears on
the basis o f m agnetism o f basaltic rocks o r sed im en ­
tary rocks. Fig. 6.11 d epicts the p o sitio n o f m ag n etic
stripes on eith er side o f the m id -o cean ic rid g e along
w ith the tim e-scale o f th eir form ation.
Fig. 11.2 : Time scale o f reversal o f geomagnetic field
It m ay be co n clu d ed , on the basis o f above
discussion, that th ere is co n tin u o u s sp read in g o f seafloor. N ew basaltic crust is co n tin u o u sly form ed
along the m id -o cean ic ridges. T h e new ly form ed
basaltic layer is div id ed into tw o equal halves and is
thus displaced aw ay from the m id-oceanic ridge.
A lternate stripes o f positive and negative m agnetic
anom alies are found on eith er side o f the m ido ceanic ridges. Such m agnetic anom alies (positive
,in(i negative) are form ed because o t tem poral re« « » H n the geom agnetic Held T he rocks f o m e d
d u rin g norm al geom agnetic Held contain posm ve
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(after A. Cox, 1969).
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GEOM ORPHOLOGY
192
m agnetic anom alies w hile the r w k s f o m e d d u n n g
reverse polarity (reversed g eo m ag n etic field ) d en o te
negative m agnetic anom alies.
T he age o f m ag n etic stripes, th e rate o f seafloor spreading and th e tim e o f d riftin g o f d iffere n t
continents are calcu lated on the basis o f above facts.
T he d ating o f th e m agnetic stripes fo rm ed upto 4.5
m illion years b efo re p resen t has been co m p leted on
the basis o f inform atio n o b tain ed from th e survey o f
palaeom agnetism o f the sea-floors o f different oceans.
T he rate o f sea-flo o r sp read in g is calcu lated on tw o
bases e.g. (i) on the basis o f the age o f is o c h ro n s
(isochrons are those lines w hich jo in th e p o in ts o f
equal dates o f the m agnetic stripes p lo tted on the
m ap) and (ii) on the basis o f d istan ce b etw een tw o
isochrons. T hus, the rates o f sp read in g (d riftin g ) o f
d ifferen t oceans have been determ in ed on the basis
o f above principles. T he A tlan tic and Indian O ceans
are spreading (expanding) very slu g g ish ly i.e. at the
rate o f 1.0 to 1.5 cm p er year w hile the P acific O cean
is expanding at the rate o f 6.0 cm p er year. It m ay be
pointed o u t th a t the rate o f seaflo o r sp read in g alw ays
m eans the rate o f ex p an sio n only on one side o f the
m id-oceanic ridge. F o r exam ple, if the rate o f seafloor spreading is rep o rted to be 1.0 cm p er year, the
total sp reading o f the co ncerned ocean w ould be
1+1=2 cm p e r year. T he recent studies have show n
that the m axim um spreading o f the P acific O cean is
6 to 9 cm p e r y ear (total expansion 12 to 18 cm /year)
along the eastern P acific ridge betw een eq u ato r and
30° S latitude, (ii) the southern A tlan tic O cean is
spreading along the southern A tlan tic rid g e at the
rate o f 2 cm p e r y ear (total ex pansion 4 cm /y ear) and
(iii) th e In d ian O cean is ex p an d in g at th e rate o f 1.5
to 3 cm p e r y e a r (total ex p an sio n bein g 3 to 6 cm /
y ear).
on th e su rface o f a sp h e re can b e re g a rd e d as a sim ple
ro tatio n o f th e p la te a b o u t a su ita b le c h o se n axis
p a ssin g th ro u g h th e c e n tre o f th e s p h e re ’ (E .R .
O x b u rg h , 1979) (fig . i 1.3). T h e ro ta tio n ax is o f
p la tes p a sse s th ro u g h th e c e n tre o f th e g lo b e. ‘A ll
p o in ts o n th e p la te tra v e l a lo n g sm all c irc le p ath s
ab o u t th e ch o se n a x is ( o f ro ta tio n ) in p a ssin g fro m
th e ir in itia l to fin al p o sitio n s. It fo llo w s th a t any
p la te b o u n d ary w h ic h is c o n s e rv a tiv e (i.e. in v o lv es
n eith er p la te g ro w th n o r d e s tru c tio n ) m u s t b e p a ra l­
lel to sm all c irc le, th e ax is o f w h ic h is th e a x is o f
ro tatio n fo r th e re la tiv e m o tio n ’ (E .R . O x b u rg h ,
1979). O n th e o th e r h an d , th e m a rg in o f th e p la te ,
w hich is n o t p arallel to sm all c irc le , b e c o m e s e ith e r
c o n stru ctiv e (acc re tin g ) o r d e s tru c tiv e (c o n s u m in g )
p late m argin.
11.4 PLATE MOTION
A ll lith o sp h eric plates co n stan tly m o v e w ith
resp ect to each o th e r w ith v ary in g rates. P late m o ­
tions are cu rren tly m easu red and m o n ito re d by u sin g
satellites and lasers. It m ay be m en tio n ed th a t each
p la te m oves as a sin g le u n it h av in g relativ ely little
c h an g es in its m id d le p art. O n ly th e p la te m arg in s
u n d erg o changes. It is also to b e stated th a t th e p la te
m o tio n is rela tiv e w ith resp ect to o th e r p la te i.e. any
c h an g e in rate o r d irec tio n o f m o tio n in o n e p la te
cau ses c o rresp o n d in g c h an g es in o th e r plates. I f th e
p la te s a re rig id b lo ck s an d m o v e on th e su rface o f th e
sp h erical earth, th e ir m otion can b e ex p la in e d in
terms o f E u ler 's g eo m etrica l th eo rem .
T h e m ech an ism o f p late m o tio n can b e e x ­
plained on the b asis o f fig . 11.3 (as d escrib ed by E .R .
O xburgh, 1979). A B E represents origin al sin g le
con tigu ou s landm ass w h ich has sp lit in to tw o b lock s
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‘E uler's g eo m etrical th eo rem show s th a t ev ery
d isp la cem e n t o f a p la te fro m o n e p o sitio n to an o th e r
Fig. 11.3 : Plate m otion according to p la te tectonic theory
based on E uler's geo m etrica l theorem . A B E
represents earlier one co ntiguous landm ass
w hich has been sp lit into tw o blocks i.e. A B C
a n d A D E blocks. A represents p o le o f rotation
w hich is also contact p o in tfo r sep a ra ted A B C
a n d A D E blocks. S o lid lin es in d ica te sm a ll
circle p a th s o r ‘lin es o f la titu d e ’ a b o u t the
p o le o f rotation (A), broken lin es-la titu d es
a n d lo n g itu d es a n d N -S d e n o te s g eo g ra p h i­
cal north a n d south pole. (a fterE .R . O xburgh,
1979).
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plate
te c to n ic s
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1 93
i.e. block X (re p re s e n te d by A B C ) and b lo c k Y
(represented by A D E ). T h e se tw o sep arated blocks
have c o n ta c t at p o in t A (p o in t o f p o le ro tatio n ).
Black so lid lin es d e n o te sm all circ le p ath s around
centre o f p o le o f ro ta tio n (A ). B ro k en lines rep resen t
geographical la titu d e s an d lo n g itu d es. P lates m ove
parallel to th e sm all c irc le p ath s aro u n d the cen tre of
pole o f ro ta tio n (A ). P la te m o tio n is alm o st zero at
the cen tre o f p o le o f ro ta tio n (A ) an d in creases aw ay
from A and b e c o m e s m a x im u m at 90° from A (i e at
0° latitu d e w h ic h re p re s e n ts sm all circ le path and not
the g e o g ra p h ic a l la titu d e ). In fig. 11.3 ps and qr
rep resen t sid e s o f th e re e n tra n t. I f th e se (sid es) are
p arallel, th e y a re a lso p a ra lle l to th e lines o f latitu d e
(sm all c irc le p a th s ) a b o u t A . T h e se sid es (ps and qr)
re p re se n t c o n s e rv a tiv e p la te m a rg in s, w h ich during
plate m o v e m e n t a re n e ith e r ac c re te d no r co nsum ed.
It m ay b e m e n tio n e d th a t lin es o f la titu d es are, in
fact, ro ta tio n a l la titu d e s .
the addition (accretio n ) o f new b asaltic cru st a t the
co n stru ctiv e plate m arg in s alo n g m id -o cean ic ridges
and co n seq u en t sea-flo o r sp read in g is su ita b ly c o m ­
pensated by loss o f cru st due to su b d u ctio n alo n g the
co n v erg in g (co n su m in g ) p late b o u n d aries.
‘A c o ro lla ry o f E u le r's th eo rem is that the
v e lo c ity o f re la tiv e m o tio n acro ss a co n stru ctiv e or
d e s tru c tiv e b o u n d a ry is p ro p o rtio n a l b o th to an g u lar
v e lo c ity a b o u t th e ax is o f ro tatio n fo r the m otion o f
th e p la te s , a n d to th e a n g u la r d ista n ce o f th e p o in t on
th e b o u n d a ry u n d e r c o n sid e ra tio n from the axis o f
ro ta tio n (fig . 11.3). It im p lie s th at v elo cities vary
c o n tin u o u s ly a lo n g all c o n stru c tiv e and d estru ctiv e
b o u n d a rie s , b e in g sm a lle s t in ‘h ig h r o ta tio n a l la ti­
t u d e s ’ a n d g re a te s t in ‘lo w r o ta t io n a l la tit u d e s ’
(E .R . O x b u rg h , 1979).
W .J. M o rg a n h as su c c e s s fu lly e x p lain ed the
s p re a d in g o f e q u a to ria l A tla n tic and p late m o v em en t
on th e b a s is o f E u le r's g e o m e tric a l th e o re m . It m ay
be m e n tio n e d th a t m id -A tla n tic rid g e crest is d is ­
p la ced on e ith e r s id e a lo n g n u m e ro u s tran sfo rm
fau lts ru n n in g in a lm o s t e a s t-w e s t d ire c tio n , w hose
im p o rta n c e h e re is th a t th ey re p re s e n t c o n serv ativ e
secto rs o f p la te b o u n d a ry s e p a ra tin g c o n stru ctiv e
se c to rs -th e s p re a d in g p a rts o f th e rid g e (m id -A tla n ­
tic rid g e ). A ll a c tiv e tra n s fo rm fau lts on the sam e
rid g e o u g h t to b e s e g m e n ts o f c o -a x ia l sm all circles
if the p la te m o d e l is v a lid ’ (E .R . O x b u rg h , 1979).
A cc o rd in g to p la te m o d e l w ith re fe re n c e to P ^ e
m otion b a s e d on E u le r 's g e o m e tric a l th e o re m all the
great c irc le s , i f d ra w n , m u s t in te rs e c t at a sin g le
point w h ic h w o u ld b e th e p o le o f rotation . W h en
W .J. M o rg a n c o n s tr u c te d g re a t c irc le s n o rm al to the
strike o f tra n s fo rm fa u lts o f th e e q u a to ria l A tla n tic
O cean (fig . 1 1 .4 ), h e fo u n d th a t all th e great circ le s
except o n e in te rs e c te d a t o n e c o m m o n p o in t P (lig11.4) at 5 7 .5 °N a n d 3 6 .5 °W . T h is re su lt th u s v a li­
dated th e m e c h a n is m o f p la te m o tio n on th e h a s i s 1
E uler's g e o m e tric a l th e o re m . It m a y b e e la rilic d th at
the s u rfa c e a re a o f th e e a rth d o e s n o t in c re a s e d u e to
plate m o v e m e n t r a th e r it re m a in s c o n s ta n t b e c a u se
Fig. 11.4 : Intersection o f great circles at a common
point P which denotes pole o f rotation, (source:
E.R. Oxburgh, 1979).
11.5 C A U SE S OF PLATE MOTION
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S ev eral m e c h a n ism s, so u rces and p o ssib le
cau ses o f p la te m o tio n (m o v e m e n t) h av e b een s u g ­
g ested by sc ie n tists but n o n e o f th e m co u ld be fully
su b sta n tia te d till now d u e to la ck o f c o n v in c in g
ev id en ces. A m a jo rity o f sc ie n tists c o n s id e r th e rm al
c o n v e c tiv e cu rren ts in sid e th e earth as p o s sib le d riv ­
ing fo rce fo r th e m o v e m e n t o f p la tes. It m ay be
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GEOMORPHOLOGY
194
it has no w b een v a lid a te d th a t th e c o n tin en ts and
o cean b a sin s h av e n e v e r b een sta tio n a ry or p erm a­
nen t at th e ir p la ces ra th e r th e se h av e alw ay s been
m o b ile th ro u g h o u t th e g e o lo g ic a l h isto ry o f the earth
and th ey are still m o v in g in re la tio n to each other.
T h e sc ie n tists h av e d is c o v e re d am p le ev id en ces to
d e m o n stra te th e o p e n in g an d c lo sin g o f ocean ba­
sins. F o r ex a m p le , the M e d ite rra n e a n S ea is the
resid u al o f o n ce v ery v ast o cea n (T e th y s S ea) and the
P acific O cean is c o n tin u o u s ly c o n tra c tin g b ecau se
o f g rad u al su b d u c tio n o f A m e ric a n p la te alo n g its
rid g es. In n u tsh ell it m ay be o p in e d th a t co n tin e n ta l
d rift h as n o w b e co m e a r e a lity on th e b a sis o f
p la te tecto n ic s. T h e d e ta ile d d e s c rip tio n o f c o n ti­
nental d isp la c e m e n t has b een p ro v id e d in c h a p te r 6
o f this b o o k (see c h a p te r 6, s u b se c tio n : p la te te c to n ­
ics and co n tin en ta l d rift).
p o in ted o u t th a t A . H o lm es p o stu lated the co n c e p t o f
risin g th erm al co n v ec tio n cu rren ts from w ith in th e
earth in 1928. T h e m ech an ism o f th erm al c o n v ec tiv e
cu rren ts in liquid m a tte r w as th e o re tic ally stu d ied by
L ord R ay leig h . C u rren tly , a h ost o f scien tists have
accorded acc ep tan c e to the m e ch an ism o f therm al
co n v ec tiv e cu rren ts on the basis o f therm al and
p ressu re c o n d itio n s o t th e in terio r o f the earth . T he
pattern o f risin g (ascen d in g ) and fallin g (d e sc e n d ­
ing) therm al co n v ec tio n cu rren ts has been sh o w n in
fig. 6.10 in ch a p te r 6 o f this book. I.G . G ass has
v alid ated the m e ch an ism o f origin and m o v e m en t o f
u n stab le th erm al co n v ectio n cu rren ts in the m an tle
(below earth 's crust). A cco rd in g to G ass the v isco s­
ity o f m a n tle d ep en d s en tirely on tem p eratu re and
pressure. T he v iscosity o f ascending m aterials caused
by u p w ard m o v e m en t o f therm al co n v ectiv e c u r­
ren ts due to high tem p eratu re d ecreases and hence
the upw ard flow velo city o f the m atter increases.
T he cen tres o f up w ard m o v em en t o f hot and liquid
m a tter w ith ascen d in g cu rren ts are g en erally located
below the m id -o cean ic ridges. T h o u g h the depth o f
such cen tres is not correctly know n but it is believed
that these are located at an averag e depth o f 3 0 0 -4 0 0
km from the earth 's su rface. T he rising co n vection
cu rren ts tran sp o rt hot and liquid m atter upw ard
w hich afte r reach in g the p o int ju s t below the crust
(p lates) sp lit and d iv erg e in o p p o site d irectio n s in the
form o f h o rizo n tal How w hich is co n fin ed to the
depth upto 2 0 0 km . T hus, the d iv erg en ce o f co n v ec­
tion cu rren ts (ju st b elow the m id -o cean ic ridges)
w ith hot and m olten m atter cau ses plate m ovem ent
in o p p o site d irec tio n s. O n the o th er hand, tw o sets o f
co n v erg in g therm al co n v ectio n cu rren ts brin g tw o
plates to g e th e r and the plate m arg in s are su b ducted.
11.7 PLATE TECTONICS AND MOUNTAIN BUILD­
ING
P late tecto n ic th eo ry h as e n a b le d s c ie n tis ts to
explain the p ro b lem o f o rig in o f fo ld e d m o u n ta in s
w hich w as h eth erto u n re so lv e d till th e p o s tu la tio n o f
this g reat scien tific th eo ry in th e d e c a d e 19 6 0 -7 0 . It
m ay be po in ted o u t th a t sev eral h y p o th e s e s h av e
been p ro p o u n d ed to so lv e th is g ig a n tic g e o lo g ic a l
problem (e.g. th erm al c o n tra c tio n h y p o th e s is by
Jeffrey s, co n tin en ta l d rift th eo ry by F .B .T a y lo r a n d
A .G . W eg en er, th erm al co n v e c tio n c u rre n t h y p o th ­
esis by A. H o lm es, slid in g c o n tin e n t h y p o th e s is by
D aly, rad io a ctiv ity h y p o th e sis by Jo ly etc) fro m tim e
to tim e b u t n o n e o f th em co u ld b e u n iv e rs a lly a c ­
cep ted b eca u se th e e x p o n e n ts o f th e se h y p o th e s e s
co u ld not p re se n t c o n v in c in g s c ie n tific e v id e n c e s in
su p p o rt o f th e ir re s p e c tiv e h y p o th e se s . N o w , the
plate te cto n ic th eo ry o ffe rs c o n v in c in g e x p la n a tio n
fo r the so lu tio n o f c o m p le x rid d le o f m o u n ta in b u ild ­
ing. T h e m o d e o f o rig in o f fo ld ed m o u n ta in s on the
basis o f p la te te c to n ic s h as b e e n d e ta ile d o u t in
c h a p te r 13 ol th is b o o k (se e c h a p te r 13, m o u n ta in s
and m o u n ta in b u ild in g ).
S o m e sc ie n tists are o f the view th at plate
m otion is c au sed d ue to high g rav ity fo rce b ecau se o f
creation o f a d d itio n al m a tter (lav a and m ag m a) on
eith er side o f the m id -o cean ic ridges. T h is high
gravity fo rce ca u se s lateral m o v e m en t o f plates in
o p p o site d irec tio n from the rid g e crests. A cco rd in g
to an o th er view the in tru sio n o f m ag m a in the m id ocean ic rid g es from b elow cau ses sep aratio n o f
oceanic p la tes from rid g e c re sts and th e ir d is p la c e ­
m ent in o p p o site d irectio n .
11 .8 PLATE TECTONICS AND VULCANICITY
B ased on p late te c to n ic s th e re is clo se relation­
ship b etw een p late b o u n d a rie s an d v u lcan icity as
m o st o f th e w o rld 's activ e v o lc a n o e s are associated
w ith p late b o u n d arie s. A b o u t 15 p e r cen t o f the
w o rld 's activ e v o lc an o es are fo u n d alo n g the con ­
str u c tiv e p la te m a r g in s o r d iv e rg e n t p la te m argins
(alo n g the m id -o cean ic rid g es, w h ere tw o p lates m ove
in o p p o site d ire c tio n s) w h ereas 8 0 p e r c e n t volcanoes
are a s s o c ia te d w ith th e d e s tr u c tiv e /c o n v e r g e n t/
c o n su m in g p la te b o u n d a r ie s (w h ere tw o plates col-
It m ay b c c o n c lu d e d th a t this is the w eak p o in t
(lack o f co m m o n ly acc ep ted c a u se o f p la te m o tio n )
in the theory o f p la te te cto n ic s w h ich is v u ln e ra b le to
severe c riticism . T h is p ro b lem o f real c a u s e o f p late
m otion needs scien tific so lu tio n .
11.6 PLATE TECTONICS AND CONTINENTAL
DRIFT
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O n the basis of the e v id e n c e s o f re c o n s tru c ­
tion o f p a la eo m a g n etism and s e a - f W s p r e a d in g
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PLATE TECTO N ICS
195
iide). S om e v o lc a n o e s a ie also lo u n d in in tra-plate
regions e.g. v o lc a n o e s o t th e H aw aii Islan d , fau lt zone
0t East A frica etc. T h e re are th ree m a jo r belts ot
volcanoes e.g. (1) m id -A tla n tic R id g e zone, (2) circum
pacific zone an d (3) m id -c o n tin e n ta l zone.
tie and fo rm atio n o f th o leiite b asalt w h ich m o v e s
u p w ard th ro u g h ascen d in g therm al c o n v e c tio n c u r­
rents and ap p ears as fissu re flow o f b asaltic lava,
T h is b asaltic th o leiite lav a afte r co o lin g and s o lid i­
fication fo rm s new o cea n ic c ru st (fig. 12.5 in c h a p te r
12). T his v o lcan ic m e ch an ism lead s to fo rm a tio n o f
ridges parallel to m id -o cean ic rid g es. T h e n ew ly
form ed b asaltic cru st is d iv id ed into tw o eq u al halv es
and arc em p laced on e ith e r sid e o f th e rid g e. T h e se
parallel basaltic strip e s p laced on e ith e r sid e o f the
ridge m ove aw ay from the m id -o c e a n ic rid g e d u e to
sea-flo o r sp read in g effected by a sc e n d in g th erm al
co n v ectio n cu rren ts and a sso ciated u p w e llin g o f
lava and (b asaltic strip e s) arc ac c re te d at the tra ilin g
m arg in s o f d iv erg en t plates. T h is is also v a lid a te d on
the basis o f p arallel but altern ate p attern o f p o sitiv e
and negative an o m alies o f p a la e o m a g n e tic strip e s
(fig. 11.5, also see figs. 6.9 and 6 .1 1 ). Ic e la n d
presents an ideal ex am p le o f th is m e c h a n ism b e­
cau se it is situ ated on both th e sid es o f m id -A tla n tic
ridge i.e. m id -A tlan tic rid g e (lo cally called as R ey kjanes ridge) p asses th ro u g h the m id d le o f Ic e la n d
th ro u g h w hich m a g m a u p w ells from tim e to tim e.
The eru p tio n o f H elg afell v o lcan o in 1973 p re s e n ts
ev id en ce in su p p o rt o f this p ro p o sitio n . T h e re is
co n tin u o u s g ro w th in the su rface area o f Ic e la n d d u e
to b asaltic lava. It is estim ated th at th e isla n d has
T h e in ten sity o f v o lc a n ic a ctiv ity is also related
to the nature o t p la te b o u n d a rie s. D iv erg en t or c o n ­
structive p la te b o u n d a rie s are a sso ciated w ith quiet
volcanic e ru p tio n k n o w n as f is s u r e e r u p tio n . The
volcanic lav a o t c o n s tru c tiv e p late m arg in s is th o le iite
w hich is in ta c t a ty p e ot b a sa lt h av in g less quantity
of potash and is fo rm e d d u e to d ifferen tial m elting.
The b asaltic la v a a s s o c ia te d w ith d iv e rg e n t (d estru c­
tive) plate b o u n d a rie s re p re se n tin g circu m -P acific
belt and m id -c o n tin e n ta l (A lp in e) belt is rich in silica
content and is m ix e d w ith an d esite, d acite and rhyolite.
The v o lc an ic la v a a sso c ia te d w ith rilt v alleys is rich in
alkalis. T h is is a lso c a lle d as a lk a lin e b a s a lt.
A c tiv e v o lc a n o e s are a sso c ia te d w ith m ido cean ic rid g e s . U n d e r th e in flu e n c e o f risin g th e r­
mal c o n v e c tio n c u rre n ts o c e a n ic p la tes (cru st) are
se p a ra te d an d tw o p la te s m o v e in o p p o site d irectio n s
from the ridtze c re s ts . B e c a u se o f d iv e rg e n c e o f tw o
plates th e c o n fin in g p re s s u re o f su p erin cu m b en t
load is re le a s e d an d c o n s e q u e n tly m e ltin g p o in t is
lo w ered w h ic h c a u s e s p a rtia l m e ltin g o f u p p er m an✓\
/ N
-----V
B
c ta n
flo o r
/ / / V / Mm
^
A scending
magma
NO R M A L MAGNETISM
R E V E R S E D MAGNETISM
fv -V v
A scending
magma
NORMAL MAGNETISM
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Fig. 1].5 : Formation o f ocean flo o r (magma) stripes on either side o f mid-oceanic ridge and magnetization. A.
Ascending magma after reaching the ridge crest is solidified on cooling and is magnetized in accordance with
the direction o f geomagnetic field. This is the present case o f normal magnetization. B. Formerly created
basaltic layer (1) moves away from llie ridge ancl new basaltic stripes form ed due to further upwelling o f
magma and the solidified stripe gets magnetized in accordance with reversed geomagnetic field (indicated by
arrow). This is the case o f reversed magnetism. C. Geomagnetic field returns to its normal position (upward
arrow) and the new ly form ed magma stripe close to the ridge is magnetized in accordance with normal
geomagnetic field, a case o f normal magnetism. The upper part o f the diagram denotes positive (shown by
+) and negative (shown b y—) magnetic anomalies. ([f'ter—M J. Bradshaw, A. J. Abbott and A. P. Gelsthorpe, 1978.
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GEOMORPHOLOGY
196
th e se v o lc an ic isla n d s arc su b m erg ed urjder sea
w av es and b eco m e sen m o u n ts or g u y o ts (fig. 11.6).
It m ay be m e n tio n e d th at not all the v o lc an ic peaks
su b m e rg e b e n e a th se a w av es as a few o f them p ro ject
fro m 1500 to 3 0 0 0 m ab o v e se a-le v el. T h e study o f
b asaltic la v a o f th e v o lc a n ic islan d s o f th e A tlan tic
O cean h as re v e a le d th e fact that v o lc a n ic islands
lo cated n eare st to th e rid g e arc c h a ra c te riz e d by
recen t lava w h ile th o se lo c ated at th e fa rth e st d is­
tan ce fro m the rid g e h a v e o ld e st lava. F o r ex am p le,
th e o d est lava o f A z o re s isla n d s lo c a te d on c ith e r
sid e o f the m id -A tla n tic rid g e is 4 m illio n y ears old
w h ile the o ld est la v a o f C a p e V e rd e isla n d lo c ated
near A frican co ast (fa rth e st from th e rid g e ) is 120
m illio n y ears old. F ig . 11.6 re p re s e n ts s e a -flo o r
sp read in g , v u lc a n ic ity , fo rm a tio n ol v o lc a n ic is­
lands and th e ir d is p la c e m e n t from th e rid g e .
g ro w n in size by 4 0 0 km sin ce the b eg in n in g o t
T e rtia ry (65 m illio n y ears B .P .) e p o ch , w h ich in d i­
c ates a v era g e g ro w th rate o f 0 .6 cm /y r. T h e age o f
la v a (b asalt) in c re ases aw ay fro m th e rid g e as recen t
la v a is fo u n d c lo se to th e rid g e, 2 m illio n y ear-o ld
la v a aw ay fro m the rid g e and 65 m illio n - y ear old
lav a at th e m a rg in o f th e island.
T h e a fo re sa id in fere n ce is also v alid ate d on
th e basis o f e v id e n c e s o f v o lcan ic islan d s situ a ted on
th e ocean floor. F o r ex am p le, th e v o lcan ic islan d s o f
A tla n tic O cean are w ith o u t d o u b t a sso ciated w ith
th e m id -A tla n tic rid g e. T h e m o st activ e v o lcan ic
islan d s are n e a re st to th e rid s e w h ereas d o rm an t and
ex tin c t v o lc a n o e s are lo cated at th e farth e st d istan ce
from the rid g e. It m ay be p o in ted o u t th a t v o lcan ic
islan d s are fo rm ed n e a r the rid g e due to u p w ellin g
o f m a g m a fro m b elo w . A s the sea flo o r sp read s these
v o lcan ic peaks m o v e aw ay from the rid g e and m ag m a
so u rce. W h en they m o v e far aw ay from the ridge the
su p p ly o f m a g m a co m es to an end and thus m o st o f
T h e islan d arcs w ith v o lc a n ic p e a k s a n d a s s o ­
ciated o cean ic tre n c h e s are fo rm e d w h en o c e a n ic
p late is su b d u cted b elo w c o n tin e n ta l b elt. S e is m ic
Fig. 11.6 : Sea-floor spreading, vulcanicity and form ation o f volcanic islands. A-form ation o f 1st volcanic islatul 70
million years ago, B-present situation, gradual shifting o f volcanic islands due to sea-floor spreading.
Volcanic island in A (shown by 1) has m oved fa r away to position 1 in B. (after M.J. Bradshaw et. al, 1978).
s h o c k s and h e a t a re g e n e ra te d at th e d e p th o f 7 0 0 km
d u e to fric tio n o f co n tin e n ta l p la te an d su b d u c te d
o c e a n ic p la te . C o n s e q u e n tly , u p p e r m a n tle , b a sa ltic
c ru s t o f o c e a n flo o r an d o v e rly in g se d im e n ts g et
m e lte d an d th u s m a g m a is fo rm e d . It m ay b e p o in te d
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o u t th a t v o lc a n ic p e a k s o f isla n d a rc s h a v e been
fo rm e d of s o d iu m -ric h b a s a lt. S u c h b a s a lt is form ed
w h en v o lc a n ic e ru p tio n o c c u rs in o c e a n ic w ater.
S o d iu m -ric h b a s a lt is c o v e re d w ith a n d e s ite o f rela­
tiv e ly le s s e r d e n s ity b u t ric h in s ilic o n in<com parison
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PLATE t e c t o n i c s
to u n d erly in g b a sa lt.
sity the resu ltan t m a g m a intrudes in the o v erly in g
crust, w ith the result folded m o u n tain s are fu rth er
uplifted and g ranitic b ath o lith s h av in g q u a rtz and
feldspar m inerals are also form ed. T he Ranchi batholiths
o f the C hotanagpur H ighlands (Bihar) m ay be associated
w ith A rachaean m o u n tain building.
R e g a id in g th e o rig in o f a n d e s ite -d a c ite rhyolite a lo n g th e c irc u m -P a c ific fo ld ed m o u n tain
chain tw o c o n tra s tin g v ie w s h av e b een floated
( 1) R ing w o o d (1 9 7 4 ) h as staled that a n d e s i l e dacite rh y o lite a re fo rm e d d u e to p artial melting o f
am p h ib o h te of s u b d u c te d B e n io ff zo n e and m ellm *
o f q u artz e c lo g ite at g re a te r d e p th in the m a n tle *
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T he origin and characteristics o f lava plateaus o f
the continents can also be explained on the basis o f
(2)
A c c o rd in g to G illu ly a n d e s ite — d ac ite — plate tectonic theory. T he form ation o f extensive basaltic
lava plateaus o f India, B razil, C olum bian plateau o f the
rhyolite are fo rm e d d u e to p a rtia l m e ltin g o f o cean ic
U
SA etc. m ay be related to continental breaking. It is
tholente o r a m p h ib o h te o r e c lo g ite an d its m ix in g
believed that lava plateaus m ight have been form ed due
/w ith se d im e n ts ol o c e a n flo o r su ch as san d sto n e
chert and ra d io la ria n o o ze.
to separation o f continents and th eir m o v em en t in
relation
to other continents e.g. D eccan lava plateau o f
A p p a ic n tly , th e e x p la n a tio n ol v o lc an o es o f
India
due
to its separation from A frica-A ustral ia and its
Hawai Is la n d (fig . 12.4) d o es n o t fit in the fra m e ­
northw
ard
m ovem ent; C olum bia lava plateau o f the
work ol p la te te c to n ic th e o ry b u t the p ro b lem m ay be
USA due to separation o f N. A m erica from E u ro p e and
solved if w e lo o k in to th e e n tire m e c h a n ism involved
w estw ard m ovem ent o f the form er; B razilean lava
in the v o lc a n ic p ro c e s s in the e a s t P a c ific O cean . T he
plateau due to separation o f S. A m erica from A frica and
Hawai Is la n d is s o u th -e a s te rn e x te n sio n o f M idw ay
w estw ard m ovem ent o f the form er etc.
Islan d -E m p ero r s e a m o u n ts — K am ch a tk a Island A rcs
R eaders are ad v ised to co n su lt c h a p te r 12 o f
and is lo c a te d fa r a w a y fro m th e E ast P acific R idge
this
book
for ex p lan atio n o f o rigin o f v o .c a n o e s o f
but H aw ai I s la n d is c h a ra c te riz e d by activ e vo lcan ic
circu m -P acific belt, m id -A tlan tic b elt, m id -c o n ti­
activities w h e re a s th e a b o v e m e n tio n e d island arcs
nental belt etc. in term s o f d iffere n t ty p e s o f p la te
are d o m in a te d by d o rm a n t v o lc a n o e s and ancient
b
o u n d aries (e.g. co n v erg en t, d iv e rg e n t an d c o n ­
lava (25 to 75 m illio n y e a rs o ld , fig. 12.4 in ch ap ter
servative
plate b o u n d aries).
12). It is b e lie v e d th a t th e re is a c tiv e p lu m e (m ag m a
source) b e n e a th H a w a i Is la n d w h ich en su res co n ­
11.9 P L A T E T E C T O N I C S AND E A R T H Q U A K E S
tinuous s u p p ly o f m o lte n m a g m a for lo n g e r duration
Seism ic events can be ex p lain ed in term s o f
of tim e. T h e re h a s b een u p w e llin g o f lava in the
plate
boundaries.
From the stan d p o in t o f m o v e m en t
H aw ai Is la n d fo r th e la st 7 0 m illio n years. D ue to
and
tectonic
events
and creation and d estru ctio n o f
plate m o v e m e n ts th e P a c ific O cean ic flo o r after
geom
aterials
the
plate
boundaries are d iv id ed into ( l )
being s e p a ra te d fro m E a s t P a c ific R id g e co n tin u ed
c o n s tru c tiv e p la te b o u n d a rie s , (ii) d e s tru c tiv e p late
to m ove in n o rth -w e s te rly d ire c tio n at the rale of 9
b o u n d a rie s , and (iii) c o n s e rv a tiv e p la te b o u n d a ­
cm per y e a r w ith th e re su lt v o lc a n ic p eak s hav in g
ries. C onstructive plate boundaries rep resent the trail­
plum e u n d e rn e a th a lso m o v e d n o rth -w estw ard . T hus,
ing ends o f divergent plates w hich m ove in o p p o site
the p lu m e b e n e a th H a w a i Isla n d c o n tin u e d to supply
directions
from the m id-o cean ic ridges, d estru ctiv e
lava to th e v o lc a n o e s of th e isla n d . O n the othei
plate boundaries are those w here tw o co n v erg e n t
hand, as th e o th e r is la n d s m o v e d ta r aw ay from the
plates collide ag ain st each o ther and the h eav ier plate
centre (p lu m e )o f la v a s u p p ly d u e to se a -flo o r sp re a d ­
boundary is subducted below relativ ely lig h ter plate
ing, the lav a s u p p ly d rie d up an d th e v o lcan o es
boundary and co n serv ativ e plate b o u n d aries are those
becam e d o rm a n t.
w here tw o plates slip past each o th er w ith o u t any
T h e fo rm a tio n ra th e r e m p la c e m e n t ot granites
collision. M ajo r tectonic ev en ts asso ciated w ith these
into co n tin en tal fo ld ed m o u n ta in s and intrusion o
plate b oundaires are ruptures and faults along the
batholiths can be e x p la in e d on the basis of plate ^ co nstructive plate b o undaries, fau ltin g and folding
tectonic theory. D u rin g th e c o llisio n of plates and
along the d estructive plate bo u n d aries and transform
form ation o f fo ld ed m o u n ta in s co n tin en tal rocks are
faults along the co n serv ativ e plate boundaries. A ll
subducted and re a c h g re a te r d ep th w here these get
sorts o f d ise q u ilib riu m are caused due to d ifferent
melted and form m a g m a T h e re is m a rk e d variatio n in
types o f plate m otions and co n sequently earthquakes
the com p osition o f m a g m a in v o lv ed in the co n tin en ­
o f varying m ag n itu d es are caused.
tal folded m o u n ta in s and lava o f v o lcan o es of island
N orm ally, m oderate earth q u ak es are caused
arcs and an d esite la v a o f su b d u ctio n zone. C o n tin en ­
along the con stru ctiv e plate b oundaries because the
tal rocks are d o m in a te d by low d en sity m atter e.g.
rate o f rupture o f the crust and co n seq u en t m o v em en t
silica and a lu m in iu m o x id e co n ten ts. W hen m elted,
o f plates aw ay from the m id -o cean ic ridges is rath e r
the resultant m a g m a is also d o m in ated by such m atter
slow and the rate o f upw elling o f lavas due to fissure
(silica and allu m in iu m o x id es). B e c a u se oi low d e n ­
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GEOMORPHOLOGY
198
flow is also slow . C onsequently, sh a llo w focus ea rth ­
q u akes arecau sed along the constructive plate bounda­
ries o r say along the m id-oceanic ridges. 'Flic depth ol
‘fo c u s ’ o f earth q u ak es associated with the co n stru c­
tive plate bo u n d aries ranges betw een 25 km to 35 km
but a few earth q u ak es have also been found to have
o c c u n o d al the depth o f 60 km . Il is, thus, o bvious that
the earth q u ak es o ccu rrin g along the m id-A tlantic
R idge, m id -Indian O cean ic R idge and East Pacific
R ise are cau sed because o f m ovem ent o f plates in
o p p o site d irectio n s (div erg en ce) and co n seq u en t for­
m ation o f faults and ruptu res and up w elling o f m agm a
or fissure flow o f basaltic lavas (fig. 11.7).
E a rth q u a k e s o f high m ag n itu d e and deep fo­
cu s are cau sed alo n g th e co n v e rg e n t o r destructive
p late b o u n d arie s b eca u se o f c o llisio n o f tw o conver­
gent plates and c o n se q u e n t su b d u ctio n o f one plate
b o u n d ary alo n g the B e n io ff zo n e. H ere m ountain
b u ild in g , fau ltin g and v io len t v o lcan ic eruptions
(cen tral ex p lo siv e ty p e o f e ru p tio n s) ca u se severe
and d isastro u s e a rth q u a k e s h a v in g th e fo cu s at the
depth upto 7 0 0 k m . T h is p ro c e ss, c o n v erg e n ce of
plates and related p late c o llisio n , e x p la in s the m axi­
m u m occurrence o fearth q u ak es o f v arying m agnitudes
alo n g the F iry R in g o f th e P a cific o r th e C in icm P a cific B elt (alo n g the w estern and ea ste rn m argins
o f th e P acific O cean or say a lo n g the w estern coastal
Ocean ridge-. Ocean trench
(spreading)! (convergence '
C o n tin e n t-
Transform
Jam
Heoled transform fault
Cool'lithtfsphere:: : : : : : : :
.......... Lithosphere —
_
Hot asthenosphere
Hot m atter rises into
ocean ridge rift
Rising magmo
Shallow earthqu akes
Deep ea rth q u a k e s
Fig. 11.7 : R elationship between earthquakes and plate boundaries, after, F. Press a n d R. Seiver, 1978.
A siatic p la te c a u s e s e a rth q u a k e s o f th e m id -co n ti­
n ental belt.
C re a tio n o f tra n s fo rm fa u lts a lo n g the con­
se rv a tiv e p la te b o u n d a rie s e x p la in s th e o ccurrence
o f sev ere e a rth q u a k e s o f C a lifo rn ia (U S A ). H ere,
o n e p art o f C a lifo rn ia m o v e s n o rth -e a stw a rd w hile
the o th e r p art m o v e s s o u th -w e s tw a rd a lo n g the fault
p la n e an d th u s is fo rm e d tra n s fo rm fau lt w hich
c a u se s e a rth q u a k e s .
T su n a m i
T h e w a v e s g e n e ra te d in th e o c e a n s triggered
by h ig h m a g n itu d e e a rth q u a k e s in th e o cean floors
(e x c e e d in g 7.5 on R ic h te r sc a le ), o r by v io len t cen­
tral v o lc a n ic e ru p tio n s (su c h as K r a k a t a o eru p tio n in.
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m a rg in s o f N o rth and S o u th A m eric as and th u s the
R o c k ie s-A n d e s M o u n tain B elt and alo n g the easte rn
c o asta l m a rg in s o f A sia and island arcs and festo o n s
p arallel to the A siatic co ast). T h e e a rth q u a k e s o f the
m id - c o n tin e n ta l b e lt alo n g the A lp in e -H im a la y a n
c h a in s arc c a u se d d ue to co llisio n o f E u ra sia n p lates
and A frican and Indian plates. T h e e a rth q u a k e s o f
th e w estern m a rg in al areas o f N o rth and S o u th
A m eric as arc cau sed b c c a u sc o fs u b d u c t ion o f A m e ri­
can p la te b e n e a th the P acific p late and the resu ltan t
te c to n ic fo rc e s w h ereas the e a rth q u a k e s o f the e a s t­
ern m a rg in s o f A sia are o rig in a te d b eca u se o f ihe
su b d u c tio n o f P a c ific p la te u n d er A siatic p lalc. S im i- '
larly , th e su b d u c tio n o f A frican p la te b elo w E u ro ­
p ean p la te an d th e s u b d u c tio n o f In d ian p la te u n d er
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PLATE t e c t o n i c s
scale, g e n e ra te d 15 m h ig h tsu n a m i an d k ille d m o re
th an 120 p e o p le in A lask a.
1883), o r by m a s s iv e la n d slid e s o f th e co a sta l lan d s
o r o f s u b m e rg e d c o n tin e n ta l sh e lv e s an d slo p e s or in
deep o c e a n ic tre n c h e s , are c a lle d tsu n a m i, w h ich is
a Ja p a n e se w o rd m e a n in g th e re b y h a r b o u r w a v e s.
T he ts u n a m is are lo n g w av es (w ith lo n g e r w a v e ­
lengths o f 100 k m o r m o re ) w h ic h trav e l at the sp eed
o f h u n d re d s o f k ilo m e te rs p e r h o u r b u t a re o f sh allo w
in d ep th in d e e p e r o c e a n s an d seas. A s th e se w av es
ap p ro ach c o a s ta l lan d , th e d e p th o f o cea n ic w ater
d ecre ase s b u t th e h e ig h t ot ts u n a m is in c re ases e n o r­
m ously a n d w h e n th e y strik e th e c o a st, th ey cau se
havoc in th e c o a s ta l area s. T h e b est e x a m p le o f
tsunam i in d u c e d by v io le n t v o lc an ic e ru p tio n is
from K ra k a ta o e ru p tio n w h ic h o c c u rre d in 1883.
S ev ere e a rth q u a k e c a u s e d by K ra k a ta o e ru p tio n g e n ­
erated fu rio u s ts u n a m i w a v e s ra n g in g in 30 to 40
m e ters in h e ig h t (a v e ra g e b e in g 120 feet or 36.5 m ).
T h ese w a v e s w e re so v io le n t th a t they rav ag ed the
c o a st o f J a v a a n d S u m a tra an d k ille d 3 6 ,0 0 0 people.
(6) S u m a t r a t s u n a m i : D e c e m b e r 2 6 ,2 0 0 4 , a
po w erfu l e a rth q u a k e o f th e m a g n itu d e o f 9 on R ic h te r
scale, o ff the c o a st o f S u m a tra w ith its e p ic e n te r at
S im e u lu e in the In d ian O cean o c c u rre d a t 0 0 ;5 8 :5 3
( G M T ) , 7 :5 8 :5 3 (In d o n esian L o cal T im e ) o r 6 .2 8 a.m .
(Indian S tan d ard T im e, 1ST) and g e n e ra te d a p o w e rfu l
tsunam i w ith a w a v e le n g th of 16 0 km and initial sp eed
o f 9 6 0 k m /hr. T h e d eep o c e a n ic e a rth q u a k e w as
cau sed d u e to su d d en su b d u ctio n o f I n d i a n p la te
below B u r m a p la te u p to 2 0 m e ters in a b o u n d a ry lin e
o f 1000 km or even m o re. T h is te c to n ic m o v e m e n t
caused 10 m rise in the o cea n ic bed w h ic h s u d d e n ly
d isp laced im m en se v o lu n e of w a te r c a u s in g k ille r
tsunam i. T h is ea rth q u a k e w ^s la rg e st (h ig h e s t on
R ich ter scale) sin ce 1950 and the 4 th la rg e st s in c e
1900 A .D . T h e A n d m an an d N ic o b a r g ro u p o f isla n d s
w ere only 128 km (80 m iles) aw ay fro m th e e p ic e n te r
(S im eulue) and the east co asts o f In d ia w e re a b o u t
1920 km (1200 m iles) aw ay from th e e p ic e n te r. T h e
furious tsunam i w ith a h eig h t o f ab o u t 10 m a d v e rs e ly
affected 12 co u n tries b o rd erin g th e In d ia n O c e a n ,
w orst affected areas in clu d ed T am il N a d u c o a s t an d
A ndm an-N icobar Islands o f India, Sri L anka. In d o n esia
and T hailand. T he stro n g tsu n am i to o k a b o u t 3 h o u rs
to strite T am il N adu coast. T h e k iller tsu n am i c la im e d
m ore than 200,000 hum an lives in the affected co u n tries
w herein Indonesia, Sri L an k a and In d ia sto o d 1st, 2nd
and 3rd in the n u m b er o f hum an ca su a litie s.
S in c e the P acific O cean is girdled by co n v er­
gent p la te b o u n d a rie s and the rin g o f earth q u ak es and
v o lc a n o e s, ts u n a m is arc m o re co m m o n in the Pacific
w ith a m in im u m freq u en cy o f 2 tsu n am is per year.
T h e g re a t ts u n a m is cau sed by the L isbon earthquake
(P o rtu g a l) o f the y e a r 1755 g en erated about 12 m high
sea w a v e s w h ic h d am ag ed m o st parts o f L isbon city
and k ille d 3 0 .0 0 0 to 6 0 ,0 0 0 p eo p le. T h e K utch
e a rth q u a k e o f Ju n e 6, 1819 g en era ted stro n g tsunam is
w h ich s u b m e rg e d the co astal areas. T he land area
m e a su rin g 2 4 km in len g th w as raised upw ard because
o f te c to n ic m o v e m e n ts . T h e raised land w as called as
A lla h 's B u n d (b u n d created by the G od).
(7) J a p a n ts u n a m i, 2011 : D ate : M arch , 11.
2 0 1 1; tim e : Japan lim e = 2.46 A. M ., 1 S T = 6 .15 A . M .;
undersea earth quake o f 8.9 m ag n itu d e; e p c e n te r 130
km off the coast ot Sendai C ity n ear L am en g V illag e
and 380 km north-east o f T o k y o , at the d ep th o f 10 km
on sea bed; tsunam i w ave height 10m; m o re than
10,000 people killed; m any cities like M iy ak o , M iy ag i,
K esen n u m a w ere fla tte n e d ; S e n d a i a ir p o rt w as
inundated w ith heaps o t cars, trucks, b u ses and m ud
deposits; aircrafts including lig h ter p lan es stan d in g on
air port w ere w ashed out by gu sh in g tsunam i w aves;
r o ta tio n s p e e d o f th e e a r th in c r e a s e d b y 16
m ic r o s e c o n d s ; d a y le n g th d e c r e a s e d b y 1.6
m icroseconds; H onshu island w as d isp laced by 2.4 m
due to m onstrous quake; earth rotational axis w as
displaced by 10 centim eters; 2 10 0 km stretch o f eastern
coastlines having several villages, cities and tow ns
w ere battered by killer tsunam i; nu clear p o w e r p lants
in F ukushim a severely dam aged resulting into leakage
o f killer radiactive radiation; m ore than 5 lakh people
in the radius o f 20 km from F ukushim a p o w er plants
w ere evacuated and shifted to safer places.
T h e fo llo w in g a re th e sig n ific a n t tsu n am is in
the seco n d h a lf o f the 2 0 th cen tu ry and 21st century :
(1 ) A l e u t i a n t s u n a m i : A p ril 1, 1946, g e n e r­
ated by A le u tia n e a rth q u a k e o f th e m a g n itu d e of 7.8
on R ic h te r s c a le , th e re s u lta n t tsu n am i w ith a h eig h t
o f 35 m k ille d m a n y p e o p le in A lask an and H aw aiian
coastal a re a s.
(2 ) K a m c h a t k a ts u n a m i : N ov. 4, 1952,
ea rth q u a k e o f th e m a g n itu d e o f 8.2, g en era ted P a ­
cific-w id e ts u n a m i w ith a w av e h e ig h t ol 15 m.
(3 )
quake o f
g enerated
ad v ersely
A le u t ia n t s u n a m i : M a rc h 9, 1957, e a rth ­
th e m a g n itu d e o l 8.3 on R ic h ter scale,
a P a c ific -w id e tsu n a m i ot 16 m h eig h t and
a ffe c te d H a w a ii isla n d s.
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(4 ) C h ile a n t s u n a m i : M ay 2 2 , I9 6 0 , a stro n g
earth q u ak e o f th e m a g n itu d e of 8.6 on R ich tei scale,
g enerated P a c ific -w id e tsu n a m is and claim e d 2 ,3 0 0
hum an liv e s in C h ile .
(5 ) A la s k a n ts u n a m i : M arch 28, 1964, a
strong e a rth q u a k e o f th e m a g n itu d e ol 8.4 on R ichter
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:
200-215
VULCANICITY AND LANDFORMS
C oncept o f vu lcanicity ; co m p o n en ts o f v o lc a n o e s ; c la s s if ic a t io n o f
v o lc a n o e s ; volcan ic types ; w orld d istrib u tion o f v o lc a n o e s ; m e c h a n is m
and cau ses o f vu lcanism ; h azardous e f f e c t s o f v o lc a n ic e r u p tio n s ;
topography produced by v u lca n icity ; g e y s e r s ; fu m a r o le s .
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CHAPTER 12
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12
VULCANICITY AND LANDFORMS
12.1 THE CONCEPT OF VULCANICITY
T h e term s v o lc an o es, m e ch an ism o f v o lc a ­
noes and v u lc an icity are m o re or less sy n o n y m to
com m on m an b u t th e se h av e d iffe re n t co n n o ta tio n s
in geology and g eo g rap h y . ‘A v o lc an o is a v en t, or
o p en in g , u su ally c irc u la r or n early c irc u la r in fo rm ,
th ro u g h w h ich heated m a teria ls c o n sistin g o f gases,
w ater, liq u id la v a an d frag m e n ts o f ro ck s are e jecte d
fro m th e h ig h ly h eated in te rio r to the su rface o f the
e a rth ’ (P .G . W o rce ster, 1948). ‘A v o lcan o is e sse n ­
tia lly a fissu re o r v ent, c o m m u n ica tin g w ith the
in te rio r, fro m w h ich flo w s o f lav a, fo u n tain s o f
in c a n d e s c e n t sp ray o r e x p lo siv e b u rsts o f g ases and
v o lc a n ic a sh e s are e ru p te d at th e s u rfa c e .’ O n the
o th e r h a n d , ‘th e te rm v u lc a n ic ity co v ers all th o se
p ro c e sse s in w h ic h m o lten ro c k m aterial o r m a g m a
rises in to th e c ru s t o r is p o u re d o u t on its su rface,
th e re to so lid ify as a c ry s ta llin e o r s e m ic ry sta llin e
r o c k ’ (S .W . W o o ld rid g e an d R .S . M o rg a n , 1959).
S o m e sc ie n tis ts h a v e a lso u sed th e te rm o f v u lc an ism
as sy n o n y m to th e te rm o f v u lc a n ic ity . F o r ex am p le,
P.G . W o rce ster (1948 ) h as m ain tain ed th at ‘vu lcan ism
in c lu d es all p h e n o m e n a c o n n e c te d w ith th e m o v e ­
e x o g e n e tic . In o th e r w o rd s, v u lc a n ic ity in c lu d e s all
th o se p ro c e s s e s an d m e c h a n is m s w h ic h a re re la te d
to th e o rig in o f m a g m a s , g a s e s a n d v a p o u r, th e ir
asc e n t an d a p p e a ra n c e o n th e e a r th 's s u rfa c e in
v ario u s fo rm s. It is e v id e n t th a t th e v u lc a n ic ity h as
tw o c o m p o n e n ts w h ic h o p e ra te b e lo w th e c ru s ta l
su rfa c e an d a b o v e th e c ru s t. T h e e n d o g e n e tic m e c h a ­
n ism o f v u lc a n ic ity in c lu d e s th e c r e a tio n o f h o t an d
liq u id m e g m a s a n d g a s e s in th e m a n tle a n d th e c ru st,
th e ir e x p a n sio n a n d u p w a rd a s c e n t, th e ir in tru s io n ,
co o lin g an d s o lid ific a tio n in v a rio u s fo rm s b e lo w
cru stal su rface (e.g. b a th o lith s, la c c o lith s , sills, d y k es,
lopoliths, p h aco lith s etc.) w h ile th e e x o g e n o u s m e c h a ­
n ism in c lu d e s th e p ro c e s s o f a p p e a ra n c e o f la v a,
v o lc an ic d u sts a n d a sh e s, fra g m e n ta l m a te ria l, m u d ,
sm o k e etc. in d iffe re n t fo rm s e .g . fis s u re flo w o r la v a
flo o d (fissu re o r q u ie t ty p e o f v o lc a n ic e ru p tio n ),
v io le n t e x p lo sio n (c e n tra l ty p e o f v o lc a n ic e r u p ­
tio n ), h o t sp rin g s, g e y se rs, fu m a ro le s , s o lfa ta ra , m u d
v o lc an o es etc. It m ay b e, th u s, c o n c lu d e d th a t th e
v u lc a n ic ity is a b ro a d e r m e c h a n is m w h ic h in c lu d e s
s e v eral e v e n ts a n d p ro c e s s e s w h ic h w o rk b e lo w th e
c ru st as w ell as a b o v e th e c ru s t w h e re a s v o lc a n o is a
m e n t o f h e a te d m a teria l fro m th e in te rio r to o r
to w ard s th e su rfa c e o f th e e a r th .’
p a rt o f v u lc a n ic ity (v u lc a n is m ).
V
1 2 .2 C O M PO N EN TS OF V O LC A N O ES
It is a p p a re n t fro m th e a b o v e d e fin itio n s o f
v o lc an o an d v u lc a n ic ity (v u lc a n ism ) th a t th e la te r
(v u lc a n ic ity ) is a b ro a d e r m e c h a n ism w h ic h is re ­
V o lc a n o e s o f e x p lo s iv e ty p e o r c e n tra l e ru p ­
tio n ty p e a re a s s o c ia te d w ith th e a c c u m u la te d vol­
c a n ic m a te ria ls in th e fo rm o f cones w h ic h a re called
as volcanic cones o r sim ply volcanic m o u n ta in s , j
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la te d to b o th the e n v iro n m e n ts, e n d o g e n e tic an d
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201
VULCANICITY AND LANDFORMS
There is a ven t or op en in g, o f circular or nearly
circular shape, alm o st in the centre o f the sum m ital
part o f the con e. T h is vent is called as volcanic vent
or volcanic m outh w h ich is con n ected w ith the
interior part o f the earth by a narrow pipe, w hich is
called as v o lc a n ic p ip e . V o lca n ic m aterials o f vari­
ous sorts are ejected through this pipe and the vent
situated at the top o f the pipe. T he enlarged form o f
the volcan ic ven t is k n ow n as v o lc a n ic cr a te r and
cald era. V o lca n ic m aterials include lavas, volcanic
dusts and a sh es, fra g m en t^ M aterials etc. (fig. 12.1).
f? &
Volcanic Vent
------* r—
> Volcanic Cruter
(2)
C la ssifica tio n o n th e B a sis o f P e r io d
o f E ru p tion s
(a) A ctive volcan oes
(b) Dormant volcanoes
(c) E x tin ct v o lcan o es
12.3
CLASSIFICATION ON THE B A SIS OF THE
NATURE OF VOLCANIC ER UPTIO NS
V olcan ic eruptions occur m o stly in tw o w ays
viz. (i) violent and e x p lo siv e type o f eruption o f
lavas, volcanic dusts, v olcan ic ash es and fragm ental
materials through a narrow p ip e and sm all op en in g
under the im pact o f violen t g a ses and (ii) cjuiet typ e
or fissure eruption along a lon g fracture or fissu re or
fault due to w eak gases and huge v o lu m e o f lavas.
Thus, on the basis o f the nature and in ten sity o f
eruptions volcanoes are d ivided into tw o ty p es e .g ..
(1) cen tra l eru p tio n ty p e o r e x p lo s iv e e r u p tio n
typ e and (2) fissu re e r u p tio n ty p e o r q u ie t e r u p ­
tion type.
(1) V o lca n o es o f C e n tr a l E r u p tio n T y p e —
Central eruption type or e x p lo siv e eruption typ e o f
volcanoes occurs through a central p ip e and sm all
opening by breaking and b lo w in g o f f crustal surface
due to violent and ex p lo siv e g a ses accu m u lated d eep
within the earth. The eruption is so rapid and v io len t
that huge quantity o f v olcan ic m aterials co n sistin g
o f lavas, volcanic dusts and ashes, fragm ental m ate­
rials etc. are ejected upto thousands o f m etres in the
sky. T hese m aterials after fa llin g d ow n accu m u late
around the volcanic vent and form v o lc a n ic c o n e s o f
various sorts. Such v o lca n o es are very d estru ctive
and are disastrous natural hazards. E x p lo siv e v o lc a ­
noes are further divided into 5 su b -ty p es on the b asis
o f difference in the intensity o f eruption, variations
in the ejected volcan ic m aterial and the period o f the
action o f volcanic even ts as g iv en b elo w .
Fig. 12.1 : Different components o f a volcano.
T here is a w id e range o f variations in the
mode o f v o lca n ic eruptions and their periodicity.
Thus, v o ca n o es are cla ssified on the basis o f (i) the
mode o f eruption and (ii) the period o f eruption and
the nature o f their activities.
(1) C la ssific a tio n o n th e B asis o f th e M od e o f
Eruptions
(i) C e n tr a l eru p tio n ty p e o r ex p losive
e r u p tio n ty p e
(a) H aw aiin type
(i)
(b) Strom bolian type
(c) V ulcanian type
(d) Peleean type
(e) V isu viu s type
(ii) F issu re eru p tio n ty p e o r q u iet e r u p ­
tio n ty p e
(a)
L a v a f lo o d o r la v a f lo w
(b) M u d flo w
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(c) Fum aroles
Hawaiin Type of Volcanoes— S u ch
canoes erupt quietly due to le ss v isc o u s lavas and
non-violent nature o f g a ses. R ounded blisters o f hot
and glo w in g m ass/b oll o f lavas (b leb s o f m olten
lava) when caught by a strong w in d g lid e in the air
like red and g lo w in g hairs. T he H aw aiin p eo p le
consider these lon g g la ssy threads o f red m olten lava
as Pele's hair (P ele is the H aw aiin g o d d ess o f fire).
Such volcan oes have been nam ed as H aw aiin type
because o f the fact that such eruptions are o f very
com m on occurrence on H aw aii island. T h e eruption '
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GEOMORPHOLOGY i
202
m ountain (M ou n t P elee) w ith great sp eed w hich
caused disastrous avalan ch es on the h illslo p e s w hich
plunged d ow n the slo p e at a sp eed o f about 100
kilom etres per hour. T he annihilating e x p lo siv e erup­
tion o f Krakatoa v o lc a n o in 1883 in Krakatoa Island
located in Sunda Strait b etw een Java and Sum atra is
another ex a m p le o f v io le n t v o lc a n ic eruption o f this
o f K ilavca v o lca n o of the southern H aw aii island in
1959-60 continued for sev en days (from N ovem b er
14 to 2 0 , 1959) w hen about 30 m illion cu b ic m etres
of lavas poured out. T he interm ilttent eruptions
continued upto D ecem b er 2 1 , 1959, w hen the v o l­
can o b ecam e dorm ant. It again erupted on January
1 3 ,1 9 6 0 and about 100 m illion cubic m etres o f lavas
w ere poured out o f on e kilom etre long fissure.
(H)
|
type.
Strombolian Type of Volcanoes— Such
(v)
Visuvious Type of Volcanoes— T he
are m ore or less sim ilar to V u lcan ian and Strom bolian
types o f v o lc a n o e s, the d iffe r e n c e lie s o n ly in the
intensity o f ex p u lsio n o f la v a s and g a se s. T here is
extrem ely v io len t ex p u lsio n o f m a g m a d u e to enor­
m ous volu m e o f e x p lo s iv e g a ses. V o lc a n ic m aterials
are thrown up to greater h eig h t in the sk y. The
ejected en orm ou s v o lu m e o f g a se s and a sh es form s
thick cloud s o f ‘c a u liflo w e r fo r m .’ T h e m o st d e­
structive type o f eruption is ca lle d as Plinian type
b ecause o f the fact that su ch ty p e o f eruption was
first ob served by P lin i in 7 9 A .D .
v o lcan oes, nam ed after Strom boli volcano o f Lipari
island in the M editerranean Sea, erupt with m oder­
ate intensity. B esid es lava, other volcanic m aterials
like p u m ice, scoria, bom bs etc. are also ejected upto
greater height in the sky. T h ese materials again fall
dow n in the volcanic craters. The eruptions are
alm ost rhythm ic or nearly continuous in nature but
so m e tim es they are interrupted by long intervals.
(iii) Vulcanian T y p e of Volcanoes— These
are named after volcano o f Lipari island in the
Mediterranean Sea. Such volcanoes erupt with great
force and intensity. The lavas are so viscou s and
pasty that these are quickly solidified and hardened
between tw o eruptions and thus they crust over
(plug) the volcanic vents. T hese lava crusts obstruct
the escape o f violent gases during next eruption.
Consequently, the violent gases break and shatter
the lava crusts into angular fragments and appear in
the sky as ash-laden volcanic clouds o f dark and
often black colour assum ing a convoluted or cau li­
flow er shape (fig. 12.2).
(2) Fissure Eruption Type of Volcanoes—
Such v o lca n o es occu r a lo n g a lo n g fracture, fault and
fissure and there is slo w u p w ellin g o f m agm a from
b elo w and the resultant la v a s spread o v e r the ground
surface. T he sp eed o f lava m o v em en t d ep en d s o n the
nature o f m agm a, v o lu m e o f m agm a, slo p e o f ground
surface and tem perature co n d itio n s. T h e L aki fis ­
sure eruption o f 1783 in Icela n d w a s so q u ick and
enorm ous that h u g e v o lu m e o f la v a s m easu rin g
about 15 cu b ic k ilo m etres w a s poured ou t from a 28km lo n g fissu re. T h e la v a flo w w a s s o en o rm o u s that
it travelled a d ista n ce o f 3 5 0 k ilo m etres.
(iv) Peleean Type of Volcanoes— T hese are
named after the Pelee volcano o f M artinique Island
in the Caribbean Sea. T hese are the m ost violen t and
m ost ex p lo siv e type o f volcanoes. The ejected lavas
are m ost viscou s and pasty. O bstructive dom es o f
lava are formed above the conduits o f the volcanoes.
Thus, every su ccessiv e eruption has to blow o ff
these lava dom es. C onsequently, each su ccessiv e
eruption occurs with greater force and intensity
m aking roaring noise. The m ost disastrous volcanic
eruption o f Mount P elee on May 8, 1902 destroyed
the w hole o f the town o f St. Pierre k illing all the
2 8 ,0 0 0 inhabitants leaving behind only tw o survi­
vors to mourn the sad d em ise o f their brethren. Such
type o f disastrous violen t eruptions are named as
nuee ardente m eaning thereby ‘glo w in g c lo u d ’ o f
hot gases, lavas etc. com in g out o f a vocan ic erup­
tion. The nuee ardente spread laterally out o f the
1 2 .4
CLASSIFICATION ON THE B A S IS O F PE­
RIODICITY O F E R U P T IO N S
V o lca n o es are d iv id e d in to 3 ty p e s on the
basis o f period o f eruption and in terval p eriod be­
tw een tw o eru p tion s o f a v o lc a n o e .g . (i) active
v o lca n o es, (ii) dorm ant v o lc a n o e s and (iii) extinct
v o lca n o es.
(i)
Active volcanoes are th o se w h ich
stantly eject v o lc a n ic la v a s, g a s e s , a sh e s and frag­
m ental m aterials. It is estim a ted that there are about
m ore than 5 0 0 v o lc a n o e s in the w orld . E tna and
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Strom boli ot the M ed iterranean S e a are the m ost
sig n ifica n t ex a m p les o f th is c a teg o r y . S trom b oli ?
V o lca n o is k n ow n as L ig h t H o u se o f th e Mediterra- ^
nean b ecau se o f co n tin u o u s e m is s io n o f burn5"
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203
lum inous incandescent gases, M o st o f the active
volcanoes are found along the m id -o cean ic ridges
representing div erg en t plate m arg in s (co n stru ctiv e
plate m argins) and co n v erg en t plate m argins (d e­
structive plate m argins rep resen ted by the eastern
and w estern m argins o f the Pacific O cean). T h e
latest eruption took place from P in atu b o volcano in
June 1991 in P hillipines.
(ii) D om ran t vo lca n o es are th o se w hich b e­
com e quiet after their eru p tio n s for som e tim e and
there are no indications for future eru p tio n b u t su d ­
denly they erupt very violently and cau se e n o rm o u s
damage to hum an health and w ealth. V isu v io u s
volcano is the best exam ple o f d o rm an t v o lcan o
w hich erupted first in 79 A .D ., then it k ep t q u iet upto
1631 A.D. when it suddenly ex p lo d ed w ith g reat
force. The subsequent eru p tio n s o ccu rred in 1803,
1 8 72,1906, 1927, 1228 and 1929.
(iii) E xtin ct volcan oes are co n sid ere d e x tin ct
when there are no indications o f fu tu re eru p tio n . T he
crater is filled up w ith w ater and lakes are form ed. It
may be pointed out that no vo lcan o can be d eclared
perm anently dead as no one know s, w h at is h a p p e n ­
ing below the ground surface.
1 2 .5 VOLCANSC M A T E R IA L S
V olcanic m aterials d isch arg ed d u rin g eru p ­
tions include gases and vapour, lav as, frag m en tal
m aterials and ashes.
V apour and G ases— S team and v ap o u r c o n ­
stitute 60 to 90 per cen t o f the total g ases d isch arg e d
during a volcanic eru p tio n . S team an d v ap o u r in ­
clude (i) ph reatic vap ou r and (ii) m a g m a tic v a ­
p our w hereas volcanic gases in clu d e carb o n d io x ­
ide, nitrogen o xides, su lp h u r d io x id e, h y d ro g en ,
carbon m onoxide etc. B esid es, c ertain c o m p o u n d s
are also ejected w ith the v o lc a n ic g a se s e .g .
sulphurated h y d ro g en , h y d ro ch lo ric acid, v o latile
chlorides o f iron, potassium and other m etallic m atter.
M agm a and L a v a— G en erally , m o lten ro ck
m aterials are called m ag m as below the e a rth ’s su r­
face w hile they are called la v as w hen they co m e at
the earth's face. L av as and m ag m as are d iv id e d on
the basis o f silica p erce n tag e into tw o g ro ups e.g. (i)
acidic m agm a (h ig h er p ercen tag e o f silica and (ii)
basic lava (low p ercen tag e o f silica). Lavas and
m agm as are also classified on the b asis o f light and
dark coloured m in erals into (i) fe lsic la v a in d (ii)
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Fig* 12,2 : Types o f Volcanoes-(l) Hawaiin type, (2)
Strombolian type, (3) Vulcanian type, (4)
Peleean type, (5) Visuvian type and (6) Fis­
sure type or Icelandic type.
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204
GEOMORPHOLOGY
in clu d e frag m e n tal m a te ria ls o f crustal rocks. On the
b asis o f size p y ro c la stic m a te ria ls are g ro u p ed into
(i) v o lc a n ic d u st (fin e s t p a rtic le s), (ii) v o lc a n ic ash
(2 m m in size), (iii) lap U li ( o f th e siz e o f peas) and
(iv) v o lc a n ic b o m b s (6 cm o r m o re in size), w hich
are o f d iffe re n t sh ap es viz. e llip s o id a l, discoidal,
cu b o id al, and irre g u la rly ro u n d e d . T h e d im e n sio n of
av erag e v o lcan ic b o m b s ra n g e s fro m th e size o f a
base ball or b a sk e t ball to g ia n t size. S o m e tim e s the
vo lcan ic b o m b s w eig h 100 to n n e s in w e ig h t a n d are
th ro w n u pto a d ista n c e o f 10 km .
m a fic la v a . B a sa ltic o r m a fic la v a is c h ara cterized
by m a x im u m flu id ity . B asa ltic lav a sp read s on the
g ro u n d su rfa c e w ith m a x im u m flo w sp eed (fro m a
few k ilo m e tre s to 100 k ilo m e tre s p er h o u r, average
flo w sp eed b ein g 45 to 65 km p er h o u r) d u e to high
flu id ity an d lo w v isco sity . B asaltic lav a is the h o ttest
la v a (1 ,000° to 1,200°C ). L a v a flo w is d iv id ed into
tw o ty p e s on the b asis o f H aw aiin lan g u ag e e.g. (i)
p a h o e h o e an d (ii) a a a a la v a flg w or b lock lava
flow . P a h o e h o e la v a has hig h flu id ity and spreads
lik e th in sh eets. T h is is also k now n as r o p y la v a . On
the o th e r h an d , aa aa lav a is m ore viscous. P ahoehoe
lav a, w h en so lid ifie d in the form o f sacks or pillow s,
is c a lle d p illo w la v a .
12.6 WORLD DISTRIBUTION OF V O LCANOES
L ik e e a rth q u ak es, the s p a tia l d is trib u tio n o f
v o lcan o es o v er the g lo b e is w ell m a rk e d a n d w ell
u nd ersto o d b ecau se v o lc a n o e s are fo u n d in a w ell
d efin ed belt or zone (fig. 12.3). T h u s, th e d is trib u ­
tional pattern o f v o lc an o es is zo n al in c h a ra c te r. If
w e lo o k at the w o rld d istrib u tio n o f v o lc a n o e s it
appears th at the v o lc an o es are a s s o c ia te d w ith th e
w eak er zones o f the earth 's c ru s t an d th e s e are
closely a sso ciated w ith seism ic e v e n ts say e a rth ­
q uakes. T h e w e a k e r zo n es o f th e e a rth are re p re -
F r a g m e n t a l o r P y r o c la s tic M a te r ia ls —
P y ro c la stic m aterials throw n durin g explosive type
o f e ru p tio n are gro u p ed into three categories, (i)
E s s e n tia l m a te r ia ls include co n so lid ated form s of.
liv e lavas. T hese are also know n as te p h r a w hich
m ean s ash. E ssential m aterial are unco n so lid ated
and th eir size is upto 2 m m . (ii) A c c e sso ry m a te ria ls
are form ed o f dead lavas, (iii) A c c id e n ta l m a te ria ls
tNuovo
Bezymian
/B o g o s lo f,
Lessen Peak
kurajirna
rcena *
Jorullu
hilippinesi
kNewGuine
Tristan da Cunna
■
C irc u m p a c ific
b e fl
M id -c o n tin e n ta l
Tarawera
B a s a ltic
b e lt
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Fig. 12.3: World distribution o f volcanoes.
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p la te a u
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VULCANICITY AND LANDFORMS
sented b y fo ld e d m o u n ta in s (w e ste rn c o rd ille ra o f
N orth A m e n c a , A n d e s , m o u n ta in s o f e a s t A s ia anH
East In d ie s ) w ith d ie e x c e p tio n s o f th e A lp s and the
H im alay as, a n d fa u lt z o n e s . V o lc a n o e s are also
asso ciated w ith th e m e e tin g z o n e s o f th e co n tin en ts
v o lcan o es are fo u n d in ch ain s e.g. th e v o lc an o es o f
the A leu tian Island, H aw aii Islan d , Jap an etc. A b o u t
22 volcanic m o u n tain s are fo u n d in g ro u p in E c u a ­
d o r w herein the h eig h t o f 15 v o lcan ic m o u n ta in s is
m ore than 45 6 0 m A M S L . C o to p ax i is the h ig h e st
volcanic m ou n tain o f the w o rld (h e ig h t b ein g 19,613
feet). T he oth er sig n ifican t v o lc an o es are F u z iy a m a
(Japan), Shasta, R ainier and H ood (w estern co rd illiera
o f N orth A m erica), a valley o f te n th o u sa n d sm o k e s
(A laska), M t. St. H elens (W ashington, U S A ), K ilav ea
(H aw aiiland), M t. T aal, P in atu b o an d M a y o n o f
P h illippines etc.
and o cea n s. O c c u r re n c e s o f m o re v o lc a n ic eru p tio n s
along c o a s ta l m a rg in s a n d d u r in g w e t se a so n d en o te
the fact th a t th e re is c lo s e re la tio n s h ip b e tw e e n w ater
and v o lc a n ic e ru p tio n s . S im ila rly , v o lc a n ic eru p ­
tions are c lo s e ly a s s o c ia te d w ith th e a c tiv ities o f
m ountain b u ild in g a n d fra c tu rin g .
B a s e d o n p la te te c to n ic s , th e re is Close
rela tio n sh ip b e tw e e n p la te m a rg in s and v u lcan icity
as m o st o f th e w o rld s a c tiv e v o lc a n o e s are asso ci­
ated w ith th e p la te b o u n d a rie s . A b o u t 15 p er cen t o f
the w o rld s a c tiv e v o lc a n o e s are fo u n d along the
c o n stru ctiv e p la te m a r g in s o r d iv e r g e n t p late
m argin s ( a lo n g th e m id -o c e a n ic rid g e s w here tw o
plates m o v e in o p p o s ite d ire c tio n s ) w h ereas 80 per
cent v o lc a n o e s a re a s s o c ia te d w ith the d estru ctiv e
or c o n v e r g e n t p la te b o u n d a r ie s (w h ere tw o plates
collide). B e s id e s , s o m e v o lc a n o e s are also found in
intraplate r e g io n s e .g . v o lc a n o e s o f the H aw aii Is­
land, fa u lt z o n e s o f E a s t A fric a etc.
H ere volcanic eru p tio n s are p rim a rily c a u se d
due to collision o f A m erican and P a c ific p la te s an d
due to subduction o f P acific P late b elo w A sia tic
plate.
(2) M id -C o n tin en ta l B elt— T h is b e lt is also
know n as ‘the v o lca n ic zo n es o f c o n v e r g e n t c o n ti­
n e n ta l p la te m e r g in s ’. T h is b elt in c lu d e s th e v o lc a ­
noes o f A lpine m o u n tain ch ain s and th e M e d ite rra ­
nean Sea and the v o lcan o es o f fa u lt zo n e o f e a s te rn
A frica. H ere, the volcanic e ru p tio n s are c a u s e d due
to convergence and co llisio n o f E u ra sia n p la te s and
A frican and Indian plates. T h e fam o u s v o lc a n o e s o f
the M editerranean Sea such as S tro m b o li, V isu v io u s,
L ik e e a rth q u a k e s , th e re are also th ree m ajor
E tna etc. and the v o lcan o es o f A eg ean S ea are
belts or z o n e s o f v o lc a n o e s in the w orld viz. (i)
included in this belt. It m ay be p o in te d o u t th a t this
circ u m -P acific b e lt, (ii) m id -c o n tin e n ta l belt and
belt
does not have the co n tin u ity o f v o lc an ic e ru p ­
(iii) m id -o c e a n ic rid g e b e lt (fig#. 12.3).
as several gaps (v o lcan ic - free z o n es) are
(1)
C ir c u m -P a c ific B e lt— T h e circ u m -P tions
a­
found along the A lps and the H im a la y a s b e c a u se o f
cific belt, a lso k n o w n as th e ‘v o lc a n ic zo n es o f the
com pact and thick cru st fo rm ed due to in ten se fo ld ­
con vergen t o c e a n ic p la te m a r g in s ’, in clu d es the
ing activity. T he im p o rtan t v o lc an o es o f th e fault
volcanoes o f th e e a s te rn an d w e ste rn co astal areas o f
zone o f eastern A frica are K ilim an jaro , M eru , E lg o n ,
the P acific O c e a n (o r th e w e ste rn co astal m arg in s o
B irunga, R ungw e etc.
North and S o u th A m e ric a s and th e eastern coj*sta
m argins o f A sia ), o f isla n d arcs a n d festo o n s o t e
east coast o f A s ia a n d o f th e v o lc a n ic islan ^ sca^
tered o v e r th e P a c ific O c e a n . T h is v o lcan ic b ell is
also called as th e F ir e G ir d le o f th e P a cific o r t e
Fire R in g o f th e P a c ific . T h is b elt b eg in s fiom
(3) M id -A tla n tic B elt— T h is b elt in c lu d es
the volcanoes m ain ly alo n g th e m id -A tla n tic ridge
w hich rep resen ts the sp littin g zo n e o f p lates. In o th e r
w ords, tw o p lates d iv erg e in o p p o site d ire c tio n s
from the m id -o cean ic ridge. T h u s, v o lc a n o e s m ain ly
o f fissu re eru tp io n ty p e o c c u r alo n g th e c o n s tru c ­
tive or d iv erg en t p late m a rg in s (b o u n d arie s). T he
m ost active v o lcan ic area is Icelan d w h ich is lo c a te d
on the m id -A tlan tic ridge. T h is b elt b eg in s fro m
H ekla volcanic m o u n tain o f Icelan d w h ere se v e ra l
fissure eru p tio n type o f v o lc an o es are fo u n d . It m ay
be po in ted out th at since Icelan d is lo c a te d o n the
m id -A tlan tic rid g e rep resen tin g th e sp littin g zon e o f
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Erebus M o u n ta in o f A n ta rc tic a and runs I' ort wa^
through A n d e s an d R o c k ie s m o u n ta in s o f ISouth a
N orth A m eric as to re a c h A laska fro m w ere is
turns to w ard s e a ste rn Asiatic c o a st to inc ui e e
volcanoes o f island a rcs and festo o n s (e.g. a a li ,
K am chatka, Japan, Phillippines etc.). T e e u i
l a t e l y m e rg e s w ith th e m i d - c o n tin en ta e ‘n
Bast Indies. M o st o f h ig h v o lc an ic co n es and vo canic m o u n tain s a re fo u n d in this belt,
os
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GEOMORPHOLOGY
206
(4)
I n tr a - P la te V o lc a n o e s— B esid es th e aforesaid w ell d e fin e d th re e z o n e s o f v o lc a n o e s, scattered
v o lc a n o e s a re a lso fo u n d in th e in n e r p a rts o f the
c o n tin en ts. S u c h d is trib u tio n a l p a tte rn s o f v o lca­
n o es are c a lle d as in tra p la te v o lc a n o e s , th e m echa­
nism o f th e ir e ru p tio n is n o t y e t p re c ise ly know n.
Fig. 12.4 d e p ic ts th e lo c a tio n o f v o lc a n o e s o f the
P acific p la te w h e re o n e b ra n c h o f v o lc a n o e s runs
fro m H aw aii to K a m c h a tk a . V u lc a n ic ity a lso b e­
co m es a c tiv e in th e in n e r p a rts o f c o n tin e n ta l plates.
M assiv e fissu re e ru p tio n o c c u rre d in th e n o rth ­
w estern p arts o f N o rth A m e r ic a d u rin g M io cen e
p erio d w h en 1 ,0 0 ,0 0 0 c u b ic k ilo m e tre s o f b asaltic
lavas w ere sp read o v e r an a re a o f 1 ,3 0 ,0 0 0 km 2 to
form C o lu m b ia n p la te a u . S im ila rly , g re a t fissure
flo w s o f lav as c o v e re d m o re th a n 5 ,0 0 ,0 0 0 k m 2 areas
o f P e n in su la r In d ia. P a ra n a o f B a ra z il a n d P arag u ay
w ere fo rm ed d ue to sp re a d o f la v a s o v e r an are a o f
A m e ric a n p la te m o v in g w e stw a rd an d E u ra sia n p late
m o v in g eastw ard , and h en ce here is co n stan t upw elling
o f m a g m a s a lo n g th e m id -o c e a n ic rid g e an d w h e r­
e v e r th e c ru s t b e c o m e s th in an d w eak , fissu re flo w o f
la v a o c c u rs b e c a u se o f fra c tu re c re a te d d u e to d iv e r­
g e n ce o f p la te s. T h e L a k i fissu re eru p tio n o f 1783
A .D . w as so q u ic k an d e n o rm o u s th at h u g e v o lu m e
o f la v a s m e a su rin g a b o u t 15 cu b ic k ilo m etres w as
p o u red o u t fro m 2 8 -k m lo n g fissu re. R ecen tly , H ek la
an d H e lg a fe ll v o lc an o es eru p te d in th e y ear 1974
and 1973 resp e c tiv e ly . O th er m o re active volcanic
areas are L e s s e r A n tilles, S o u th ern A ntilles, A zores,
St. H e le n a etc. T h e d read fu l and disastro u s eru p tio n
o f M o u n t P ele e o ccu rred on M ay 8 ,1 9 0 2 in the tow n
o f St. P ierre on the M artin iq u e Island o f W est Indies
in th e C arib b ean Sea. A ll the 28 ,0 0 0 inh ab itan ts,
ex ce p t tw o persons, w ere killed by the k iller v o l­
7 ,5 0 ,0 0 0 k m 2.
can ic eruption.
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Fig. 12.4 : Volcanic-ridge-chain on Pacific plate.
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V U L C A N IC IT Y A N D L A N D F O R M S
207
12.7 MECHANISMS AND C A U S E S OF V U L C A N IC
asso ciated w ith plate b oundaries. It m ay be p o in ted
th at the types o f plate m o v em en ts and p late
b o undaries also d eterm in e the n atu re and intensity
o f volcanic eruptions. M o st o f the activ e fissure
volcanoes are found along the m id -o cean ic rid g es
w hich rep resen t sp littin g zones o f d iv e rg e n t p late
boundaries (fig. 12.5). T w o p lates m ove in opp o site
d irectio n s from the m id -o cean ic rid g es due to th e r­
mal conv ectiv e cu rren ts w hich are o rig in ated in the
m antle below the cru st (plates). T h is sp littin g and
lateral spreading o f plates creates fractu res and faults
(transform faults) w hich cau se p re ssu re re le a se and
low ering o f m elting p oint and th u s m a te ria ls o f
upper m antle lying below the m id -o c e a n ic rid g e s are
m elted and m ove u p w ard as m a g m as u n d e r the
im pact o f enorm ous volum e o f acc u m u la te d g ases
and vapour. T his rise o f m ag m as a lo n g th e m id oceanic ridges (constructive or divergent plate b o u n d a­
ries) causes fissure eru p tio n s o f v o lc an o es an d th e re
is constant upw elling o f lavas. T h ese lavas are c o o le d
and solidified and are ad d ed to th e trailin g e n d s o f
d ivergent plate b o undaries and th u s th e re is c o n s ta n t
creation o f new basaltic crust, T he v o lcan ic e ru p ­
tions o f Iceland and the islands lo c ated alo n g the
m id-A tlantic ridge are cau sed b eca u se o f se a -flo o r
spreading and d iv erg en ce o f p lates. It is o b v io u s th at
divergent or co n stru ctiv e p late b o u n d arie s are a l­
w ays associated w ith q u iet type o f fissure flo w s o f
lavas because the p ressure release o f su p erin cu m b en t
load due to d iv erg en ce o f p lates and fo rm atio n o f
fissures and faults is a slow and g rad u al p ro cess.
•
t S/
a >hd e a r ‘r
t h e V O ,C a n ic e r u P t i o n s a r eo ut
a s s o c i a t e d w i t h w e a k e r z o n e s o f t h e e a r t h surface
r e p r e se n te d b y m o u n t a in b u ild in g a t th e d e s tr u c tiv e
or c o n v e r g e n t p la t e m a r g in s a n d fr a c tu r e z o n e s r e n
r e se n te d b y c o n s t r u c t iv e o r d iv e r g e n t p la te b o u n d a ­
r ie s a t t h e s p l i t t i n g z o n e s o f m i d - o c e a n i c r i d g e s a n d
th e z o n e s o f t r a n s f o r m
s e r v a tiv e
v u lc a n ic it y
p la te
fa u lts r e p r e se n te d b v co n
b o u n d a r ie s .
(v u lc a n is m ) a n d
The
m e c h a n is m
of
v o l c a n i c e r u p tio n s is
c lo s e ly a s s o c ia t e d w it h s e v e r a l in t e r c o n n e c t e d p r o c ­
esses su ch
a s ( i ) g r a d u a l in c r e a s e o f te m p e r a tu r e
with in c re a s in g d e p th a t th e ra te o f 1°C p er 32 m due
to heat g e n e ra te d fro m th e d isin te g ra tio n o f rad io a c­
tive e le m e n ts d e e p w ith in th e earth , (ii) o rigin o f
m agm a b e c a u s e o f lo w e rin g o t m e ltin g p o in t caused
by r e d u c t i o n in th e p r e s s u r e o f o v e r ly in g
su p erin cu m b en t lo a d d u e to fra c tu re cau sed by sp lit­
ting o f p la te s a n d th e ir m o v e m e n t in o p p o site d irec­
tion. (iii) o rig in o f g a s e s an d v a p o u r d u e to heating
of w ater w h ic h re a c h e s u n d e rg ro u n d th ro u g h p erco ­
lation o f ra in w a te r a n d m e it-w a te r (w ater derived
through the m e ltin g o f ice an d sn o w ), (iv) the ascent
of m agm a fo rc e d by e n o rm o u s v o lu m e o f gases and
vapour an d (v ) fin a lly th e o c c u rre n c e o f volcanic
eruptions o f e ith e r v io le n t e x p lo siv e cen tral type or
quiet fissu re ty p e d e p e n d in g up o n the intensity o f
gases and v a p o u r an d th e n a tu re o f cru stal surface.
T h e o r y o f p l a t e te c to n ic s now very well
explains th e m e c h a n is m o f v u lc an ism and volcanic
eruptions. In fact, v o lc a n ic e ru p tio n s are very closely
12.5 : Illustration o f constructive (divergent) and destructive (convergent) plate boundaries and their relationship
with vulcanicity.
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GEOMORPHOLOGY
208
It is apparent from the above d iscussion that
the m id-oceanic ridges, rep resen tin g splitting zones,
are associated w ith active volcanoes w herein the
supply o f lav a com es from the upper m antle ju st
below the ridge because o f differen tial m elting o f the
rocks into th oleiitic b asa lts. Since there is constant
supply o f basaltic lavas from below the m id-oceanic
ridges and hence the v o lcan o es are active near the
ridges but the supply o f lavas d ecreases w ith increas­
ing distance from the m id -o cean ic ridges and there­
fore the volcanoes becom e inactive, d orm ant and
extinct d epending on th eir distances from the source
o f lava supply, e.g. m id -o cean ic ridges. This fact has
been validated on the basis o f the study o f the
b asaltic floor o f the A tlantic O cean and the lavas of
several Islands. It has been found that the islands
n earer to the m id -A tlan tic R idge have younger lavas
w hereas the islands aw ay from the ridge have older
lavas. F or exam ple, the lavas o f A zores islands
situated on eith er side o f the m id-A tlantic Ridge are
4 -m illion y ear old w hereas the lavas o f Cape V erde
Island, located far aw ay from the said ridge, are 120m illion year old.
D estru ctive or con vergent plate b ou nd a­
ries are associated w ith explosive type o f volcanic
eruptions. W hen tw o convergent plates collide along
B en io ff zon e (subduction zone), co m p aratively
heavier plate m argin (boundary) is subducted be­
neath com paratively lighter plate boundary. The
subducted plate m argin, after reaching a depth o f
100 km or m ore in the upper m antle, is m elted and
thus m agm a is form ed. T his m agm a is forced to
ascend by the enorm ous volum e o f accum ulated
explosive gases and thus m agm a appears as violent
volcanic eruption on the earth's surface. Such type o f
volcanic eruption is very com m on along the d estru c­
tive or co n v erg en t plate boundaries w hich rep resen t
the volcanoes o f the circu m -P a cific b elt and the
m id -con tin en tal belt. T he volcanoes o f the island
arcs and festoons (o ff the east co ast o f A sia) are
caused due to subductio n o f oceanic cru st (p late) say
Pacific plate below the co n tin en tal plate, say A siatic
plate near Japan T rench.
12.8 HAZARDOUS EFFECTS OF VOLCANIC
ERUPTIONS
(1) H uge vo lu m es o f h o t and liq u id lavas
m oving at co n sid erab ly fast sp eed (reco rd ed speed is
48 km per hour) bury h um an stru ctu re s, kill people
and anim als, destro y ag ricu ltu ral farm s and pas­
tures, plug rivers and lakes, b u m an d d estro y forest
etc. The great eru p tio n o f M t. L o a on H aaw aii
poured out such a huge vo lu m e o f lav as th a t these
covered a distance o f 53 km dow n the slo p e. E n o r­
m ous Laki lava flow o f 1783 A .D . tra v e lle d a d is­
tance of 350 km eng u lfin g tw o c h u rch es, 15 a g ric u l­
tural farm s and k illin g 24 p er c e n t o f th e total
population o f Iceland. T he cases o f M t. P elee e ru p ­
tion o f 1902 in M artinique Islan d (in C a rib b e a n S ea)
(total death 28,000) and St. H elen s eru p tio n o f 1980
(W ashington, U SA ) are rep resen tativ e e x a m p le s o f
dam ages done by lav a m o v em en t. T h e th ic k c o v ers
o f green and dense fo rests on th e flan k s o f M t. St.
H elens w ere com p letely d e stro y e d d u e to sev ere
forest fires kindled by h o t lav as.
(2) F allo u t o f im m en se q u an tity o f v o lc an ic
m aterials including frag m en tal m a teria ls (p y ro clastic
m aterials), dusts and ashes, sm o k es etc. c o v e rs la rg e
ground su rface and th u s d estro y s cro p s, v e g e ta tio n
and buildings, d isru p ts an d d iv e rts n a tu ra l d ra in a g e
system s, creates h ealth h azard s d u e to p o iso n o u s
gases em itted d u rin g the eru p tio n , and c a u s e s k ille r
acid rains.
(3) A ll ty p es o f v o lc an ic e ru p tio n s , if n o t
predicted w ell in ad v an c e, c a u se s tre m e n d o u s lo sses
to p recio u s h um an lives. S u d d en e m p tio n o f v io le n t
and ex p lo siv e type th ro u g h c en tral p ip e d o e s n o t
give any tim e to h u m an b ein g s to e v a c u a te th e m ­
selves and th u s to save th e m se lv e s fro m th e c lu tc h e s
o f d eath lo o m in g larg e o v er th em . S u d d en e m p tio n
o f M t. P elee on the Islan d o f M a rtin iq u e , W e st In d ie s
in the C arib b ean S ea, on M ay 8, 1902 d e stro y e d th e
w hole o f St. P ierre tow n and k illed all the 2 8 ,0 0 0
in h ab itan ts leav in g b eh in d o n ly tw o su rv iv o rs to
m ourn the sad d e m ise o f th e ir b re th re n . T h e heavy
rain fa ll, asso c ia te d w ith v o lc a n ic e ru p tio n s , m ixin g
w ith fallin g v o lc a n ic d u sts an d ga ses ca u ses e n o r­
m o u s m u d flo w o r ‘la h a r * on th e s te e p slo p es o f
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V olcanic eru p tio n s cau se heavy d am ag e to
h u m an lives and pro p erty th ro u g h ad v an cin g hot
lavas and fallo u t o f vo lcan ic m a terials; d estru ctio n
to hum an structures such as b u ild in g s, factories,
roads, rails, airp o rts, dam s and reserv o irs through
hot lavas and fires cau sed by h o t lav as; flo o ds in the
rivers and clim atic ch an g es. A few o f the severe
dam ages w ro u g h t by v o lcan ic eru p tio n s m ay be
sum m arized as given b elo w —
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209
VULCANICITY AND LANDFORMS
(5 )
V o lca n ic eruptions also ch an ge the radia­
tion balance o f the earth and the atm osphere and thus
help in causing clim atic ch an ges. Greater con cen tra­
tion o f volcan ic dusts and ash es in the sky red u ces
the am ount o f insolation reaching the earth s su rface
(4 )
E arthquakes ca u sed b efore and after the
as they scatter and reflect som e am ount o f in co m in g
volcanic eruptions gen erate d e s tr u c ti v e tsu n a m is
shortwave solar radiation. D u st v e ils , on the other
seism ic w a v es w h ic h create m o st d estructive and
hand, do not hinder in the lo ss o f heat o f the eart s
disastrous sea w a v e s ca u sin g innum erable deaths o f
surface through ou tgoin g lo n g w a v e terrestrial ra
hum an b ein gs in the a ffe c te d co a sta l areas. O n ly the
diation. The ejection o f nearly 2 0 cu b ic k ilo m etres
exam ple o f K rakatoa in 1883 w o u ld be su fficien t
o f fragm ental m aterials, dusts and a sh es u p to
e
enough to d em on stra te the d isastrous im pact o f
height o f 23 km in the sky during the v i o le n t eru p tion
tsunam is w h ich gen erated en orm ou s sea w a v es o f 30
o f Krakatoa volcano on A u gu st 2 7 , 1 8 8 3 , fo rm e a
to 40 m h eig h t w h ic h k illed 3 6 ,0 0 0 p eop le in the
thick dust veil in the stratosphere w h ich c a u sed a
coastal areas o f Java and Sum atra.
volcanic c o n e s w h ich c a u ses sudden deaths o f human b ein gs. For ex a m p le , great m ud flo w created on
the steep slo p es o f K elu t v o lc a n o in Japan in the year
1919 killed 5 ,5 0 0 p eo p le.
C uldera w ith C in d e r
Volcanic Neck with Rnclianf inf> Dikes
C one
E ro d e d L acco lith
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nroduced during volcanic activities.
fig. 12.6: Different types o ,f .landforms proauc
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geom orphology
210
m a tio n o f c in d e r c o n e s is in itia te d d u e to accu m u la­
tio n o f fin e r p a rtic le s a ro u n d v o lc a n ic v e n t in the
form o f tin y m o u n d , say ‘a n t m o u n t’ w h ich varies
(6)
A g ro u p o f scien tists b eliev es th a t v o l­
in h e ig h t fro m a few c e n tim e tre s to a few m etres in
canic eru p tio n s and fallo u t o f d u sts and ash es cau se
the b eg in n in g . T h e size o f th e c o n e g rad u ally in­
m ass ex tin ctio n o f a few sp ecies o f an im als. B ased
cre a se s d u e to c o n tin u o u s a c c u m u la tio n o f volcanic
on this h y p o th e sis th e m ass ex tin ctio n o f d in o sau rs
m aterials m in u s la v as. S o m e tim e s , th e ra te o f grow th
ab o u t 60 m illio n y ears ago has been rela ted to
o f the co n e is so h ig h th a t it g a in s h e ig h t o f 100 m or
increased w o rld -w id e v o lcan ic activity. A cid rains
m o re w ith in a w eek . T h e s lo p e s o f c in d e r cones
acco m p an ied by v o lcan ic eru p tio n s cau se largeran g e b etw ee n 30° an d 45°. L a rg e r p a rtic le s are •
scale d estru ctio n o f p la n ts and anim als.
arran g ed n ear th e c ra te rs a n d re s t a t th e a n g le be­
tw een 40° and 45° an d th e fin e r p a rtic le s a re d epos- *
12.9TOPOGRAPHY PRODUCED BY VULCANICITY
ited at the o u te r m a rg in s o f th e c o n e s . S in c e such
N u m e ro u s ty p es o f lan d fo rm s are created due
co n es are fo rm ed o f u n c o n s o lid a te d la rg e r p article s
to co o lin g an d so lid ificatio n o f m ag m as b elow the
and are seld o m c o m p a c te d by la v a s a n d h e n c e they
earth 's su rface and lav as at the earth 's surface and
are p erm eab le to w ater.
du e to acc u m u la tio n o f frag m en tal m aterials, dusts
and ashes w ith lav as such as d ifferen t types o f
Such co n es are on an a v e ra g e le ss s u s c e p itb le
v o lcan ic co n es. T h e cones and craters are n ot alw ays
to ero sio n and h e n ce th ey m a in ta in th e ir o rig in a l
p erm a n e n t lan d fo rm s b ecau se they are ch an g ed and
form s fo r h u n d red s o f y ears p ro v id e d th a t th e y a re
m o d ified d u rin g every su ccessiv e eruption. E x p lo ­
n ot d estro y ed by en su in g v io le n t e x p lo s io n . T h e
sive ty p e o f volcan ic eru p tio n s helps in the fo rm a­
v o lcanic co n es o f M t. Jo ru llo o f M e x ic o , M t. Iz a lc o
tio n o f several types o f volcanic cones w hereas
o f San S alv ad o r, M t. C a m ig u in o f L u z o n Is la n d o f
fissu re flow s resu lt in the fo rm ation o f lava plateaus
P h illip p in es etc. are ty p ic a l e x a m p le s o f c in d e r c o n e s
and lav a plains due to accu m u latio n o f th ick layers
(fig. 12.7(1).
o f basaltic lavas over ex ten siv e areas. T he to p o ­
(ii) C o m p o site c o n e s a re th e h ig h e s t o f all
graphic features produced by the entire process o f
volcan ic cones. T h e se are fo rm e d d u e to a c c u m u la ­
vulcanicity are grouped into tw o broad categ o ries
tion
o f d iffe re n t la y ers o f v a rio u s v o lc a n ic m a te ria ls
viz. (1) extru sive to p o g ra p h y and (ii) in tru sive
and h en ce th e se are a lso c a lle d as s tr a to -c o n e s (fig.
top ograp h y. Fig. 12.6 depicts m ajo r ch aracteristic
12.7(2). In fact, th e se c o n e s a re fo rm e d d u e to
volcanic landform s.
d
ep o sitio n o f a lte rn a te la y e rs o f la v a ai^d fra g m e n ta l
(1) E xtru sive V olcan ic T op ograp h y
(p h y ro cla stic) m a te ria ls w h e re in la v a a c ts as c e ­
(i) F rom exp losive type o f eru p tion s
m en tin g m a teria ls fo r th e c o m p a c tio n o f fra g m e n ta l
(a) E levated form s, e.g. volcanic cones
m aterials. T h e co n e b e c o m e s c o m p a ra tiv e ly r e s is t­
(b) D epressed form s, e.g. craters and
an t to ero sio n if it is c o ated by th ic k la y e r o f la v a . O n
calderas
the o th er h an d , if th e o u te r la y e r is c o m p o s e d o f
(ii) F rom fissu re eru p tion s
frag m en tal m a te ria ls, the c o m p o s ite c o n e is s u b ­
je c te d to sev ere ero sio n . M o s t o f th e h ig h e s t s y m ­
(a) L av a p lateau s and dom es
(b) L av a plains
m etrical and e x te n siv e v o lc a n ic c o n e s o f th e w o rld
co m e u n d er th is c a te g o ry e.g . M t. S h a s ta , M t. R a n ie r,
(2) In tru siv e V o lca n ic T o p o g ra p h y
global d ecre ase o f so lar ra d ia tio n re c e iv e d at the
earth 's su rface by 10 to 20 p er cent.
M t. H o o d (U S A ), M t. M a y o n o f P h illip p in e s , M t.
(i)
in tru siv e lav a d o m es, (ii) b ath o lith s, (iii)
F u z iy a m a o t Ja p a n , M t. C o to p a x i o f E c u a d o r etc.
lacco lith s, (iv) p h aco lith s, (vi) lo p o lith s, (vi) sills,
(vii) d ik es, (viii) v o lcan ic p lu g s and sto ck s etc.
(iii) P a r a s ite c o n e s- S e v e ra l b ra n c h e s o f pipes
c o m e o u t fro m th e m a in c e n tra l p ip e o f th e v o lc a n o
w h en the v o lc a n ic c o n e s are e n o rm o u s ly e n la rg e d .
VOLCANIC CONES
L av a s an d o th e r v o lc a n ic m a te ria ls c o m e o u t from
(i)
C in d e r o r a sh c o n e s are u su ally o f low
th e se m in o r p ip e s a n d th e se m a te ria ls a re d e p o s ite d
height and are form ed o f volcanic d u sts an d ashes and
a ro u n d n e w ly fo rm e d v e n ts lo c a te d o n th e o u te r
p y ro c la stic m a tte r (fra g m e n ta l m a te ria ls). T h e fo r­
su rfa c e o f th e m a in c o n e a n d th u s s e v e ra l s m a lle r
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yyiXANICTTY AND LANDFORMS
cones are form ed on m ajo r cone (fig. 12.7(3)). T hese
cones are called p arasite cones b ecau se the supply o f
lava for these cones com es from the m ain pipe.
T hese cones are also know n as a d v en tiv e or lateral
con es. S hastina cone is a p arasite co n e o f M t. S h asta
o f the U SA .
(iv) B asic lava con e is fo rm ed o f lig h t an d
less viscous lava w ith less q u an tity o f silica. In fact,
w hen the lava co m in g o ut o f fissu se flow is d e fic ie n t
in silica and is ch aracterized by h ig h d eg ree o f
fluidity, it cools and so lid ifies afte r sp read in g o v er
larger area. Thus, a long co n e w ith sig n ifican tly low
h eig h t is form ed. Such cones are also c a lle d as sh ield
cones because o f th eir sh ap es re se m b lin g a sh ield .
Since these cones are co m p o sed o f b a sa ltic la v as,
they are also called as b asic la v a co n es. T h e se are
also know n as H aw an a ty p e o f co n es (fig. 12.7(4)).
(v) A cid la v a co n es are fo rm ed w h e re the
lavas com ing out o f v o lcan ic e ru p tio n s are h ig h ly
viscous and rich in silica co n ten t. In fact, such
viscous lavas have very low m o b ility an d h e n ce th e y
are im m ediately cooled and so lid ified a fte r th e ir
appearance on the earth's su rface. T h u s, h ig h c o n e s
of steep slopes are form ed. S u ch co n es are very o fte n
know n as S tro m b o lia n ty p e o f co n es (fig. 12.7(5).
(vi) L ava d om es are in fa c t sim ila r to sh ield
cones in one w ay or the other. L av a d o m e s d iffe r
from shield cones as reg ard s th eir size. A c tu a lly ,
lava dom es are larg er and m o re ex te n siv e in size th a n
the shield cones. T h ese are fo rm ed d u e to a c c u m u la ­
tion o f so lid ified lavas aro u n d the v o lc a n ic ven ts.
B ased on the m o d e o f o rigin and the p la c e o f fo rm a ­
tion lava dom es are d iv id ed into 3 c a te g o rie s e.g. (A )
p lu g d om e (fo rm ed o f lav as d u e to fillin g o f v o l­
canic vents), (B ) en d o g en o u s d o m e (fo rm e d o f
silica rich v isco u s lavas) an d (c) e x o g e n o u s d o m e
(form ed o f s ilica-d eficien t la v a w ith h ig h d e g re e o f
fluidity).
(vii) L ava p lu g s are fo rm e d d u e to p lu g g in g
o f volcanic pipes and v en ts w h en v o lc a n o e s b e c o m e
extinct. T h ese v ertical c o lu m n s o f s o lid ifie d la v a s
ap p ear on the earth 's su rface w h en th e v o lc a n ic
cones are ero d ed aw ay. T h e la v a -fille d v o lc a n ic
piple is called as v o lc a n ic n e c k (fig . 1 2 .7 (6 )). G en ­
erally, volcan ic n eck s are c y lin d ric a l sh a p e d a n d
m easu re 50 to 6 0 m in h e ig h t (a b o v e th e g ro u n d
su rface) and 3 0 0 to 6 0 0 m in d ia m e te r. S o m e tim e s
d ia trem e term is u sed to in d ic a te v o lc a n ic neck or
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% 12,7: Different types o f volcanic cones.
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GEOMORPHOLOGY
212
len t ex am p le o f a diatreme ex p o se d by the erosion
o f its en clo sin g sed im en tary ro c k s’ (F. P ress and R.
pipe filled w ith breccia. ‘S h ip ro ck * w hich tow ers
515 m etres (1700 feet) ov er the su rrounding, flatlying sedim entary rocks o f N ew M exico, is an excel-
S iev er 1974) (fig. 12.8).
Fig. 12.8 : Shiprock (New Mexico, USA), an example o f diatreme or volcanic neck.
Depressed Forms
th eir size e.g. craters ran g e fro m sm all craterlets
Craters— T he d ep ressio n fo rm ed at hthe
av in g a d ia m ete r o f a few h u n d re d m e tre s to la rg e
m outh o f a volcanic vent is called a crater o r a
craters h av in g th e d ia m e te r o f a few k ilo m e tre s. T he
volcanic mouth, w h ich is usually funnel shaped.
c rater o f e x tin c t A n ia k c h a k v o lc a n o o f A la sk a h as a
(i)
T he slope o f the c ra te r d ep en d s upon th e vo lcan ic
cone in w hich c ra te r is fo rm ed . N o rm ally , a c rater
fo rm ed in a c in d e r co n e slo p es at the an g le b etw een
25° an d 30°. T h e size o f a c ra te r in c re ases w ith
in c re ase an d e x p an sio n o f its co n e. A c ra te r m ay be
d iffe re n tia te d fro m a c a ld e ra on the b asis o f size and
m o d e o f fo rm atio n . A n av era g e c ra te r m e asu re s 300
m in d ia m e te r an d 3 0 0 m in d e p th b u t th e re is w ide
ra n g e o f v aria tio n s in c ra te rs fro m th e sta n d p o in t o f
d ia m e te r o f 9 .6 km (6 m iles) a n d th e sid e w alls are
364 m to 91 2 m (1 2 0 0 to 3 0 0 0 fe e t) hig h . I f the
Crater Lake o f th e state o f O re g o n (U S A ) is a c ­
ce p te d as a c ra te r, it b e c o m e s o n e o f th e m o st
e x te n siv e c ra te rs o f th e w o rld , th o u g h m an y sc ie n ­
tists c o n s id e r it as an e x a m p le o f a ca ld e ra . W h en a
c ra te r is filled w ith w a te r, it b e c o m e s a crater lake.
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W hen the crater o f v o lca n o b ecom es very
ex ten siv e and if there are fe w eruptions o f very sm all
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213
VULCANICITY AND LANDFORM S
intensity a fte r long tim e, sev eral sm aller co n es are
form ed w ithin the e x ten siv e o ld er c ra te r and thus
several sm all-sized craters are fo rm ed at the m o u th
o f each v o lcan ic v en t in sid e the e x ten siv e crater.
Such craters or craterlets are called ‘nested cra­
ters’ or ‘craters within the crater’ o r ‘grouped
craters’. S uch c ra te rs are fo rm ed only w hen the next
eruption is sm a lle r in in ten sity than the p rev io u s
one. T he c ra te rs fo rm ed at the m o u th o f v o lcan ic
vents o f p arasite co n es d e v e lo p e d o v er an ex ten siv e
volcanic co n e is ca lle d adventive crater. T h ree
sm aller c ra te rs are fo u n d w ith in the e x ten siv e crater
o f M t. T aal o f P h illip p in e s. S im ilarly , th ree and tw o
craters are fo u n d w ith in th e craters o f V isu v iu s and
E tna v o lcan o es.
T arso Y eg a (20 km x 14 k m ) in S h a ra (A fric a ), A so
San (23 km x 14 k m ) in Ja p a n , A lb a n (11 km x 10
km ) in Italy , C ra te r L ak e (1 0 km x 10 k m ) in U S A ,
K rak ato a (7 km x 6 k m ) in In d o n e sia , K ila u e a (5 km
x 3 k m ) in H aw aii etc. S m a lle r c a ld e ra s h o u se d in a
big c a ld era are c a lle d nested calderas o r grouped
calderas (fig. 12.9).
C aldera
(ii)
Calderas— G e n e ra lly , en larg ed form
crater is c a lle d ca ld e ra . T h e re are tw o p arallel c o n ­
cepts fo r th e o rig in o f ca ld e ra s. A cco rd in g to the first
group o f sc ie n tists a c a ld e ra is an en larg ed form o f a
cra te r and it is s u rro u n d e d by steep w alls from all
sid es. T h e c a ld e ra is fo rm ed d u e to su b sid en ce o f a
c rater. T h is c o n c e p t has been p ro p o u n d ed by the
U .S. G e o lo g ic a l S u rv ey . It is b eliev ed acco rd in g to
this c o n c e p t th a t A so c ra te r o f Jap an and C rater L ake
o f the U S A are th e re su lt o f su b sid en ce. T he second
g ro u p o f s c ie n tis ts has o p in e d th at the cald eras are
fo rm ed d u e to v io le n t a n d ex p lo siv e eru p tio n s o f
v o lcan o es.
of a
Fig. 12.9 : Exam ple o f nested cladera.
Intrusive Topography
W hen g ases an d v a p o u r a re n o t v e ry m u c h
strong d u rin g v o lc an ic a c tiv ity , th e a s c e n d in g m a g ­
m as do not eru p t as lav as ra th e r th e se are in tru d e d in
viods b elow the cru stal su rfa c e a n d a fte r c o o lin g a n d
so lid ificatio n a ssu m e s e v e ra l in te re s tin g fo rm s lik e
batholiths, laccoliths, phacoliths, lopoliths, sills
and dykes. T h e se in tru siv e v o lc a n ic fo rm s a re seen
only w hen th e s u p e rin c u m b e n t lo a d s o f o v e rly in g
co u n try ro ck s are re m o v e d th ro u g h p ro lo n g e d e r o ­
sion. T h ese featu res h av e a lre a d y b e e n d is c u s s e d in
the p rece d in g c h a p te r 8 on rocks.
D aly, the le a d in g a d v o c a te o f ‘eruption hy­
pothesis’ o f th e o rig in o f c a ld e ra s, b eliev es th at the
to p o g rap h ic fe a tu re s fo rm e d by su b sid e n c e are ‘vol­
canic sinks.’ A c c o rd in g to th e a d v o c a te s o f this
Geysers
h y p o th esis if c a ld e ra s are fo rm ed d u e to su b sid en ce
there sh o u ld n o t be any d e p o s it o f p y ro c la stic m a te ­
G ey ser, in fact, is a sp e c ia l ty p e o f hot spring
w h ich sp o u ts h o t w a te r an d v a p o u r fro m tim e to
rials and v o lc a n ic a sh e s re la te d to a p a rtic u la r v o l­
tim e. T h e w o rd g e y s e r h as b e e n d e riv e d fro m an
Icelan d ic w o rd ‘geysir’ w h ic h m e a n s gusher o r
canic co n e n e a r the c a ld e ra b u t e v id e n c e s h av e
revealed th a t th e re m a in s o f v o lc a n ic m a te ria ls re ­
spouter. T h is w o rd w as u sed to in d ic a te the sp o u tin g
w ater o f a h o t s p rin g o f Ic e la n d k n o w n as Great
Geyser o r Gesir.
lated to a p a rtic u la r c o n e are fo u n d n o t o n ly n e a r the
concerned c a ld e ra but a re a lso fo u n d s ev eral k ilo m e ­
tres aw ay from the c a ld e ra . F o r e x a m p le , v o lc an ic
G e y se r, re p re s e n tin g a m in o r form o f the
b ro a d e r p ro c e ss o f v u lc a n ic ity , h as b een v a rio u sly
m aterials h av e been fo u n d at th e d is ta n c e o f 128 km
from the c a ld e ra o f C ra te r L ak e (U S A ). T h e s ig n ifi­
cant c ald eras o f th e w o rld are (fig u re in th e b ra c k e ts
denote dim ension in k ilo m e tre s)L a k e T o b a o fS u m a tra
(50 km x 50 k m ) in S u m a tra , A ira (25 km x 24 km )
in Japan, L ak e K u tc h a io (2 6 km x 2 0 k m ) in Ja p a n ,
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d efin ed by the scien tists. F o re x a m p le , A rth u rH o le m s
has d e fin e d g e y s e r in th e fo llo w in g manner-. “G e y ­
sers are h o t sp rin g s fro m w h ic h a co lu m n o f h o t
w a te r an d steam is e x p lo siv e ly d isch arg e d a t in te r­
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G EO M O R PHOLOGY
214
vals, sp o u tin g in so m e cases to h eig h ts o f h u n d red s
p ie, G ra n d G e y s e r o f Ic e la n d s p o u ts w a te r for 30
o f fe e t.” A cco rd in g to P .G . W o rc e ste r “G e y se rs are
m in u te s in c o n tin u a tio n b e fo re th e n ex t interval
in te rm itte n t ho t sp rin g s th a t fro m tim e to tim e sp o u t
p e rio d sta rts) an d (iv ) f e e b le g e y s e r (w h erein the
steam and hot w a te r fro m th e ir c ra te rs .”
activ e p e rio d o f w a te r s p o u tin g is v ery sh o rt). C o n ­
"
tin u o u s ly a c tiv e g e y s e r s are, in fa c t, hot springs
T h e d iffe re n c e b etw ee n h o t sp rin g s and g e y ­
w hich spout w ater w ith o u t an y in terv al. T h e Excelsior
ser lies in the fact th a t th ere is co n tin u o u s sp o u tin g
G ey ser o f the Y ello w S to n e N a tio n a l P ark o f the
o f h o t w a te r fro m th e f o r m e r w h ile th e re is
U S A is th e e x am p le o f th is c a te g o ry .
in te rm itte n t(w ith in terv al) sp o u tin g o f w ater from
the alter. A g ey ser sp o u ts w ater from a sm all and
T h ere is no certain o b s e rv a b le d istrib u tio n a l
n arrow vent w h ich is c o n n ec ted by a circu ito u s pipe
p attern o f g e y se rs o v er th e g lo b e as th ey are found
w ith the u n d erg ro u n d aquifers. T h is pipe is called as
g e y s e r p ip e or g e y s e r tu b e . T he length o f g ey ser
tube ran g es betw een 30 to 100 m at d ifferent places.
T h e te m p eratu re o f w ater co m in g out o f a g ey ser
in alm o st all the c o n tin e n ts an d in a lm o s t all the
clim atic zones. T he g ey sers o f the U S A , Ic e la n d and
N ew Z ealan d are m o st w id ely stu d ie d g e y se rs. G e y ­
sers are found in g ro u p s in the Y ello w S to n e N a ­
tional Park (U SA ). A b o u t one h u n d re d g e y se rs h ave
ran g es betw een 75° to 90°C.
G eysers are classified into tw o types viz. (i)
pool type o f geyser and (ii) nozzle t]/pe o f geyser.
W hen a geyser spouts w ater through an open and
relatively large pool, it is called po o l ty p e o f g ey ser.
Such geysers spout larger volum e o f w ater and
vapour through long geyser tubes. N o deposits are
possible around the geyser pools. N ozzle ty p e o f
g ey sers spout w ater and vapour through a very small
and constricted vent. Em itted m aterials are d ep o s­
ited around the geyser vents and thus g ey ser cones
are form ed.
been nam ed and an o th er h u n d red g e y se rs are k n o w n
to the scien tists. T h ere are fo u r m a jo r b a sin s o f
g ey sers viz. (i) N o rris B asin, (ii) U p p e r L a k e B asin ,
(iii) L o w er L ake B asin and (iv ) H eart L ak e B asin.
T he m ajo r g ey ser o f N ew Z e a la n d is lo c ated in the
w estern region o f the n o rth ern Islan d w h ich is also
dom in ated by v o lcan ic ac tiv itie s. T h e g e y se rs and
hot springs are spread o v er an a re a o f 1786 km 2
(5000 square m iles) in Iceland. T h e m o st s ig n ific a n t
g eyser o f Iceland is G ran d G ey ser.
Som e scientists do not agree to accept hot
sp rin g s and g e y se rs as tw o sep arate fo rm s o f
vulcanicity rather they believe that both are the
sam e, the difference is only o f periodicity o f sp o u t­
ing o f water. Thus, they have grouped geysers into
two categories viz. (1) no n -co n tin u o u s geysers or
geysers w ith interm ittent spouting and (2) co n tin u ­
ously active geysers. The in term itten t geysers are
further divided into (i) geysers o f equal intervals
between two successive period o f spouting (w herein
interval period betw een two successive active p eri­
ods of spouting is certain and fixed, such geysers are,
thus, considered to be reliable as regards the p eriods
of interval and spouting, exam ple, O ld F aithful G ey ­
ser of the Y ellow Stone N ational Park, U SA ), (ii)
v a ria b le geysers (w herein the interval period b e ­
tw een tw o successive periods o f spouting is not
certain), (iii) lo n g -p e rio d g ey sers (w herein the ac­
tive period o f spouting is longest o f all the geysers,
ranging betw een a few m inutes to one hour, exam -
F um arole m ean s such a v en t th ro u g h w hich
there is em ission of g ases and w a te r v ap o u r. It
appears from a d istan t place th a t th ere is em issio n o f
enorm ous volum e o f sm o k es from a p a rtic u la r c e n ­
tre. Thus, sm oke o r gas e m ittin g v en ts are called
fum aroles. In fact, fu m aro les are d ire c tly lin k e d w ith
volcanic activ ities. E m issio n o f g ases and v a p o u r
12.10 FUM AROLES
begins after the em issio n of v o lc an ic m a teria ls is
term inated in an active v o lcan o . S o m e tim es the
em ission o f gases and v ap o u r is c o n tin u o u s but in
m ajority o f the cases em issio n o ccu rs a fte r intervals.
It is believ ed that g ases and v a p o u r are g en era ted due
to co o lin g and co n tractio n o f m a g m a afte r the term i­
nation o f the eru p tio n o f a v o lcano. T h e se gases and
vap o u r ap p ear at the earth 's su rface th ro u g h a narrow
and co n stricted pipe (tube). It m ay be po in ted out
th at fu m aro les are the last sig n s o f th e activ en ess o f
a volcano.
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N u m ero u s fu m aro les are fo u n d in groups
near Katm ai volcano o f A laska (U SA ). H ere fum aroles
|
J
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VULCANICITY AND LANDFORMS
215
are found in groups in ex ten siv e v a lley zone, w hich
is called a v a lle y o f te n th o u sa n d s m o k e s ’ w hich
9 8 .4 to 9 8 .9 9 p ercen t o f the total g a ses em itted from
fum aroles. Other g a ses in clu d e carbon d io x id e, h y ­
m eans fu m aroles appear from 10 ,0 0 0 vents the d i­
drochloric acid, hydrogen su lp h id e, nitrogen, som e
o x y g en and am m onia. S o m e m inerals are a lso em it­
ameter o f w h ich is around 3 m etres. Here fum aroles
ted w ith g a s e s and v a p o u r fro m f u m a r o le s .
Sulphur is the m ost im portant m ineral. F um aroles
dom inated by sulphur are ca lled s o lfa ta r a or s u l­
are found along a linear fracture. Elsew here, fumaroles
are found a lo n g the v o lca n ic craters. The tem pera­
ture o f vapour em itted from fum aroles is around
645°C . It m ay be m en tio n ed that vapour constitutes
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p h u r fu m a r o le s.
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:
MOUNTAIN BUILDING
216*246
In tr o d u c tio n ; c la s s ific a tio n o f m o u n ta in s ; b lo ck m o u n ta in s ; fo ld e d
m o u n ta in s ; g e o s y n c lin e s ; th eo ries o f m o u n tain b u ild in g - g e o s y n c lin a l
th e o r y o f K o b e r ; therm al co n tra ctio n th eory o f J e ffrey s ; slid in g c o n ti­
n e n t th e o r y o f D a ly ; therm al c o n v e c tio n current thery o f H o lm e s ;
r a d ia c tiv ity th eo ry o f J o ly ; p late te c to n ic th eory.
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CHAPTER 13
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13
MOUNTAIN BUILDING
ta in o u s re d o n o f th e w e s te rn p a r t o f N o rth A m e ric a
13.1 INTRODUCTION
M o u n ta in s ir e sig n ific a n t re lie f fe a tu re s o f
the seco n d o rd er on th e earth s s u rta c e . A m o u n ta in
m av have sev eral fo rm s viz. (1) m o u n ta in rid g e, iji)
m o u n tain ran g e. viiO m o u n ta in ch ain , (iv ) m o u n tain
system , (v) m o u n ta in g ro u p an d (v i) c o rd illera . A
m o u n ta in rid g e is a sy stem o f lo n g , narro w and hig h
hills. G en erally , th e slope o f one side o f a rid g e is
steep w hile die o th e r side is o f m o d erate slope but a
ridge m ay also h av e sy m m e trica l slo p es on b o th the
sides. A m o u n ta in ra n g e is a sy stem o f m o u n tain s
and hiTk h av in g several rid g es, p eaks and su m m its
and valleys. In fact, a m o u n tain range stretch es in a
lin ear m anner. In o th e r w o rd s, a m o u n tain range
rep resen ts a long but narrow strip o f m o u n tain s and
hills. A ll o f the hills o f a m o u n tain ran g e are o f the
sam e age but there are stru ctu ral v aria tio n s in d iffe r­
ent m e m b ers o f the range. A m o u n ta in ch a in c o n ­
sists o f sev eral p arallel long and n arro w m o u n tain s
is th e b est e x a m p le o f a c o r d ille r a .
13.2 CLASSIFICATION O F MOUNTAINS
1. On the Basis of Height
(i) lo w m o u n ta in s; h e ig h t ra n g e s b e tw e e n 7 0 0
to 1.00 m .
(ii) rough m ountains; height-1000 m to 1.500 m
(iii) rugged m ountains; h eig h t-1.500 to 2.0 0 0 m
(iv) h ig h m o u n ta in s; h e ig h t a b o v e 2 .0 0 0 m
2. On the Basis of Location
(i) C o n tin en ta l m o u n ta in s
(a) coastal m o u n ta in s, e x a m p le s: A p p la c h ia n s,
R ockies, A lpine m o u n tain ch ain s. W e ste rn a n d E a s te rn
G h ats o f In d ia etc.
(b) in la n d m o u n ta in s, e x a m p le s ; U ra l m o u n ­
tain s (R u ssia). V o sg e s and B la c k F o r e s t b lo c k m o u n ­
tains (E urope). H im alay as, A ra v a llis, S a tp u ra . M aik al,
K aim u rs etc. (In d ia ), K u n lu n , T ie n s h a n , A lta i etc.
(A sia) etc.
o f different periods. S om e tim es. the m o u n tain ranges
are separated by flat upland or plateaus. A m o u n ta in
sy ste m con sists o f different m ountain ranges o f the
sam e period. D ifferen t m ountain ranges are sepa­
(ii)
O c e a n ic m o u n ta in s -m o s t o f the o cea n
m ountains are b elo w w ater su rface (b e lo w sea
lev el). O cean ic m ountains are lo ca ted on continental
sh elv es and ocean flo o rs. S o m e o c e a n ic m ountains
are also w ell a b o v e the sea le v e l. If the h eigh t o f the
m ountains is co n sid ered from the o c e a n ic flo o r and
not from se a -le v e l, m any o f the o c e a n ic m ountains
rated by valleys. A m o u n ta in g ro u p co n sists o f
several unsystem atic patterns o f different m ountain
system s. C o r d ille r a co n sists o f several m ountain
groups and system s. In fact, cordillera is a co m m u ­
nity o f m ountains having d ifferent ridges, ranges,
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m ountain ch ain s and m ountain system s. The m oun­
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m o u n t a in b u il d in g
217
will becom e m u c h h ig h e r than the M ount E verest
For ex am p le M a u n a K ea volcanic m ountain o f
Hawaii Islan d is 4 2 0 0 m h ig h from the sea level but
if its h eig h t is c o n sid e re d from th e sea bottom it
height b eco m es 9 1 4 0 m w hich is h ig h er than ’the
highest m o u n ta in , M o u n t E v erest (8848 m AMSL1
K ilam ean m ountains etc. (N orth A m erica), m oun­
tains o f Feno-Scandia, N orth-W est H ighlands and
A nglesey etc. (Europe).
(2) Caledonian mountains: mountains formed
during Silurian and D evonian periods, exam ples :
T aconic m ountains o f the A pplachian system , m oun­
tains of Scottland, Ireland and Scandinavia (E u­
rope), B razilid es o f S outh A m erica, A ravallis,
M ahadeo, Satpura etc. o f India.
SLmilar,y’ th£ An,ilean Mountain
system is 3 0 0 0 m a b o v e sea-lev el b u t it is also 5400
m below se a -le v e l, an d th u s its total height from the
oceanic flo o r b e c o m e s 8 4 0 0 m. M o st o f the oceanic
m o u n ta in s a re v o l c a n i c m o u n ta in s .
(3) H e rc y r ;an m ountains: m ountains form ed
during Perm ian and Perm ocarboniferous periods,
exam ples: m ountains o f Iberian peninsula, Ireland,
Spanish M esseta, B rittany of France, S outh W ales,
Cornw all, M endips, Paris basin, B elgian coalfields,
Rhine M ass, B ohem ian plateau, V osges and B lack
F o re s t, p la te a u re g io n o f c e n tr a l F r a n c e ,
T h u ringenw ald, F ran k en w ald , H a rtz m o u n ta in ,
Donbas coalfield (all in E urope); V ariscan m o u n ­
tains o f A sia include A ltai, Sayan, B aikal A rcs, T ien
Shan, Khingan, m ountains o f D zu n g arian b asin,
Tarim basin, N anshan, A lai and T ran s A lai m o u n ­
tains o f A m ur basin, M o n golia and G obi etc; A u ­
stralian V ariscan m ountains include the scattered
hills in the Eastern C ordillera, N ew E n g lan d o f N ew
Southerw ales; N orth A m erican V ariscan m o u n tain s
include A pplachians; S outh A m eric an V a riscan
m ountains are A ustrian and S aalian fo ld s o f S an
Juan and M endoza, m ountains o f Puna arc o f A tacam a,
G ondw anides o f A rgentina etc.
3. On the B a sis of Mode of Origin
(1) O rig in a l or tecton ic m ountains are caused
due to t e c t o n ic f o r c e s e.g. c o m p r e s s iv e and tensile
forces m o t o r e d b y e n d o g e n e t ic fo rces c o m in g from
d e e p w i t h in th e e a rth . T h e s e m o u n ta in s are further
di v id e d i n to 4 t y p e s o n th e b a s is o f o ro g en e tic forces
r e s p o n s i b le f o r th e o r ig in o f a p a rtic u la r type o f
m o u n ta in .
(1) F o ld e d m o u n ta in s are fu rth er divided
in to 3 s u b - t y p e s o n the b a s is o f their area. T h e se are
o r ig i n a te d by c o m p r e s s i v e forces.
(A ) y o u n g fo ld e d m o u n ta in s
(B ) m a t u r e fo ld e d m o u n ta in s
(C ) o ld fo ld e d m o u n ta in s
(ii) B lo c k m o u n ta in s are originated by ten ­
sile f o r c e s l e a d i n g to th e fo rm a tio n o f rift valleys.
T h e y a r e a ls o c a lle d as h o r s t m o u n ta in s .
(4) A lpine m ou n tain s : m o u n tain s fo rm ed
during Teritary period, ex am ples: R o ck ies (N o rth
A m erica), A ndes (S outh A m erica), A lp in e m o u n ­
tain system s o f E urope (m ain A lp s, C arp ath ian s,
Pyrenees, B alkans, C au casu s, C an tab rian s, A pen­
nines, D inaric A lps etc.), A tlas m o u n tain s o f n o rth ­
w est A frica; H im alayas and m o u n tain s c o m in g out
o f Pam ir K not o f A sia (T au ru s, P au n tic, Z agros,
Elburz, K unlum etc.).
(iii) D o m e m o u n ta in s are o rig in ated by
m a g m a t i c i n tr u s io n s a n d u p w a r p in g o f the crustal
surface. E xam ples, normal domes, lava domes,
batholithic dom es, laccolithic domes, salt domes
etc.
(iv ) M o u n ta in s o f a c c u m u la tio n s are form ed
d u e to a c c u m u la tio n o f v o lc a n ic m ate ria ls. T hus,
th e s e a re a ls o c a lle d as v o lc a n ic m o u n ta in s. D iffe r­
en t ty p e s o f v o lc a n ic c o n e s (e.g. cin d er cones, co m p o s­
Block Mountains
ite c o n e s , a c id la v a c o n e s , b a sic lava c o n e s etc.)
B lock m o untains, also know n as fau ltb lo ck
m ou n tain s, are the resu lt o f fau ltin g cau sed by
te n s ile a n d c o m p r e s s iv e f o r c e s m o to r e d by
endogenetic forces co m in g from w ithin the earth.
B lock m ountains represent the u p stan ding p arts of
the ground betw een tw o faults o r on e ith er side of a
rift valley or a graben. E ssen tially , b lo ck mountains
are form ed due to faulting in the g ro u n d surface.
c o m e u n d e r th is c a te g o r y .
(2) C i r c u m - e ^ o s i o n a l o r re lic t m o u n ta in s :
exam ples, V in d h y ach al ranges, A rav alh s, Satpura,
E astern G h ats, W estern G h ats etc. (all from India).
4. On the basis of period of origin
(1 ) P r e -C a m b r ia n m o u n ta in s : examples,
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L a u ren tia n m o u n ta in s, A lg o m a n m o u n ta in s,
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geom o rph
218
r e p r e s e n te d b y fa u lt s c a r p a n d o n e g e n tle side and(ii)
lifted block m ountains r e p r e s e n t real horst and are
c h a r a c te r iz e d by f la tte n e d s u m m i t o f tab u la r shape
Horst
and very steep side slo p e s re p re sen te d by tw o boundary
fault s c a rp s . B lo c k m o u n ta in s a re a lso called as
horst mountains (fig. 13.1).
If
B lo c k m o u n ta in s a re found in all the conti.
n e n ts e.g. (i) y o u n g b lo c k m o u n ta in s a ro u n d Albert,
W a r n e r an d K la m a th lak e s in the S te e n s M ountain
D istrict o f S o u th e r n O r e g o n , W a s a t c h R a n g e in the
U tah p r o v in c e etc. in the U S A , (ii) V o s g e s and Black
F o re st m o u n ta in s b o r d e r in g the fa u lte d R h in e Rift
va lle y in E u r o p e , (iii) S a lt R a n g e o f P a k ista n etc.
S ierra N a v a d a m o u n ta in o f C a l if o r n ia ( U S A ) is
c o n s id e r e d to be the m o s t e x te n s iv e b lo c k m ountain
o f the w orld. T h is m o u n ta in e x te n d s fo r a length of
6 4 0 km ( 4 0 0 m ile s ) h a v in g a w id th o f 80 km (50
m iles) and the h e ig h t o f 2 ,4 0 0 to 3 ,6 6 0 m (8 ,0 0 0 to
12,000 feet). T h e r e is d if f e r e n c e o f o p i n io n s am o n g
the sc ie n tists r e g a r d in g the o rig in o f b lo c k m o u n ­
tains. T h e re are tw o th eo ries for the o rig in o f these
m o u n ta in s viz. ( 1) f a u lt th e o ry a n d (ii) ero sio n
th e o ry .
F ault T heory
M o st o f the g e o lo g is ts are o f the o p in io n that
block m o u n ta in s are fo rm e d d u e to faulting. T h e
structural pa tte rn s o f G re a t B asin R a n g e m o u n ta in s
o f U tah p r o v in c e ( U S A ) w e re c lo se ly s tu d ie d by
C la re n c e K ing and G .K . G ilb e r t w h o n a m e d these
m o u n ta in s as f a u l t e d b l o c k s ( b e tw e e n 1870 and
1875 A .D .). S in c e then the m o u n ta in s f o rm e d d u e to
larg e -sc a le fa u ltin g w e re n a m e d b lo c k m o u n ta in s.
L a ter on G .D . L o u d e r b a c k o p in e d that B asin Range
m o u n ta in s w e re fo rm e d d u e to f a u ltin g a n d tilting in
the g ro u n d s u rfa c e . W .M .D a vis a lso a d v o c a te d for
the fault th eo ry o f the o rig in o f b lo ck m ountains.
B lo c k m o u n ta in s are fo rm e d in a n u m b e r o f ways.
C
niock Mountain
mock
M o u n t a in
(i) B lo c k m o u n ta in s are f o rm e d due to up­
w ard m o v e m e n t o f m id d le b lo ck b e tw e e n tw o nor­
mal faults (fig. 13.1 ). T h e u p th ro w n block is also
ca lled as horst. T h e s u m m ita l a rea o f such block
m o u n ta in is o f Hat s u rf a c e but the side slopes are
very steep.
Fig. 13.1 : A-lilock mountain form ed due to rise o f m id­
dle block, /?-form ation o f block mountain due
to downward movement o f side blocks and Cform ation o f block mountain due to down­
ward movement o f middle block-due to rift
vailey formation.
(ii) B lo c k m o u n ta in s m ay be fo rm e d when the
side b locks o f tw o faults m o v e d o w n w a rd whereas
the m id d le b lo ck re m a in s s ta b le at its place (fig1 3.1B). It is a p p a re n t that the m id d le b lock projects
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B lo c k m o u n ta in s are g e n e ra lly o f tw o basic types
e.g. (i) tilted b lock m ou n tain s ha v in g o n e steep side
j
^
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m ou nta in b u il d in g
219
above the surrounding surface because o f downward
m ovem ent o f sid e b lo ck s. Su ch b lock m ountains are
generally form ed ,n h igh plateaus or broad dom es
c u m bent folds caused by pow erful com pressive forces.
( 2 ) F o ld e d m o u n ta in s are c la s s ifie d in to (i)
you n g fold ed m ou n tain s (w h ic h a re le a s t a ffe c te d *
by d e n u d a tio n al p ro c e s s e s) a n d (ii) m atu re fo ld ed
m ou n tain s. It m a y be p o in te d o u t th a t it is d iffic u lt
to find true y o u n g fo ld e d m o u n ta in s b e c a u s e th e
p ro ce ss o f m o u n ta in b u ild in g is e x c e e d in g ly slo w
p ro ce ss and thus d e n u d a tio n a l p r o c e s s e s s a rt d e ­
n u d in g the m o u n ta in s rig h t fro m the b e g in l i n g o f
their origin. M a tu re fo ld e d m o u n ta in s a re ch lra c te rized by m o n o c lin a l rid g e s a n d v a lle y s. T h is classifi­
cation is b a s e d on the a g e factor.
u
J hi' ) ® lo c k m o u n ta in s m ay b e fo rm ed w hen
the m id d le b lo c k b e tw e e n tw o n o rm al fau lts m oves
dow nw ard. T h u s , th e s id e b lo c k s b e c o m e horsts and
block m o u n t a i n s ( fig . 13 i q S u r h
♦ •
• . . , ■, . f
^ u c n m o u n ta in s are
a s s o c ia te d w i t h t h e f o r m a t i o n o f r ift v a lle y s
Erosion Theory
J .F . S p u r r , o n th e b a s is o f d e ta i le d stu d y o f
G reat B a s i n R a n g e m o u n t a i n s o f the U S A o p in e d
that th e s e m o u n t a i n s w e r e n o t f o r m e d d u e t o ’fau ltin g
and tiltin g , r a t h e r t h e y w e r e f o r m e d d u e to d if f e r e n ­
tial e r o s i o n . A c c o r d i n g to S p u r r th e m o u n ta in s , after
their o r i g i n in M e s o z o i c e ra , w e r e s u b je c te d to
intense e r o s i o n . C o n s e q u e n t l y , d iff e re n tia l e ro s io n
re s u lte d i n t o t h e f o r m a t i o n of e x is t in g d e n u d e d G re a t
B asin R a n g e m o u n t a i n s . It m a y be p o in te d out that
e ro s io n t h e o r y o f t h e o r i g i n o f b l o c k m o u n ta in s is not
a c c e p ta b le to m o s t o f th e s c ie n t is ts b e c a u s e they
b e lie v e t h a t d e n u d a t i o n m a y m o d i f y m o u n ta ns but
c a n n o t f o r m a m o u n t a i n . In fact, d e f o r m a t o r y p r o c ­
ess p la y m a j o r r o l e in th e o r i g i n o f b l o c k m o u n ta in s .
(3) O n the basis o f the p e r io d o f o r ig in f o ld e d
m o u n ta in s are d iv id e d into (i) o ld fo ld ed m o u n ta in s
a nd (ii) new fold ed m o u n ta in s. All th e o ld f o ld e d
m o u n ta in s w e re o r ig in a te d b e fo r e T e r ti a r y p e r i o d .
T h e folded m o u n ta in s o f C a l e d o n ia n a n d H e r c y n i a n
m o u n ta in b u ild in g p e rio d s c o m e u n d e r th is c a t ­
egory. T h e s e m o u n ta in s h a v e b e e n so g r e a tly d e ­
n u ded that they h a v e n o w b e c o m e r e lic t -folded
m o u n t a i n s , for e x a m p le , A r a v a llis , V i n d h y a c h a l
etc. T he Alpi ne fo ld e d m o u n ta in s o f T ertiary' p e r i o d
are g ro u p e d u n d e r the c a te g o r y o f n e w f o ld e d m o u n ­
tains, for e x a m p le , R o c k ie s , A n d e s , A lp s , H i m a l a ­
yas etc.
Folded Mountains
F o l d e d m o u n t a i n s a re f o r m e d d u e to fo ld in g
C h a ra c te ris tic s o f F o ld e d M o u n ta in s
(1) F o ld e d m o u n ta in s a re th e y o u n g e s t m o u n ­
tains on the e a rth 's s u rfa c e.
o f c ru s ta l r o c k s b y c o m p r e s s i v e fo rc e s g e n e ra te d by
e n d o g e n e t ic f o r c e s c o m i n g f r o m w ith in the earth.
T h e s e a re t h e h i g h e s t a n d m o s t e x te n s i v e m o u n ta in s
o f the w o r l d a n d a r e f o u n d in all the c o n tin e n ts . T h e
d is tr ib u tio n a l p a t t e r n o f f o l d e d m o u n t a i n s o v e r the
g lo b e d e n o t e s th e f a c t t h a t th e y a re g e n e r a lly fo u n d
a long th e m a r g i n s o f t h e c o n t i n e n t s e it h e r in n o r t
south d i r e c t i o n o r e a s t - w e s t d i r e c ti o n . R o c k ie s , A n ­
des, A lp s , H i m a l a y a s , A t l a s e tc . a re th e e x a m p l e s o t
fo ld e d m o u n t a i n s . F o l d e d m o u n t a i n s a re c assi le
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(2) T h e lith o lo g ic a l c h a r a c te r is t ic s o f f o l d e d
m o u n ta in s rev e a l th a t th e s e h a v e b e e n f o r m e d d u e to
fo ld in g o f s e d im e n ta ry r o c k s by s tr o n g c o m p r e s s i v e
forces. T h e fo ss ils fo u n d in th e r o c k s o f f o ld e d
m o u n ta in s d e n o te the fa c t th a t th e s e d i m e n t a r y r o c k s
o f these m o u n ta in s w e re f o r m e d d u e to d e p o s i t i o n
and c o n so lid a tio n o f s e d im e n ts in w a t e r b o d ie s m a in ly
in o c e a n ic e n v i r o n m e n t b e c a u s e th e a r g i l l a c e o u s
on v a rio u s b a s e s a f o ll o w s .
r o c k s o f fo ld e d m o u n ta in s c o n t a i n m a r i n e fo s s ils .
(1)
F o l d e d m o u n t a i n s a r e d i v i d e d into 2 b ro a d
(3) S e d i m e n ts a re f o u n d u p t o g r e a t e r d e p t h s ,
c a te g o rie s on t h e b a s i s oi t h e n a tu r e o
o s.
th o u s a n d s o f m e t r e s ( m o r e th a n 12 ,000 m e t r e s ) .
S im p le fo ld e d m o u n t a i n s w i t h o p en 0 s
u
B a s e d on this fa c t s o m e s c ie n tis ts h a v e o p i n e d th a t
m o u n ta in s a r e c haracterized by w e ll d e v e o p e sys
the s e d im e n ts i n v o lv e d in th e f o r m a t i o n o f s e d i m e n ­
tern o f a n ti c li n e s and synclines w h e r e in o s a
tary r o c k s o f f o ld e d m o u n t a i n s m i g h t h a v e b e e n
a rra n g e d in w a v e - l i k e p a t t e r n . T h e s e m o u n ta in s h a v e
d e p o s ite d in d e e p o c e a n i c a re a s b u t th e m a r i n e
o pen a n d r e l a ti v e ly s i m p l e fols. (ii) C o m p ex o e
fo ss ils f o u n d in th e r o c k s b e lo n g to s u c h m a r i n e o r ­
m o u n ta in s r e p r e s e n t v e r y c o m p l e x s tr u c tu r e o in^
g a n is m s w h i c h c a n s u r v i v e o n ly in s h a l l o w w ater or
ten se ly c o m p r e s s e d f o ld s . S u c h c o m p l e x stru c ure
s h a llo w sea. It m e a n s t h a t th e s e d i m e n t a r y rock s o f
o f folds is c a l l e d ‘n a p p e ’ . In fac t, c o m p l e x fo ld
fo ld e d m o u n ta in s w e r e d e p o s i t e d in sh allow seas.
m ountains are f o r m e d due to the f o r m a ti o n o re
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GEOMORPHOLOGY
220
O n an average, a g eo sy n clin e m eans a water
d ep ressio n c h a ra c te riz e d by se d im e n ta tio n . It has
now b een acc ep ted by m a jo rity o f th e g eo lo g ists and
g e o g ra p h e rs th a t all th e m o u n ta in s h a v e co m e o u t o f
the g eo sy n c lin e s an d th e ro c k s o f th e m ountains
o rig in ated as se d im e n ts w ere d e p o s ite d and later on
c o n s o lid a te d in s in k in g s e a s , n o w k n o w n as
g eo sy n clin es. I f w e c o n s id e r th e h e ig h t and thick­
ness o f sed im en ts o f the y o u n g fo ld e d m o u n tain s of
T ertiary p erio d (e.g. R o c k ie s, A n d e s, A lp s, H im ala­
yas etc.), then it ap p ea rs th at th e g e o s y n c lin e s should
have been very d eep w a te r b o d ie s b u t th e m arine
fossils found in the se d im en tary ro c k s o f th e se folded
m o u n tain s b elo n g to the c a te g o ry o f m a rin e organ­
ism s o f sh allo w seas. It is, th u s, o b v io u s th at the
g eo sy n clin es are sh allo w w a te r b o d ie s ch aracterized
by grad u al sed im en tatio n an d su b sid e n c e . B ased on
above facts g eo sy n clin es can n o w be d efin ed as
T h e sea bottom s w ere su b jected to co n tin u o u s su b ­
sidence due to gradual sed im en tatio n . T hus, the
greater thickness o f sed im en ts co u ld be p o ssib le due
to continuous sed im en tatio n and su b sid en ce and
consequent co n so lid atio n o f sed im en ts d u e to ev er
increasing su p erin cu m b en t load.
(4) Folded m ountains extend for greater lengths
but their w idths are far sm aller than th eir len g th s, F or
ex am ple, the H im alay as ex ten d from w est to east for
a length o f 2400 km (1 5 0 0 m iles) but th eir northsoutjf w idth is only 400 km (250 m iles). It m eans that
folded m o u n tain s have been form ed in long, n arrow
and shallow seas. S uch w ater bodies have been
term ed g eo sy n clin es ^nd it has been estab lish ed that
‘o u t o f .geosyn clin es h ave co m e ou t th e m o u n ­
ta in s ’ or ‘g eo sy n clin es h ave b een crad les o f m o u n ­
t a in s / A ccording to P.G . W o rcester ‘all g reat folded
m ountains stand on the sites o f fo rm er g eo sy n clin es’.
follow s-
(5) F olded m o u n tain s are g enerally round in
arch shape having one side concave slope and the
other side convex slope.
‘G eo sy n clin e s are lo n g b u t n arro w and shal­
low w ater d ep ressio n s c h a ra c te riz e d by sed im en ta­
tion and su b sid e n c e ’.
(6) F olded m o u n tain s are found along the
m argins o f the contin en ts facing oceans. F or exam ple^R ockies and A ndes are located along the w est­
ern m argins o f N orth and S outh A m ericas resp ec­
tively and face Pacific O cean. T hey are lo cated in
tw o directions e.g. n o rth-south (e.g. R ockies and
A ndes) and w est-east d irectio n s (e.g. H im alayas).
T he A lpine m ountains are lo cated along the southern
m a rg in ! o f E urope facing M ed iterran ean sea. If we
consider form er T ethys Sea, then the H im alayas
w ere also located along the m argins o f the continent.
J.A . S teers (1 9 3 2 ) has ap tly rem ark ed , ‘the
g eo sy n clin es h ave been long and rela tiv e ly narrow
d ep ressio n s w h ich seem to have su b sid ed d u rin g the
accu m u latio n o f sed im en ts in th e m .’
T he fo llo w in g are the g en eral ch aracteristics
o f g eo sy n clin es.
(1) G e o sy n clin e s are lo ng, narrow and shal­
low d ep ressio n s o f w ater.
(2) T h ese are c h a ra c te riz e d by g rad u al sedi­
m en tatio n and su b sid en ce.
(3) T he n atu re and p attern s o f geosynclines
have not rem ain ed the sam e th ro u g h o u t geological
h istory rath er th ese have w id ely c h an g e d . In fact, the
location, shape, d im en sio n and ex ten t o f geosynclines
have co n sid erab ly ch an g e d d u e to e a rth m ovem ents
and geological p ro cess.
13.3 GEOSYNCLINES
Meaning and C oncep t
The geological history o f the continents and
ocean basins denotes the fact that in the beginning
our globe was characterized by two im portant fea­
tures viz. (i) rigid m asses and (ii) geosyn clin es.
Rigid m asses representing the ancient nuclei o f the
present continents, have rem ained stable for co n sid ­
erably longer periods o f time. T hese rigid m asses are
supposed to have been surrounded by m obile zones
o f w ater characterized by extensive sedim entation.
These m obile zones o f w ater have been term ed
‘geosynclines’ w hich have now been converted by
com pressive forces into folded m ountain ranges.
(4) G eo sy n clin e s are m o b ile z o n e s o f water.
(5) G eo sy n clin es are g e n e ra lly b o rd ered by
tw o rigid m asses w h ich are ca lle d fo rela n d s.
E volution of th e C o n cep t
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T he co n cep t o f g e o sy n clin es w as given by
Jam es Hall and D ana but the co n c e p t w as elaborated
and further d ev elo p ed by H aug. J.A . S teers (1932)
has rem arked, ‘w hile the th eo ry o f g eo sy n clin es is
M
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m o u n ta in b u i l d i n g
221
due to H au g , th e c o n c e p t o f id e a b e lo n g s to H all and
D ana’. It is d e s ira b le to d is c u s s th e c o n c e p t o f
geo sy n clin es d e v e lo p e d b y d iffe re n t e x p o n e n ts.
m o u n tain s. H e o p in e d th a t th e ro ck s o f fo ld e d m o u n ­
tain s w ere d e p o site d in sh allo w seas. A c c o rd in g to
H all th e b ed s o f g e o sy n c lin e s are su b je c te d to su b ­
sid en ce d u e to c o n tin u o u s se d im e n ta tio n b u t the
(1)
Concept o f Hall and Dana- D a n a stu d ied
d ep th o f w ater in th e g e o sy n c lin e s re m a in s th e sam e
the folded m o u n tain s and p o stu lated th at the sedim ents
(fig. 13.2). G e o sy n c lin e s are m u ch lo n g e r th a n th e ir
of the ro c k s o f fo ld e d m o u n ta in s w ere o f m arin e
origin. T h e s e ro c k s are d e p o s ite d in lo n g , narro w
and sh allo w seas. D a n a n a m e d su ch w a te r b o d ies as
geosynclines. H e d e f in e d , f o r th e f ir s t tim e ,
g eo sy n clin es as lo n g , n a rro w an d sh allo w and sin k ­
ing beds o f seas.
Fig. 13.2 : Sinking beds o f geosynclines due to sedimen­
tation and subsidence.
H all e la b o ra te d th e co n ce p t o f geosynclines
as ad v an c ed by D an a. H e p resen te d am ple evidences
to show re la tio n sh ip b etw een geosynclines and folded
w idths.
(2)
Concept of E . Haug - ‘I f th e id
g eo sy n clin es is d u e to H all an d D an a, the th e o ry o f
th eir d ev elo p m en t is really d u e to H a u g ’. H e d e fin e d
geo sy n clin es as long and d eep w a te r b o d ie s. A c ­
co rd in g to H au g ‘g eo sy n clin es are re la tiv e ly d e e p
w ater areas and they are m u ch lo n g e r th a n th e y are
w id e.’ He drew the p a la e o g e o g ra p h ic a l m a p s o f th e
w orld and d ep icted long and n arro w o c e a n ic tra c ts to
d em o n strate the facts th at th ese w a te r tra c ts w ere
subsequently folded into m o u n tain ra n g e s (fig. 13.3).
He further postulated th at the p o sitio n s o f th e p re se n tday m ountains w ere p rev io u sly o c c u p ie d by o c e a n ic
tracts i.e. g eo sy n clin es. G e o sy n c lin e s e x iste d as
m obile zones o f w ater b etw een rig id m a sse s. H e
identified 5 m ajo r rig id m asses d u rin g M e s o z o ic e ra
e.g. (i) N orth A tlan tic M ass, (ii) S in o -S ib e ria n M a s s,
(iii) A frica-B razil M ass, (iv) A u stra lia -In d ia -M a d a gascar M ass and (v) P acific M ass. H e lo c a te d 4
geosynclines betw een th ese a n c ie n t rig id m a s s e s
North Atlantic Continent
180° Eq«»tor
Pacific Continent
Ojy1 A frkun -Hrazlleun Continent
(Jeosyncline —
vr.A
AstraHnii-liulian M adagascar G
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Fig. 13-3 ; distribution o f rigid masses and geosynclines during Mesozoic era as depicted by £ H aug
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GEOMORPHOLOGY
H im a lay as, this d ep ressio n w as la ter on filled with
sed im en ts to form In d o -G an g etic P lain s), (iii) it may
be alo n g th e m a rg in s o f th e co n tin en ts, (iv) it may be
in front o f a riv er m o u th etc. A cco rd in g to E vans all
the g eo sy n clin es irre sp e c tiv e o f th e ir v arying forms,
sh ap es and lo catio n s are c h a ra c te riz e d by tw in proc­
esses o f sedim entation and su b sid en ce. G eosynclines,
after long p erio d o f sed im en tatio n , are sq u eezed and
folded into m o u n tain ran g es.
e.g. (i) R o ck ies g eo sy n clin e, (ii) U ral g eo sy n clin e,
(iii) T eth y s g e o sy n clin e and (iv) C irc u m -P acific
g eo sy n clin e.
A cco rd in g to H au g th ere is sy stem a tic s e d i­
m e n tatio n in the g eo sy n c lin e s. T h e litto ral m arg in s
o f the g e o sy n c lin e s are a ffected by tran sg ressio n al
and re g re ssio n a l p h ases o f th e seas. T h e m arginal
areas o f the g e o sy n c lin e s h av e sh allo w w ater w h ere­
in la rg e r se d im e n ts are d ep o site d w h ereas fin er
se d im e n ts are d e p o s ite d in ce n tra l p arts o f the
g eo sy n clin es. T h e sed im en ts are sq u eezed and folded
in to m o u n ta in ran g es d u e to c o m p re ssiv e forces
c o m in g fro m the m a rg in s o f the g eo sy n clin es. He
h as fu rth e r re m a rk e d th a t it is n o t alw ay s necessary
th a t all the g e o sy n c lin e s m ay pass th ro u g h the co m ­
p le te c y c le o f the p ro cesses o f sed im en tatio n , su b ­
sid en ce, c o m p re ssio n and fo lding o f sedim ents. Som e
tim es, no m o u n tain s are form ed from the geosynclines
in sp ite o f c o n tin u o u s sed im en tatio n for long d u ra­
tio n o f g e o lo g ical tim e.
(4)
V iew s o f S ch u ch ert- H e attem p ted t
classify g eo sy n clin es on the b asis o f th e ir character­
istics related to th e ir size, lo catio n , evolutionary
history etc. H e has d iv id ed g e o sy n c lin e s into 3
categories, (i) M o n o g eo sy n clin e s are exceptionally
long and narrow but sh allo w w a te r trac ts as con­
ceived by H all and D ana. T h e g e o sy n clin al beds are
subjected to co n tin u o u s su b sid en ce d u e to gradual
sedim entation and resu ltan t load. S u ch g eo sy n clin es
are situated eith er w ithin a c o n tin en t o r along its
borders. T hese are called m ono b ecau se they pass
through only one cycle o f sed im en tatio n an d m o u n ­
tain building. A pplachian g eo sy n clin e is co n sid ered
to be the best exam ple o f m o n o g e o sy n clin es. In
place o f the A p plachians (U S A ) there ex isted a long
and narrow A ppalachian g eo sy n clin e d u rin g preC am brian period. T he g eo sy n clin e w as b o rd ered by
highland m ass know n as A p p lach ia in the east.
A pplachian geosynclines were folded from O rdovician
to P erm ian periods.
T h o u g h th e co n trib u tio n s o f H aug in this
re g a rd are p ra isew o rth y as he d ev elo p ed the concept
o f g e o sy n c lin e s b u t his th eo ry suffers from certain
serio u s d ra w b a c k s an d co n fu sin g ideas about them .
H is p a la e o g e o g ra p h ic a l m ap (fig. 13.3) o f M esozoic
era d ep icted unbelievable larger extent o f rigid m asses
(lan d area s) in co m p ariso n to g eo sy n clin es (oceanic
areas). Q u estio n s arise, as to w h at h ap p en ed to such
e x te n siv e land m asses afte r M eso zo ic era ? W here
d id they d isa p p e a r ? H aug co u ld not ex p lain these
and m any m o re Q uestio n s. H is g eo sy n clin es as very
d eep o cean ic tracts are also not accep tab le because
the m arin e fossils found in the fo ld ed m o untains
belong to the g ro u p o f m arin e o rg an ism s o f shallow
(ii) P o ly g e o sy n c lin e s w ere long and wide
w ater bodies. T hese w ere d efin itely b ro a d e r than the
m o n o g eo sy n clin es. T h ese g eo sy n clin es ex isted for
relativ ely lo n g er period than the m o n o g eo sy n clin es
and these have p assed th ro u g h c o m p lex ev o lu tio n ­
ary h istories. T hese are c o n sid ere d to have experi­
seas.
m ore than one phase o f o ro g e n e sis, conse­
(3)
C o n c ep t o f J .W . E van s- A cco rd in g enced
to
q uently they ‘m ay have been d iv e rsifie d by the
E vans the g eo sy n clin es are so varied th at it becom es
p roduction o f one or m ore p arallel g ean ticlin es aris­
d ifficu lt to p resen t th e ir d efin ite form and location.
ing from th eir floors in the sq u e e z in g p ro c e ss’. They
T he beds o f g eo sy n clin es are su b jected to gradual
o r ig i n a te d in p o s i t i o n s s i m i l a r to th o s e of
subsidence b ecause o f sed im en tatio n . T h e form and
m o n o g eo sy n clin es. R ocky and U ral geosynclines
shape o f g eo sy n clin es ch an g e w ith ch an g in g en v i­
are q u o te d as th e r e p r e s e n ta tiv e e x a m p le s of
p o ly g eo sy n clin es.
ronm ental conditions. A g eo sy n clin e m ay be narrow
o r wide. It m ay be o f d ifferen t shapes. T h ere m ay be
several alternative situ atio n s o f g eo sy n clin es e.g. (i)
it m aybe betw een tw o land m asses (ex am p le, T eth y s
geosyncline betw een L au rasia and G o n d w an alan d ),
(ii) it m ay be in front o f a m o untain o r a plateau (for
exam ple, resu ltan t long tren ch after the origin o f the
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(iii) M e so g e o sy n c lin e s are very long, narrow
and m o b ile o cean b asin s w h ich are bordered by
c o n tin en ts from all sides. T h ey are characterized by
g reat ab y ssal d ep th and long and co m p lex geologic^
histo ries. T h ese g e o sy n c lin e s pass through sever
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-/ "'l~ -I'M
MOUNTAIN BUILDING
223
g e o s y n c lin a l p h a s e s e .g . p h a s e s o f s e d im e n ta tio n ,
s u b s id e n c e a n d fo ld in g . M e s o g e o s y n c lin e s are s im i­
la r to th e g e o s y n c lin e s c o n c e iv e d by H a u g . T e th y s
g e o s y n c lin e is the. ty p ic a l e x a m p le o f su c h ty p e.
M e d i t e r r a n e a n S e a is th e r e m n a n t o f T e th y s
g e o s y n c lin e . T h is g e o s y n c lin e w a s fo ld e d in to A l­
p in e m o u n ta in s o t E u ro p e an d th e H im a la y a s o f
A sia. T h e u n fo ld e d r e m a in in g p o rtio n o f T e th y s
g e o s y n c lin e b e c a m e M e d ite r r a n e a n sea, an e x a m p le
o f median mass o f K o b e r.
ty p ic a l e x a m p le s o f su ch g e o s y n c lin e . T h is con cep t
o f H o lm e s h as been s e v e re ly c ritic is e d b e c a u se the
tra n s fe r an d d is p la c e m e n t o f m a g m a s c a n n o t cause
s u b sid e n c e to form g e o s y n c lin e s .
(ii) F o r m a tio n o f G e o sy n c lin e s d u e to M e ta ­
m o r p h is m - A c c o rd in g to H o lm e s th e ro c k s o f the
lo w e r la y e r o f th e c ru s t, as re fe rre d to a b o v e , are
m e ta m o rp h o s e d d u e to c o m p re s s io n c a u s e d by c o n ­
v e rg in g c o n v e c tiv e c u rre n ts. T h is m a ta m o rp h is m
in c re a se s th e d e n sity o f ro c k s, w ith th e re s u lt the
(5)
Concept of Arthur H o lm e s -B e s id e s dlo
e ­w er la y er o f th e c ru s t is s u b je c te d to s u b sid e n c e
s c rib in g m a in c h a r a c te r is tic s o f g e o s y n c lin s , A.
an d th u s a g e o s y n c lin e is fo rm e d . C a rib b e a n S ea, th e
w estern M e d ite rra n e a n S e a an d B a n d a S e a h a v e
H o lm e s h a s a ls o e la b o r a te d th e c a u s e s o f th e o rig in
b
e e n q u o te d as e x a m p le s o f th is c a t e g o r y o f
o f d if f e r e n t ty p e s o f g e o s y n c lin e s . H e h as a lso d e ­
g e o sy n c lin e s. T h is c o n c e p t h a s b e e n re je c te d o n th e
s c rib e d th e d e ta ile d p r o c e s s e s an d m e c h a n ism s o f
g ro u n d th at c o m p re ss io n c a u s e d by c o n v e r g e n t c o n ­
s e d im e n ta t io n a n d s u b s id e n c e a n d c o n s e q u e n t
v ectiv e c u rre n ts w o u ld n o t c a u s e m e ta m o rp h is m
o ro g e n e s is . A c c o r d in g to h im no d o u b t s e d im e n ta ­
ra th e r it w o u ld ca u se m e ltin g o f ro c k s d u e to r e s u lt­
tio n le a d s to s u b s id e n c e b u t th is p ro c e ss can n o t
an t h ig h te m p eratu re.
a c c o u n t fo r th e g r e a te r th ic k n e s s o f se d im e n ts in
g e o s y n c lin e s r a th e r e a r th m o v e m e n ts can cau se su b ­
(iii) F o rm a tio n o f G e o s y n c lin e s d u e to C o m ­
s id e n c e o f h ig h m a g n itu d e in th e g e o sy n c lin a l beds.
p ressio n -S o m e g e o s y n c lin e s a re f o rm e d d u e to
H e f u rth e r p o in te d o u t th a t th e p ro c e s s o f su b sid en ce
c o m p re ssio n an d re s u lta n t s u b s id e n c e o f o u te r la y e r
o f th e g e o s y n c lin a l b e d s w as n o t a su d d en p ro cess
o f the c ru st c au sed by c o n v e rg e n t c o n v e c tiv e cu r­
r a th e r it w a s a g ra d u a l p ro c e s s . T h e d e p o sitio n o f
rents. P ersian G u lf an d I n d o -G a n g e tic tro u g h a re
s e d im e n ts u p to th e th ic k n e s s o t 12,160 m (4 0 ,0 0 0
c o n sid e re d to be ty p ical e x a m p le s o f th is g ro u p o f
fe e t) in th e A p p la c h ia n g e o s y n c lin e c o u ld be p o s si­
g eo sy n clin es.
b le d u rin g a lo n g p e rio d o f 3 ,0 0 0 .0 0 0 ,0 0 0 y ears from
(i v) F o rm a tio n o f G e o s y n c lin e s d u e to T h in ­
C a m b ria n p e rio d to e a rly P e rm ia n p erio d at the rate
n in g o f S ia lic L a y e r - A c c o rd in g to H o lm e s th e re
o f o n e fo o t o f s e d im e n ta tio n ev e ry 7 ,5 0 0 years.
m ay be tw o p o s sib ilitie s if a c o lu m n o f ris in g c o n ­
H o lm e s h a s id e n tif ie d 4 m a jo r ty p e s o f g eo sy n clin es
v ectiv e c u rre n ts d iv e rg e s a fte r r e a c h in g th e lo w e r
a n d h a s d e s c r ib e d th e m o d e o f th e ir o rig in sep arately
lay er o f th e c ru s t in o p p o s ite d ire c tio n s , (i) T h e s ia lic
as g iv e n b e lo w la y er is stre tc h e d a p art d u e to te n s ile fo rc e s e x e r te d
(i) F o r m a t i o n o f G e o s y n c lin e s d u e to M i­
by d iv e rg in g c o n v ec tiv e c u rre n ts. T h is p ro c e s s c a u s e s
g r a t io n o f M a g m a - A c c o rd in g to H o lm e s the cru st
th in n in g o f sialic la y e r w h ic h re s u lts in th e c r e a tio n
o f th e e a rth is c o m p o s e d of 3 sh ells o f ro ck s. Ju st
o f a g e o sy n c lin e . T h e fo rm e r T e th y s g e o s y n c lin e is
b e lo w th e o u te r th in s e d im e n ta ry la y e r lies (.) o u te r
c o n sid e re d to hav e b een fo rm e d in th is m a n n e r, (ii)
la y e r o f g ra n o d io rite ( th ic k n e s s , 10 to 12 k m ), fo l­
A lte rn a tiv e ly , the c o n tin e n ta l m a s s m a y be s e p a ­
lo w e d by (ii) an in te rm e d ia te la y e r ol a m p h ib o lite
rated d u e to e n o rm o u s te n sile fo rc e g e n e r a te d by
( th ic k n e s s , 2 0 -2 5 k m ), an d ( i i i ) . lo w e r la y e r o
d iv e rg e n t c o n v e c tiv e c u r r e n ts . F o r m e r U ra l
e c lo g ite a n d s o m e p e rid o tite . H e h as fu rth er p o in ted
g e o sy n c lin e is s u p p o se d to h a v e b e e n fo rm e d d u e to
o u t th a t m ig ra tio n o f m a g m a s fro m th e in te rm e d ia te
th is m e c h a n ism .
la y er to n e ig h b o u rin g a re a s c a u s e s co lla p se and
(6 )
V ie w s o f O th e r s - D u s t a r h a s c la s s i
s u b s id e n c e o f u p p e r o r o u te r la y e r an d th u s is fo rm ed
g e o s y n c lin e s in to 3 ty p e s on th e b a s is o f s tru c tu re o f
a g e o s y n c lin e . It m a y be s u m m a ii/.e d that som e
m o u n tain ran g es.
g e o s y n c lin e s a rc fo rm e d d u e to d is p la c e m e n t ol
lig h t m a g m a s and c o n s e q u e n t su b sid e n c e o f cru stal
(i)
ln te r -c o n tin e n ta l g e o s y n c lin e s a re
w ays situ ated betw een tw o c o n tin e n ta l o r la n d m a ss e s
S c h u c h e rt's m e so g e o s y n c lin e is s im ila r to th is ty p e
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s u rfa c e P re s e n t C o ra l S ea, T a s m a n S ea. A rafu ra
S ea W e d d e ll S e a a n d R o ss S ea hav e been q u o te d as
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224
U ral G eo sy n c lin e is q u o te d as th e re p re se n ta tiv e
ex am p le, (ii) C ir c u m -c o n tin e n ta l g e o sy n c lin e s are
g en erally situ a ted alo n g the m a rg in s o f th e c o n ti­
nen ts. S ch u ch ert's m o n o g e o sy n c lin e is th e e x a m ­
ple. (iii) C ir c u m -o c e a n ic g e o sy n c lin e s are g e n e r­
ally fo u n d a lo n g the m a rg in a l areas o f th e o cean s
w h ere c o n tin e n ta l m a rg in s m e e t w ith the o cea n ic
m arg in s. S tille h as n a m e d su ch g e o sy n c lin e as m a r ­
g in a l g e o sy n c lin e w h ile o th e rs h av e called it sp ecia l
ty p e o f g e o s y n c lin e o r u n iq u e g e o sy n c lin e . M o re
ex te n siv e g e o sy n c lin e s h av e b een n am ed by S tille as
o r th o g e o s y n c lin e s. S tille h as fu rth e r cla ssifie d the
g e o sy n c lin e s on th e b asis o f in te rm itte n t v o lcan ic
a c tiv ity d u rin g th e ir in fillin g in to (i) e u g eo sy n clin e s
an d (ii) m io g e o s y n c lin e s . E u g e o sy n c lin e s h av e re la ­
tiv e ly h ig h a m o u n t o f v o lc an ic p ro d u cts (G reek
p re fix eu m e a n s h ig h statu s o f ig n eo u s activ ity )
w h ile m io g e o s y n c lin e s h av e lo w v o lcan ic p ro d u cts
(m io m e a n s low ).
Folded Ranges
Com
----------- V
Landmass
Fig. 13.5 :Stage o f orogenesis : squeezing and folding o f
geosynclinal sediments due to compressive
forces; the whole o f geosyndinal sediments
are folded when the compressive forces com­
ing from the sides o f geosyncline is enormous
and acute.
Stages of Geosynclines
T h e g e o sy n c lin a l h isto ry is d iv id ed into three
sta g e s viz. (i) lith o g e n e sis (th e stag e o f crea tio n o f
g e o sy n c lin e s, se d im e n ta tio n an d su b sid en ce o f the
b ed s o f g e o sy n c lin e s, fig. 13.4), (ii) o r o g e n e sis (the
stag e o f sq u e e z in g an d fo ld in g o f g e o sy n c lin a l
sed im en ts into m o u n tain ran g es, figs. 13.5 and 13.6),
M arginal Ranges
M a rg in al R anges
Fig. 13.6 : Folding o f marginal sediments into marginal
ranges and formation o f median mass when
the compressive forces are moderate.
tain s o f a c c u m u la tio n ) is m o re o r le s s w e ll u n d er­
sto o d b ut the p ro b le m o f th e o rig in o f fo ld e d m o u n ­
ta in s is very m u c h c o m p le x a n d c o m p lic a te d . D iffe r­
en t h y p g th e se s a n d th e o rie s h a v e b e e n p o stu la te d
fro m tim e to tim e by v a rio u s s c ie n tis ts fo r th e e x p la­
n atio n o f th e o rig in o f fo ld e d m o u n ta in s b u t n o n e o f
th e m c o u ld b e c o m e c o m m o n ly a c c e p ta b le to m a jo r­
ity o f the s c ie n tis ts . R e c e n tly , p la te te c to n ic theory
h as, to la rg e r e x te n t, s o lv e d th e p ro b le m o f m o u n ­
tain b u ild in g at g lo b a l s c a le . T h e h y p o th e s e s and
th e o rie s re la te d to m o u n ta in b u ild in g a re d iv id ed
in to tw o g ro u p s , (i) th e o rie s b a s e d o n h o rizo n tal
fo rces an d (ii) th e o rie s b a s e d o n v e rtic a l fo rces.
Fig. 13.4 : The stage o f lithogenesis : creation o f
geosyncline followed by sedimentation and
subsidence.
an d (iii) g lip to g e n e s is (th e sta g e o f g rad u al rise o f
m o u n ta in s , and th e ir d e n u d a tio n an d c o n s e q u e n t
lo w e rin g o f th e ir h e ig h ts). T h e s e sta g e s w o u ld be
e la b o ra te d d u rin g th e d is c u ssio n o f g e o sy n c lin a l
th e o ry o f K o b er.
1 3 .4 THEORIES OF MOUNTAIN BUILDING
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T h e p ro c e s s o f the o rig in o f b lo ck m o u n ta in s ,
d o m e m o u n ta in s , an d v o lc a n ic m o u n ta in s (m o u n -
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MOUNTAIN b u i l d i n g
225
(1) T he first group includes those theories
which postulate the origin o f m o untains due to
horizontal crustal m ov em en t and co n seq u en t c o n ­
traction and folding o f crustal surface into m o u n ­
tains. This group is fu rther sub d iv id ed into tw o
subgroups e.g. (i) the group o f co n tractio n theories
(i.e. horizontal m ov em en ts are caused due to co n ­
traction o f the earth b ecau se o f co o lin g ) and (ii) the
group of drift theories (i.e. the h o rizontal m o v e­
m ents are caused due to continental disp lacem en t
and drift). T herm al contraction theory o f Jeffreys
and geosynclinal T heory o f K ober belong to the
group o f co n tractio n theories w hereas C ontinental
Drift th eo ries o f F.B . T aylor, and A.G. W egener,
Therm al C o n v ectio n C u rren t T heory o f A. H olm es,
Sliding C o n tin en ts T h eo ry o f D aly, R adioactivity
Theory o f Joly and P late T ectonic Theory are in­
cluded in the g ro u p o f d rift theories. (2) The second
group includes those th eo ries w hich are based on
vertical m o v em en ts co m in g from w ithin the earth,
e.g. U ndulation and O scillatio n Theory o f H arm on.
Theories o f F.B . T ay lo r and A .G . W egener have
already been d iscu ssed in ch ap ter 6 o f this book.
earth. He believes in the contraction history o f the
earth. A ccording to J.A. Steers (1932) ‘K ober is
definitely a contractionist, contraction providing the
m otive force for the com pressive stress’. In other
w ords, the force o f contraction generated due to
cooling o f the earth causes horizontal m ovem ents o f
the rigid m asses or forelands w hich squeeze, buckle
and fold the sedim ents into m ountain ranges.
B a se of the Theory
A ccording to K ober there w ere m o b ile zones
o f w ater in the places o f p resen t-d ay m o u n tain s. H e
called m obile zo n es o f w ater as g eo sy n clin es or
orogen (the place o f m ountain b u ild in g ). T h e se
m obile zones o f g eo synclines w ere s u rr o u n d e d by
rigid m asses w h ich w ere te rm ed by K o b e r as
‘k r a to g e n ’. The old rigid m asses in clu d ed C a n a d ia n
Shield, B altic Shield or R ussian M assif, S ib e ria n
Shield, C hinese M assif, P en in su la r In d ia, A fric a n
Shield, B razilian M ass, A u stralian and A n ta rc tic
rig id m asses. A cco rd in g to K o b e r m id -P a c ific
geosyncline separated north and so u th P a c ific fo re ­
lands w hich w ere later on fo u n d ered to fo rm P a c ific
O cean. Eight m o rphotectonic u n its can be id e n tifie d
on the basis o f the descrip tio n o f the su rface fe a tu re s
of the earth during M eso zo ic era as p re s e n te d by
K ober e.g. (i) A frica to g eth er w ith so m e p a rts o f
A tlantic and Indian O ceans, (ii) In d ian A u s tra lia n
land m ass, (iii) E urasia, (iv) N o rth P a c ific c o n tin e n t,
(v) South Pacific co n tin en t, (vi) S o u th A m e ric a a n d
A ntarctica etc.
(1) G E O S Y N C L IN A L O R O G EN T H E O R Y O F K O B ER
O bjectives
F am ous G erm an g eo lo g ist K ober has pre­
sented a d etailed and sy stem atic d escription o f the
surface features o f the earth in his book ‘D e r B au
d e r E r d e \ H is m ain o b jectiv e was to establish
relationship betw een an cien t rigid m asses or tab le­
lands and m ore m o b ile zones or g eo sy n clin es, w hich
he called ‘o r o g e n .’ K o b er not only attem p ted to
explain the origin o f the m o u n tain s on the basis o f his
geosynclinal theory but he also attem p ted to elab o ­
rate the various asp ects o f m o u n tain b u ild in g e.g.
form ation o f m o u n tain s, th eir g eo lo g ical history and
evolution and d ev elo p m en t. H e co n sid ered the old
rigid m asses as the fo u n d atio n sto n es o f the p resent
continents. A cco rd in g to him presen t co n tin en ts
have grow n out o f rigid m asses. He d efin ed the
process o f m ountain b u ild in g or o ro g en esis as that
process w hich links rigid m asses w ith g eo sy n clin es.
In other w ords, m o u n tain s are form ed from the
geosynclines due to the im p acts o f rigid m asses.
K ober has id entified 6 m a jo r p e rio d s o f m o u n ­
tain building. T hree m ountain b u ild in g p e rio d s, a b o u t
w hich very little is k n ow n, are re p o rte d to h a v e
occurred during p re-C am b rian p erio d . P a la e o z o ic
era saw tw o m ajo r m o u n tain b u ild in g p e rio d s - th e
C aledonian o ro g en esis w as c o m p le te d by th e e n d o f
S ilurian period and the V ariscan o ro g e n y w as c u lm i­
nated in P erm o -C arb o n ifero u s p erio d . T h e la s t (6 th )
orogenic activity k now n as A lp in e o ro g e n y w as
com pleted d u rin g T ertiary ep o ch .
K ober has op in ed th at m o u n ta in s a re fo rm e d
out o f geosynclines. A ccording to K o b er g eo sy n clin es,
the places o f m o u n tain fo rm atio n (k n o w n as o ro g e n )
are long and w ide w ater areas c h a ra c te riz e d by
sed im en tatio n and su b sid en ce. A c c o rd in g .to J .A
Steers (1932), ‘K o b er’s v iew s (on g e o s y n c lin e s a n d
o ro g en esis) are, then, a c o m b in a tio n o f th e o ld
g eo sy n clin al h y p o th esis o f H all a n d D a n a , which
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O rogenetic Force
K ober's g eosy n clin al theory is based on the
forces o f co n tractio n p ro d u ced by the co o lin g o f the
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■ •
226.
GEOMORPHOLOGY
m asses or forelands are subjected to continuous
erosion by fluVial processes and eroded materials
are deposited in the geosynclines. This process of
sedim ent deposition is called sedim entation. The
everincreasing w eight o f sedim ents due to gradual
sedim entation exerts enorm ous pressure on the beds
o f g e o s y n c lin e s , w ith th e r e s u lt th e b ed s o f
geosynclines are subjected to gradual subsidence.
This process is know n as the process o f subsidence.
These twin processes o f sedim entation and resultant
subsidence result in the d eposition o f enorm ous
volum e o f sedim ents and attain m en t o f great thick­
ness o f sedim ents in the g eosynclines.
was developed later by H aug, and his own views on
orogenesis.’
M echanism of the Theory
A ccording to K ober the w hole process o f
mountain building passes thorugh three closely linked
stages o f lithogenesis, orogenesis and gliptogenesis.
The firststage is related to the creation of geosynclines
due to the force o f contraction caused by cooling of
the earth. This preparatory stage o f m ountain build­
ing is called lith ogen esis. The geosynclines are long
and w ide m obile zones o f w ater w hich are bordered
by rigid m asses, w hich have been nam ed by Kober
as forelan d s or kratogen . T hese upstanding land
M a rg in a l R a n g es
M a rg in a l R a n g es
Fig. 13.7 : Illustration o f Kober's geosynclinal theory o f mountain building through a block diagram.
m arginal sed im en ts o f the g e o sy n clin e are fo ld ed to
form tw o m arg in al ran d k etten (m arg in al ran g es) and
m id d le po rtio n o f the g eo sy n clin e rem a in s unaf­
fected by fo ld in g activ ity (th u s re m a in s unfolded).
T h is u n f o ld e d m id d le p o i i i o n is c a lle d
z w is c h e n g e b irg e (b e tw ix t-m o u n ta in s) o r m e d ia n
m a s s (figs. 13.6 an d 13.7). A lte rn a tiv e ly , if the
c o m p r e s s iv e f o r c e s a r e a c u te , th e w h o le o f
g eo sy n clin al se d im e n ts are c o m p re sse d , squeezed,
b u ck led and u ltim a te ly fo ld ed (fig . 13.5) and both
the fo rela n d s are clo se te d . T h is p ro c e ss introduces
co m p le x ity in th e m o u n ta in s b ec a u se acute com ­
p ressio n re su lts in the fo rm a tio n o f recu m b en t folds
T he S econ d S tage is related to m ountain
b uilding and is called the sta g e o f o ro g en esis. B oth
the forelands start to m ove to w ard s each other b e­
cause o f h o rizontal m o v em en ts cau sed by the force
o f co n tractio n resu ltin g from the co o lin g o f the
earth. T he co m p re ssiv e forces g en erated by the
m o vem ent o f fo relan d s to g eth er cau se co n tractio n ,
squeezing and u ltim ately fo ld in g o f g eo sy n clin al
sedim ents to form m o u n tain ranges. T h e parallel
ranges form ed on eith e r sid e o f the g eo sy n clin e have
been term ed by K o b er as ra n d k etten (m arg in al
ranges) (figs. 13.6 and 13.7).
A ccording to K ober folding o f entire sedim ents
o f the g eo sy n clin e or part th e re o f d ep en d s upon the
intensity o f co m p ressiv e forces. If the co m p re ssiv e
forces are norm al and o f m o d e ra te in ten sity , on ly the
an d n ap p es.
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K o b er h a s a tte m p te d to e x p la in th e form s and
stru c tu re s o f fo ld e d m o u n ta in s on th e b asis o f h is - .J
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m o u n ta in b u i ld i n g
227
typical m edian m ass. ‘R eally, K ober's typical “orogen”
(g eo sy n clin es) w ell ex p lain s the orig in o f m o u n ­
tain s’. ‘T h e id ea o f m e d ian m ass o f K o b er fully
explains the p ro c e ss o f m o u n tain b u ild in g ’. A cco rd ­
ing to K o b er the A lp in e m o u n tain ch ain s o f E urope
can w ell be e x p la in e d on the basis o f m ed ian m asses.
A ccording to him T eth y s g eo sy n clin e w as b ordered
by E u ro p ean lan d m ass in the n orth and by A frican
rigid m ass in the so u th . T h e sed im en ts o f T ethys
gco sy n clin e w ere c o m p re sse d and folded due to
m o v e m en t o f E u ro p e a n lan d m ass (fo relan d ) and
A frican rig id m a ss (fo rela n d ) to g e th er in the form of
A lp in e m ountain system . A cco rd in g to K ober the
A lpine m ountain chains w ere form ed because o f
co m p ressiv e forces com ing from tw o sides (north
and south). Betic C o rd illera, Pyrenees, Province
ranges, A lps- proper, C arp ath ian s, B alkan m oun­
tains and C aucasus m o untains w ere form ed due to
northw ard m ovem ent o f A frican foreland (fig. 13.8).
On the other hand, A tlas m ountain (north-w est
A frica), A p en n in es, D in a rid e s, H e lle n id e s and
T aurides w ere form ed due to so u th w ard m o v em en t
o f E uropean landm ass (fig. 13.8).
C a i p a tfiiu n s
Fig. 13.8 : The directions o f folding in Alpine mountains o f Europe. Arrows indicate directions (based on Kober).
m ountain ranges take so u th erly tren d in th e fo rm o f
B urm ese hills. A siatic A lp in e ran g es b eg in fro m
A sia m inor and run upto S u n d a Isla n d in th e E a s t
Indies. K ober has also ex p lain ed th e o rie n ta tio n o f
thrust or com pression o f A siatic fo ld e d m o u n ta in s
7'he m e d ian m asses located in the A lpine
m ountain sy stem very w ell ex p lain the m echanism
o f m o u n tain b u ild in g . It is ap p are n t from fig. 13.8
that the d ire c tio n o f fo ld in g in the C arp p ath ian s and
D inaric A lp s (D in a rid e s) is north and south resp ec­
tively, w hich m e a n s th a t H u n g arian m edian m ass is
located b etw een tw o m o u n tain ranges having o p p o ­
site d irec tio n s o f fo ld in g . M ed iterran ean S ea is in
fact an e x am p le o f m ed ian m ass betw een PyrenessProvence R an g es in the north and A tlas m ountains
and their eastern e x te n sio n in the south. C o rsica and
Sardinia are rem n a n ts o f th is m edian m ass. A natolian
plateau b etw een P an tic and T au ru s ran g es is a n o th er
exam ple o f m edian m ass. S im ilarly , further e a st­
ward, Iranian p lateau is a m edian m ass betw een
Zagros and E lb u rz m o u n tain s.
A lpine m o u n tain s fu rth er ex ten d into A sia
w here m ountain ran g es fo llo w latitudinal directio n s
e -g. w est-east o rien tatio n b u t th e latitu d in al pattern
is broken in n o rth -eastern hill region o t In d ia w here
on the basis o f his f o r e la n d th e o r y . A sia tic fo ld ed
m ountains including the H im a la y a w ere fo rm e d due
to com pression and folding o f s e d im e n ts o f T e th y s
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geosyncline caused by the m o v e m e n t o f A n g a ra la n d
and G ondw ana F o relan d s to g e th e r (fig. 13.9). T w o
m arginal ranges (ran d k etten ) w ere fo rm ed on e ith e r
side o f the g eo sy n clin e and u n fo ld ed m id d le p o rtio n
rem ained as m edian m ass. A c c o rd in g to K o b e r A si­
atic A lpine folded m o u n tain s can be g ro u p e d in to
tw o categ o ries on the b asis o f o rie n ta tio n o f fo ld s i.e.
(i) the ranges, w hich w ere fo rm ed by th e n o rth w a rd
co m p ressio n , in clu d e C a u c a su s, P an tic an d T a u ru s
(o f T urkey), K unlun, Y an n an an d A n n a n ra n g e s , a n d
(ii) the ranges, w h ich w ere fo rm e d b y th e s o u th w a rd
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GEOMORPHOLOGY
228
betan p lateau is a fine e x a m p le o f median mass
b etw een K u n lu n -T ie n -S h a n and the Himalayas.
com pression, include Zagros and Elburz o f Iran,
Oman ranges, H im alayas, B urm ese ranges etc. T i­
S F o ld e d M a r g in a l R a n g e s
N M a rg in a l F o ld e d R anges
K u n lu n M t
Til>etan P la te a u
H im a la y a
"G eosyncline
\;°r f
M l
Fig. 13.9 : Illustration o f Kober s median mass through Tibetan plateau between Kunlun and Himalaya.
The median mass m ay be in various form s
e.g. (i) in the form o f plateau (exam ples, T ibetan
plateau betw een K unlun and H im alaya, Iranian p la­
teau betw een Z agros and E lburz, A natolian plateau
betw een P antic and T aurus, B asin R ange betw een
W asatch ranges and S eirra N av ad a in the U S A ) ; (i)
in the form o f plain (exam ple, H ungarian plain
betw een C arpathian s and D inaric A lps), and (iii) in
the form o f seas (exam ples, M ed iterran ean S ea b e­
tw een A frican A tlas m o u n tain s and E u ro p ean A l­
p in e m o u n tain s, C arib b ean S ea b etw een the m o u n ­
tain ran g es o f m id d le A m erica and W est Indies).
Third Stage
the A lps, the H im a lay as, th e R o c k ie s and the A ndes
can n o t be fo rm ed by the fo rce o f c o n tra c tio n gener­
ated by co o lin g o f th e earth .
(2) A cco rd in g to S u e ss o n ly o n e side o f the
g eo sy n clin e m o v es w h e re a s th e o th e r sid e rem ains
stable. T h e m o v in g sid e h as b e e n te rm e d by S uess as
backland w h ereas stab le sid e h a s b e e n c a lle d fore­
land. A cco rd in g to S u ess th e H im a la y a s w ere form ed
due to so u th w ard m o v e m e n t o f A n g a ra la n d . The
G o n d w a n alan d re m a in e d s ta tio n a ry . T h is o b serv a­
tion o f S u ess g a in e d m u c h fa v o u r p re v io u s ly but
after the p o stu la tio n o f plate tectonic theory his
o f m o u n ta in b u ild in g is c h a r a c ­
view s h av e b e c o m e m e a n in g le s s a n d th e c o n c e p t of
t e r iz e d by g r a d u a l rise o f m o u n ta in s a n d th e ir d e n u ­
K ober, that b o th th e fo re la n d s r r o v e to g e th e r, has
d a tio n by flu v ia l a n d o t h e r p r o c e s s e s . C o n t i n u o u s
been
d e n u d a t i o n r e s u l ts in g r a d u a l l o w e r i n g o f the h e ig h t
v a lid a te d
b ecau se
a m p le
e v id e n c e s of
p alaeo m ag n etism an d s e a -flo o r sp re a d in g h av e shown
o f m o u n ta in s .
th at b o th A siatic a n d I n d ia n p la te s are m o v in g to­
w ard s e a c h o th e r.
Evaluation of the theory
T h o u g h K o b e r's g e o sy n c lin a l th eo ry s a tis fa c ­
to rily e x p la in s a few a sp e c ts o f m o u n tain b u ild in g
b u t the th e o ry su ffe rs from c e rta in w e a k n e sse s and
la c u n a e .
(3 ) K o b e r's th e o ry so m e h o w e x p la in s the
w e s t-e a st e x te n d in g m o u n ta in s b u t n o rth -s o u th ex­
te n d in g m o u n ta in s (R o c k ie s a n d A n d e s ) c a n n o t be
e x p la in e d on th e b a s is o f th is th e o ry . In s p ite o f a few
by
in h e re n t lim ita tio n s a n d w e a k n e s s e s K o b e r is given
c re d it fo r a d v a n c in g th e id e a o f th e fo rm a tio n o f
m o u n ta in s fro m g e o s y n c lin a l s e d im e n ts
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(1 )
T h e fo rce o f c o n tra c tio n , as e n v isa g e d
K o b e r, is n o t s u ffic ie n t to c a u se m o u n ta in b u ild in g .
In fa c t, v ery e x te n s iv e an d g ig a n tic m o u n ta in s lik e
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b ecau se
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m o u n t a in b u il d in g
g eosyncline fo u n d b e rth in a lm o s t all th e su b se q u e n t
theories e v en in p la te te c to n ic th eo ry .
trac tio n c au sed b y g rad u al c o o lin g o f th e ea rth d u e to
loss o f h eat th ro u g h rad ia tio n fro m th e v ery b e g in ­
n in g o f its o rig in . H e h as m a th e m a tic a lly c a lc u la te d
th e e x te n t o f c o n tra c tio n o n c o o lin g . A d e c re a se o f
te m p eratu re u p to 400°C in th e 4 0 0 k m th ic k o u te r
shell o f the earth w o u ld ca u se sh o rte n in g o f th e
d ia m ete r o f the ea rth by 2 0 km a n d th e c irc u m fe r­
en ce by 130 km d u e to c o o lin g a n d re s u lta n t c o n tra c ­
tion. H e c a lc u la te d th e m a x im u m s h o rte n in g o f th e
cru st d ue to c o n tra c tio n to b e 2 0 0 k m a n d th e r e d u c ­
tion in su rface a re a u p to 5 x 1 0 16 c m 2.
(2) THERMAL CONTRACTION THEORY OF JEFFREYS
Objectives
J e ffre y s, a s tro n g e x p o n e n t o f co n tra ctio n
theory, p o stu la te d h is ‘thermal contraction theory’
to explain th e o rig in a n d e v o lu tio n o f m a jo r reliefs o f
the earth 's s u rfa c e (c o n tin e n ts , o cea n b asin s, m o u n ­
tains, isla n d a rc s a n d fe s to o n s ) b u t h is m a jo r o b je c ­
tive w as to e x p la in th e o rig in and d istrib u tio n a l
patterns o f m o u n ta in sy ste m s o f th e g lo b e. Jeffrey s
was a c o n tra c tio n is t. H is th e o ry w as b a se d on m a th ­
em atical re a s o n in g . H e p o stu la te d h is co n tractio n
theory b e c a u s e he c o u ld n o t fin d any stro n g reason
in the c o n tin e n ta l d rift th e o ry w h ic h ad v o ca ted h o ri­
zontal m o v e m e n t o f th e c o n tin e n ts d ue to tid al force
of the sun a n d th e m o o n an d th e g rav itatio n al force
as e n v isa g e d by A .G . W e g e n e r.
A cco rd in g to Je ffre y s th e e a rth is c o m p o s e d
o f several co n ce n tric sh ells (la y e rs). T h e c o o lin g a n d
resu ltan t co n tra ctio n tak e p la c e la y e r a fte r la y e r b u t
the co o lin g is effe c tiv e u p to th e d e p th o f o n ly 7 0 0 k m
from the earth 's su rface. “T h e re g io n o f th e e a rth
from the cen tre to so m ew h ere a b o u t 7 0 0 k ilo m e tre s
from the su rface m ay h av e u n d e rg o n e n o a p p r e c i­
able ch an g e o f te m p e ra tu re , an d c o n s e q u e n tly n o
m arked change in v o lu m e” (J.A . S teers, 1932). W ith in
the zone o f 70 0 km from th e e a rth 's s u rfa c e e v e ry
uper lay er has co o led e a rlie r an d m o re th a n th e la y e r
im m ediately b elo w the u p p e r la y er. T h u s , e a c h u p ­
p er layer co n tra cted m o re th a n th e la y e r j u s t b e lo w
it. F u rther, each u p p er la y e r c o n tin u e d to c o o l u n le s s
o bstructed by th e im m e d ia te lo w e r la y e r. T h e o u te r
layer began to cool first d u e to lo ss o f h e a t th ro u g h
radiation. It m ay be p o in te d o u t th a t th e re is a lim it
o f cooling b ey o n d w h ich no fu rth e r c o o lin g is p o s ­
sible. A fter m ax im u m c o o lin g a n d r e s u lta n t c o n tr a c ­
tion o f the uper la y e r lo w e r la y e r ju s t ly in g b e lo w th e
upper lay er b eg in s to co o l a n d c o n tra c t, w ith th e
resu lt alread y co o led an d c o n tra c te d u p p e r la y e r
b eco m es too larg e to fit in w ith th e s till c o o lin g a n d
co n tractin g lo w er lay er. T h e c o re o f th e e a rth is n o t
affected by c o o lin g b e c a u se o f e x c e p tio n a lly h ig h
tem p eratu re p re v a ilin g th e re . T h u s , th e c o re o b ­
structs the c o n tra c tio n o f th e la y e r ly in g a b o v e it.
T h e co o lin g an d c o n tra c tin g la y e r ly in g b e lo w th e
alread y c o o le d an d c o n tra c te d la y e r b e c o m e to o b ig
to fit in w ith th e c o re o f th e e a rth . T h e re is s u c h a
ay er etw ee n th e u p p e r a n d lo w e r la y e r w h e re
co n tra ctio n is su ch th a t th e in te rm e d ia te la y e r c a n fit
m w ith the lo w e r la y er. T h is la y e r is c a lle d level o f
Orogenetic Force
J e ffre y s u sed th e fo rc e o f co n tra ctio n resu lt­
ing p artly fro m c o o lin g o f the earth due to loss o f
heat th ro u g h ra d ia tio n fro m the earth 's su rface and
partly fro m th e d e c re a s e o f the speed o f the earth's
rotation. In fact, th e fo rc e s in v o k ed by Jeffrey s are
divided in to tw o g ro u p s. (1 ) F o rce co m in g through
the c o o lin g o f th e e a rth . T h e earth , afte r being
form ed, sta rte d c o o lin g d u e to loss o f h eat through
rad iatio n . T h is p ro c e s s re su lte d in the gradual d e ­
crease o f th e size o f th e earth d u e to co n tractio n on
cooling. T h e re s u lta n t co n tra c tio n p ro v id ed adequate
force (as b e lie v e d by Je ffre y s ) to form vario u s re lie f
features in c lu d in g m o u n ta in s. (2) F o rce co m in g
through d e c re a s e in th e sp eed o f earth s rotatio n .
A bout 1600 m illio n y e a rs a g o th e earth co m p leted its
one ro ta tio n in a b o u t 0 .8 4 h o u r w h ereas it p resen tly
com pletes o n e ro ta tio n in a b o u t 2 4 h o u rs. T h e d e­
crease in th e ro ta tio n a l sp e e d ca u se d co n tra ctio n in
the e q u ato rial c irc u m fe re n c e o f th e earth . It m ay he
con clu d ed th a t th e fo rc e o f c o n tra c tio n w as d e rived
through th e c o n tra c tio n o f th e earth d ue to (i) co o lin g
o f the e a rth an d (ii) d u e to d e c re a se in th e speed o f
earth's ro tatio n .
no strain.
•Jeffreys' th e o ry is b a se d essen tially on the
history o f th e c o n tra c tio n o f the earth . A cco rd in g to
Jeffreys th e e a rth b eg an to sh rin k b eca u se o f c o n ­
T h e la y e r ly in g o v e r th e le v e l o f n o s tra in is
too b ig to fit w ith th e lo w e r la y e r a n d h e n c e th e u p p e r
ay er has to co lla p se on th e lo w e r la y e r so th a t it c a n
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Mechanism of the Theory
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GEOMORPHOLOGY
f it w ith th e lo w e r la y e r. T h is p ro c e s s (c o lla p s e o f
re g io n s h a v in g h a rd an d le ss e la stic ro c k s are af­
fe c te d by te n sile fo rc e s an d th u s se v e ra l fau lts and
fra c tu re s a rc fo rm e d b e c a u se su c h ro ck s are easily
b ro k e n in to b lo c k s. It is, th u s, a p p a re n t th a t m oun­
tain b u ild in g is lo c a liz e d in c e rta in zo n es o f the
g lo b e .
u p p e r la y e r on lo w e r la y e r ) re s u lts in th e d e c re a s e in
th e ra d iu s o f th e e a rth w h ic h c a u s e s h o riz o n ta l
c o m p re s s iv e s tre s s w h ic h le a d s to b u c k lin g and
fo ld in g o f th e ro c k s o f u p p e r la y e r. T h u s , th e m o u n ­
ta in s a re fo rm e d . T h e lo w e r la y e r b e lo w th e lev el o f
n o s tra in is to o s h o rt to fit w ith th e c o re o f th e e a rth
a n d h e n c e th e lo w e r la y e r h a s to s tre tc h h o riz o n ta lly .
T h is p ro c e s s im p lie s a la te ra l s p re a d in g an d th in n in g
o u t o f th e m a te ria ls o f th e lo w e r la y e r b e lo w th e level
Direction o f the Force- A c c o rd in g to Jeffreys
n o t all th e a re a s b elo w th e e a rth s u rfa c e are equally
affe c te d by the m e c h a n ism o f c o o lin g a n d co n tra c­
tion. T h e c o o lin g p ro c e ss w as m o re a c tiv e b e lo w the
o c e a n ic c ru s t th an th e c o n tin e n ta l c ru s t b e c a u se o f
d is s im ila r s tru c tu re o f th e se tw o z o n e s. T h u s, the
ro ck s b elo w th e o c e a n ic c ru s t e x p e rie n c e d m o re
c o o lin g and c o n tra c tio n than th e ro c k s b e lo w the
c o n tin en ta l cru st. T h u s, the fo rc e o f c o n tra c tio n is
d ire c te d fro m th e o c ea n ic c ru st to w a rd s th e c o n ti­
n ental cru st. T h is m e c h a n ism re su lts in th e fo rm a ­
tion o f m o u n tain s alo n g th e c o n tin e n ta l m a rg in s
p arallel to ,th e o cean s. R o ck ies an d A n d e s are the
e x am p les o f such situ atio n .
o f n o s tra in . T h e s p re a d in g a n d th in n in g o f th e lo w er
la y e r in tro d u c e s a s ta te o f s tre s s w h ic h c a u s e s fra c ­
tu re s a n d fis s u re s re s u ltin g in to b re a k in g o f ro ck s.
T h is m e c h a n is m a llo w s fu rth e r c o lla p s e o f th e a l­
re a d y c o o le d o u te r la y e r a n d th u s a lre a d y fo rm ed
m o u n ta in s a re s u b je c te d to fu rth e r rise in h eig h t.
J e ffre y 's h a s a lso e x p la in e d v a rio u s asp ects o f
m o u n ta in b u ild in g e.g . p e rio d o f m o u n ta in b u ild in g ,
z o n e s o f m o u n ta in b u ild in g , d ire c tio n o f m o u n tain s,
e tc .
Period o f Mountain Building- A cco rd in g to
D ire c tio n o f M o u n ta in s - A cco rd ing to Jeffreys
the co m p re ssiv e fo rce g en era ted by c o n tra c tio n o f
the earth d u e to co o lin g w as d ire c te d fro m o cean ic
areas to w ard s the co n tin e n ta l area s a lm o s t at right
an g le and thus the m o u n tain ra n g e s w ere form ed
p arallel to th e o cean ic areas. T h e la y o u t an d d irec­
tion o f the R o ck ies and A n d es m o u n ta in s are very
w ell ex p la in e d on the b asis o f th is th e o ry because
th ese m o u n tain s run n o rth to so u th a lo n g th e w estern
m a rg in s o f N o rth and S o u th A m e ric a resp ectiv ely
an d are p arallel to the P acific O c e a n b u t the w esteast e x ten t o f th e A lp s an d th e H im a la y a s can n o t be
e x p la in e d on the b asis o f th is th e o ry .
J e ffre y s th e p ro c e ss o f a fo re sa id m e ch an ism o f m o u n ­
ta in b u ild in g is n o t a lw a y s a c tiv e th ro u g h o u t the
g e o lo g ic a l p e rio d s ra th e r is c o n fin e d to c e rta in p e ri­
o d s o n ly . T h e re is c o n tin u o u s a c c u m u la tio n o f
c o m p re s s iv e and te n sile fo rc e s re su ltin g from c o n ­
tra c tio n o f th e e a rth d u e to c o o lin g an d th is p ro cess
c o n tin u e s until th e a c c u m u la te d fo rces ex c e e d the
ro c k stre n g th . W h en , th is sta te (w h en a c c u m u la ted
c o m p re s s iv e an d te n sile fo rc e s e x c e e d th e ro ck
s tre n g th ) is re a c h e d , fo ld in g an d fau ltin g are in tro ­
d u c e d a n d th e p ro c e ss o f m o u n ta in b u ild in g sets in
a n d th is p ro c e s s c o n tin u e s till th e c o m p re ssiv e and
te n s ile fo rc e s a re s tro n g an d activ e. W h e n th ese
fo rc e s b e c o m e w eak , m o u n ta in b u ild in g sto p s and
th e p e rio d o f q u ie s c e n c e sets in. A g ain th e p ro c e ss o f
a c c u m u la tio n o f c o m p re ss iv e and te n sile fo rces starts
and th e n e x t p ro c e s s o f m o u n ta in b u ild in g b egins
when th e s e fo rc e s a g a in b eco m e stro n g en o u g h to
fold th e c ru s ta l ro ck s. T h u s , tw o p e rio d s o f m o u n tain
building a re s e p a ra te d by a lo n g p erio d o f q u ie s ­
Evaluation of the Theory
T h o u g h Je ffre y s h as a tte m p te d to ex p lain the
o rig in an d e v o lu tio n o f su rface fe a tu re s o f the earth
an d h as p re se n te d sev eral e v id e n c e s in su p p o rt o f his
th erm al c o n tra c tio n th e o ry b u t h is th e o ry h as been
sev erely .c ritic is e d and a tta c k e d on the follow ing
g ro u n d s.
( I)
T h e fo rce o f c o n tra c tio n re su ltin g from th
c o o lin g o f the ea rth is not s u ffic ie n t en o u g h to
a c c o u n t for th e o rig in an d e v o lu tio n o f m a j o r surface
cence.
Zones o f M ountain Building- A c c o rd in g to
J e ffre y s m o u n ta in b u ild in g d e p e n d s u p o n the n atu re
a n d stre n g th o f ro c k s. T h e a re a s h a v in g so ft and
e la s tic ro c k s are m o s t a ffe c te d by the p ro c e ss o f
m o u n ta in b u ild in g as th e ro c k s are e a sily fo ld e d by
c o m p re s s iv e fo rc e s c a u s e d by c o n tra c tio n b u t the
re lie fs o f the g lo b e. A H o lm e s h as re m a rk e d that ‘the
c a lc u la te d re d u c tio n o f a re a (by J e ffre y s) is seri­
o u sly in d e fic it o f the a m o u n t to e x p la in m ountain
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b u ild in g .’
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MOUNTAIN b u i l d i n g
231
(2) The concept o f cooling o f the earth in the
system o f concentric shells (layers) is erroneous and
is not acceptable.
processes o f m ountain building. H e attem pted to
explain salient aspects o f folded m ountains e.g.
origin, successive upheavals, distributional patterns
and orientation and extent.
(3) The im pact o f decrease in the speed of
rotation o f the earth on m ountain building is doubt­
ful. J.A. Steers (1932) has aptly rem arked, ‘It may,
in fact, be safely concluded that w hatever effects the
changing speed o f rotation in geological tim es may
have had, it w as totally inadequate to influence
m ountain building in any m arked w ay .’
Orogenetic force
The main force im plied by Daly for the origin
of the m ountains has been the force o f gravity. The
w hole theory o f D aly is based on the nature and rate
o f dow nw ard slide o f the continents fostered by
gravitational force. ‘The key to the D aly's view s is
the idea that there has been dow nhill sliding m ove­
m ent o f continental m asses. In other w ords, the
controlling factor has been g rav ity ’ (J.A . S teers,
1932). Daly h im self proclaim ed th at his theory based
on gravitational force w as co m p eten t to deal w ith all
the problem s o f m ountain bu ild in g satisfacto rily .
(4) It is im p ro p er to believe that contraction
would have been so im m ense about 200 m illion
years ago so that it m ight have form ed such gigantic
m ountains o f T ertiary period as the R ockies, the
A ndes, the A lps, the H im layas etc.
(5) A s per th erm al con tractio n theory o f
Jeffreys the co n tin en ts and oceans should have been
uniform ly d istrib u ted as the earth w as contracted
from all sid es but p resen tly there is uneven d istribu­
tion o f co n tin en ts and oceans.
Axioms of the Theory
Daly has assum ed certain ax io m s (se lf p ro v ed
facts) in support o f his theory. If w e lo o k into the
history it appears that ‘a m ajo r p art o f the th e o ry is
based on self proved facts or a x io m s’ . It m ay b e
pointed out that D aly did n ot elab o rate h is ax io m s.
He adm itted h im se lf th at his th eo ry can w ell e x p la in
the problem s o f o ro g en esis on th e fo rce o f g rav ity
alo n e.’
(6) A cco rd in g to this theory the situation o f
m ountains sh o u ld alw ay s be parallel to the oceans.
The arran g em en t o f the R ockies and A ndes is ju s ti­
fied on the basis o f this th eory but the arrangem ent
o f E uropean A lp in e m o u n tain s and the H im alayas
cannot be ex p lain ed .
A cco rd in g to D aly a so lid c ru st w as fo rm e d
ju s t after the o rigin o f the earth . H e n am ed th is so lid •
crust as primitive crust. In early tim es th e re e x iste d
a series o f an cien t rig id m a sses w h ic h w ere g e n e ra lly
situ ated near th e p o les and a ro u n d th e e q u a to r. T h e s e
rigid m asses h av e b een n am ed by D aly as polar and
equatorial domes. T h u s, th e re w ere th re e b e lts o f
rig id m asses e.g . (i) n o rth p o la r d o m e s, (ii) e q u a to ­
rial d o m es and (iii) so u th p o la r d o m e s. T h e s e th ree
belts o f rig id m a sses w ere s e p a ra te d by d e p re sse d
reg io n s w h ich w ere c a lle d by D a ly as midlatitude
furrows an d primeval Pacific Ocean. T h e s e d e ­
p re sse d re g io n s w ere, in fact, o c e a n ic a re a s (o r say
g e o sy n c lin e s) th e b e d s o f w h ic h w ere fo rm e d o f
primitive crust w h ic h w as fo rm e d w ith th e o rig in o f
th e earth .
(7) If we b eliev e in the co m p eten ce o f the
force o f co n tra ctio n to form m o u n tain s it cannot
produce g reat ran g es o f m o u n tain s as they are found
at p resen t o v e r the g lo b e but it w ould p roduce a
larger n u m b e r o f sm all p u ck ers or m in o r folds.
(8) A c c o rd in g to this th eo ry there sh o u ld not
be any d efin itiv e d istrib u tio n a l p attern o f m o u n tain s
as they m ay be fo rm ed e v ery w h ere b eca u se all parts
o f earth's c ru st e x p e rie n c e d co n tra ctio n b u t c o n trary
to this m o u n tain s are fo u n d in certain p a tte rn s e.g.
along the m a rg in s o f the c o n tin e n ts e x te n d in g eith er
n o rth -so u th w ard o r w est-eastw ard .
(3) SUDING CONTINENT THEORY OF DALY
Objectives
D aly p o stu la te d h is th e o ry o f sliding conti­
nents in his b o o k ‘Our Mobile Earth* in th e y e a r
T h e c ru st, a c c o rd in g to D aly , co m p o se d o f
g ra n ite s, w as h e a v ie r th a n th e ro c k s o f su b stratu m
b elo w th e c ru st. T h e c ru s t w as c o m p o se d o f h eav ier
g ra n ite s w h ile th e s u b stra tu m w as fo rm ed o f lig h ter
g la ssy b asalt. It m ay b e p o in te d o u t th a t this view o f
D aly is iso sta tic a lly to ta lly w ro n g . H e fu rth er as­
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1926 to e x p la in the o rig in an d e v o lu tio n o f d iffe re n t
relief featu res o f th e e a rth 's su rface. T h o u g h D aly
attem pted to th ro w lig h t on m a jo r re lie fs o f th e g lo b e
but his m ain o b je c tiv e w as to e x p la in th e c a u s e s an d
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GEOMORPHOLOGY
su m ed th a t th e w a te r b o d ies o ccu p ied ab o u t h a lf o f
th e g lo b e an d T eth y s g eo sy n clin e (n o rth ern m id ­
la titu d e fu rro w b etw een n o rth p o la r d o m e and e q u a ­
to rial d o m e ) ‘w as a m a rk e d featu re th ro u g h o u t m uch
o f g eo lo g ical tim e .’ L an d m a sses (p o lar eq u ato rial
d o m e s - rig id m a sse s) p ro je c te d ab o v e the w ater
b o d ies an d th e p o la r an d eq u ato rial d o m es w ere
slo p in g to w a rd s m id -la titu d e fu rro w s (w h ich w ere
in fa c t g e o sy n c lin a l trac ts) an d th e P acific O cean.
sed im en ts, the size o f d o m es co n tin u ed to increase.
It m ay be p o in ted o u t th at g rad u al in crease in the
lateral p ressu re resu ltin g from co n tin u o u s dow n­
w ard m o v em en t o f g eo sy n clin al b ed s w as responsi­
ble fo r in crease in the size an d h e ig h t o f the continen­
tal dom es.
T h e sed im en ts o f th e c o n tin e n ta l dom es be­
gan to ex p an d b eca u se o f in c re a se in th e size and
h eig h t o f the d o m e s an d c o n se q u e n tly sedim ents o f
the d o m es b eg an to lo se w e ig h t an d b e c a m e lighter
in w eight. In o rd e r to c o m p e n sa te th e lo ss o f w eight
o f sed im en ts o f the c o n tin e n ta l d o m e s th e re began
u n d erg ro u n d flo w ag e o f d e n se m a te ria ls fro m below
the o cean ic (g e o sy n c lin a l b ed s) b ed s to w ard s the
co n tin en tal d o m es. B e c a u se o f th is p ro c e s s denser
m aterials began to a c c u m u la te in th e co n tin en tal
d o m es from b elo w . B e c a u se o f th e re p e titio n o f the
above p ro cesses th e c o n tin e n ta l d o m e s co n tin u ed to
gro w in size an d h e ig h t, ‘p ro b a b ly n o t as rapidly in
the c en tre as to w a rd s th e ir p e rip h e rie s ’. T he increase
in th e size o f d o m e s cau sed p ressu re on the crust
u n d er the o c ea n ic beds (g e o sy n c lin a l beds). A s the
size o f d o m e s co n tin u e d to e x p a n d , the re su lta n t
p ressu re on o c e a n ic b ed s also c o n tin u e d to in crease.
W h en the to le ra n c e lim it o f th e o c e a n ic cru st to
Mechanism of the Theory
D aly h as b e lie v e d in the co llap se o f the p rim i­
tiv e c ru st b u t has no t e la b o rated the m ech an ism o f
c o lla p se . It m a y be su rm ise d th at the p rim itiv e cru st
w o u ld h av e b een p ro b ab ly b ad c o n d u cto r o f h eat and
so th e su rfa c e te m p e ra tu re w o u ld have fallen soon to
th a t o f the p re se n t tim e b u t th e loss o f h eat from the
in te rio r into the e x te rio r p art co n tin u ed and hence
th e in te rio r p art c o n tra cted aw ay from the o u ter shell
o r crust. C o n seq u en tly , the o u te r cru st w o u ld have
co llap sed on the still co n tra c tin g in terio r d u e to (i)
the w eig h t o f the o cea n ic w ater, (ii) the w eig h t o f
g eo sy n ciin al se d im e n ts an d (iii) g rav itatio n al fo rce
o f the cen tre o f the earth . It m ay be p o in ted o u t th at
the im p act o f g ra v ita tio n a l fo rce w as m o re u n d er the
o cean ic c ru st than the co n tin e n ta l d o m es b ecau se the
fo rm e r w as n eare r to the earth 's cen tre. It ap p ears
(th o u g h not d e sc rib e d by D aly ) that the m id -la titu d i­
nal fu rro w s w ere fo rm ed as g eo sy n c lin e s due to
c o lla p se o f o u te r c ru s t on th e c o n tra c tin g in te rio r o f
th e e a rth an d d u e to th e g rav ita tio n a l force c o m in g
fro m the c e n tre o f the earth .
T h e se d im e n ts d e riv e d th ro u g h the ero sio n o f
p o la r an d e q u a to ria l d o m e s (m o re p recise ly c o n ti­
n en tal d o m e s) w e re d e p o s ite d by th e riv ers in to the
m id - la titu d in a l fu rr o w s a n d th e P a c ific O c e a n
(g eosynclines). C o n tin u o u s sed im en tatio n and w eig h t
o f the o c ea n ic w a te rs e x e rte d e n o rm o u s p re ssu re on
the beds o f o c e a n s (g e o s y n c lin e s ) w ith th e re su lt
their b eds w ere s u b je c te d to c o n tin u o u s su b sid e n c e .
T h u s , d o w n w a r d p r e s s u r e o n th e o c e a n i c
(g eosynclinal) b e d s d u e to c o n tin u o u s s e d im e n ta ­
tion and resu lta n t s u b s id e n c e o f g e o s y n c lin a l b ed s
caused lateral p re s s u re on th e c o n tin e n ta l m a sse s,
w ith the resu lt th e y w ere tra n s fo rm e d in to b ro a d
co n tinental d o m e s k n o w n as p o la r an d e q u a to ria l
Folded Mi - -
om es. A s the o cea n ic b e d s w e re d e p re s s e d d o w n ­
Fig. 1 3 .1 0 : Illustration o f slid in g
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w ard due to g rav ita tio n a l fo rc e o f th e e a rth 's c e n tre ,
an w eig h t o f o c e a n ic w a te r a n d g e o s y n c lin a l
Daly.
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c o n tin e n t
theory ofR-A-
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m o u n t a in b u i l d i n g
233
w ithstand the ev erin creasin g pressure w as crossed,
the oceanic beds began to rupture and break. Thus,
the support o f the co n tin en tal dom es w as rem oved
due to rupture o f the ocean ic beds w hich introduced
strong tensional m o v em en ts d ue to w hich larger
blocks o f continen tal m ass began to slide tow ards
the geosynclines. T he geo sy n clin al sedim ents were
thus squeezed and fo ld ed due to com pressive force
coming from the slid in g continental blocks (fig.
13.10) giving birth to folded m ountains.
building in a sim ple m anner yet it does not present
coherent account o f the problem as the theory does
not go into details and there is a w ide gap betw een
theoretical and practical aspects o f the theory. M ajor
part o f the theory is based on self proved facts
(axiom s). The theory suffers from the follow ing
lim itations.
(1) The sliding continent theory presents er­
roneous concepts about the structure o f the interior
o f the earth. His concept, th at the outer crust is
denser than the substratum , is against the evidences
of seism ology because it is now proven fact that the
density increases with increasing dep th in the in te ­
rior o f the earth.
A ccording to D aly the broken continental
blocks and parts o f o ceanic crust founder in the
substratum b ecau se the density o f outer crust is more
than th e s u b s tra tu m . Or: th e o th e r h an d , the
geosynclinal sed im en ts do not founder in the sub­
stratum rath er th ese flo at on the substratum because
these are less d en se than the substratum . Because of
this fact geo sy n clin al sedim ents are m ore folded by
the com pressive fo rces g enerated by sliding conti­
nental blocks, it is, thus, obvious that greater the
am ount o f slip p in g o f co n tin en tal blocks, the more
geosynclinal sed im en ts are squeezed and m ore and
greater folded m o u n tain s are form ed. D aly has fur­
ther pointed out that the foundered continental blocks
in the su b stratu m are m elted due to high tem perature
and thus rise in the volum e o f m olten continental
blocks cau ses fu rth er rise in the m ountains.
(2) D aly’s theory is based on sev eral guesses
and surm ises. W hy did the earth ’s cru st b eco m e
asym m etrical? W hy the co n tin en tal do m es w ere
sloping towards m id-latitude furrow s (geosynclines)?
How was the Pacific O cean form ed ? D aly d o es not
offer any convincing ex p lanation to th ese an d m an y
more questions.
(3) This theory presents erro n eo u s view s ab o u t
geosynclines because these are g en erally c o n sid e re d
as long, narrow and relatively sh allo w d e p re ssio n s
of water but D aly's geosynclines w ere in fa c t o c e a n s
(e.g. m id-latitude furrow s and P acific O c e a n ). If
these are accepted as g eo sy n clin es they w o u ld h a v e
never been filled w ith sed im en ts and th u s no m o u n ­
tains could have been form ed.
T he sliding c o n tin en t theory o f Daly also well
explains the d istrib u tio n a l patterns o f folded m oun­
tains e.g. n o rth -so u th and w est-east extents. A ccord­
ing to D aly fo ld ed m o u n tain s are form ed because ol
squeezing and fo ld in g ol geosynclinal sedim ents by
com pressive fo rces cau sed by sliding o f the conti­
nental blocks to w ard s the geosynclines. Thus, westeast ex ten d in g m o u n tain s (e.g. A lpine chains and the
H im alayas) w ere fo rm ed due to sliding o f polar and
equatorial dom es tow ards m id-latitude furrow (Tethys
geosyncline) and n o rth -so u th extending m ountains
(e.g. R ockies and A n d es) w ere form ed due to sliding
of continental m asses to w ard s P acific O cean. Sim i­
larly, the island arcs and festoons parallel to the
A siatic co ast w ere form ed due to sliding o f A siatic
(4) Daly has also p resen te d c o n fu s in g id e a s
and erroneous co n cep ts ab o u t the m e c h a n is m o f
m ountain building. In fact, this th e o ry d o e s n o t ca re
for the extension and dep th o t o cea n s an d a m o u n t o f
sedim ents d eposited in them b u t e x p e c ts m o u n ta in s
from every ocean (g eo sy n clin e).
(5) T he theory p ro v id es w ro n g v ie w s a b o u t
the m echanism and p ro cess o f g rav ity . T h e th e o ry
does not throw light on the te rm in a tio n o f p u llin g
effects of gravity and the b e g in n in g o f th e ru p tu re o f
the beds o f the g eo sy n clin es. T h u s, th e re is n o
coherence b etw een d iffe re n t e v en ts re la te d to m o u n ­
tain building as en v isag e d by th e slid in g c o n tin e n t
theory. In fact, the th eo ry p resen ts so m e p ie c e m e a l
m ass tow ards P acific O cean.
Evaluation of the Theory
analysis o f m o u n tain b u ild in g ra th e r th a n a c o m p le te
or perfect p ersp ectiv e.
T h o u g h the ‘sliding co n tinent th e o ry ’ by Daly
is based on w ell know n principle o f gravitational
force and tries to explain the problem o f m ountain
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(6) The theory to certain exten t b e lie v e s in
such distribution o f land and sea (m id latitud e fur­
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GEOMORPHOLOGY
234
p art o f the lo w er layer. C ru st and substratum are
co m p o sed o f sial and sim a resp ectiv ely . G enerally,
sial is ab sen t in the oceanic areas.
row s and P acific O cean an d p o la r and equatorial
continental dom es) as to suit its ow n purpose.
W ooldridge and M org an have aptly rem arked,
‘com plete rejectio n o f the id ea m ay be prem ature,
b ut it is a fair co m m en t to say th at the cause o f
prim ary “b u lg e s” w h ich start the slipping has in no
sense been satisfacto rily in d ic a te d .’
T h e origin o f co n v ec tiv e cu rren ts w ithin the
earth d epends on the p resen ce o f rad io a ctiv e ele­
m ents in the rocks. T h e d isin teg ratio n o f radioactive
elem en ts g en erates h eat w h ich cau ses convective
currents. A cco rd in g to H o lm es th e re is m axim um
co n cen tratio n o f rad io activ e elem en ts in the crust
but tem p eratu re is not so high b eca u se there is
gradual loss o f h eat th ro u g h co n d u ctio n and radia­
tion from the u pper su rface at the rate o f 60 calories
per square cen tim eter per year. ‘T h is is ap p ro x i­
m ately equal to the rad io activ e en erg y p ro d u ced by
a layer 14 km thick o f granite, 16.5 km o f granodiorite,
52 km o f p lateau b asalt or g ab b ro and 6 0 km o f
p erid o tite’ (J.A . Steers, 1932). A cco rd in g to H olm es
the loss o f heat from the earth ’s su rface is co m p en ­
sated by the heat produced by a cru stal shell o f 60 km
thickness. Thus, there is no acc u m u latio n o f addi­
tional heat in the earth's cru st in sp ite o f m axim um
concentration o f radioactive elem en ts. On the other
hand, though there is very low co n cen tratio n o f
radioactive elem ents in the substratum but the gradual
accum ulation o f heat p roduced by the rad io activ e
elem ents causes convective cu rren ts. T h e c o n v e c ­
tive currents depend on tw o facto rs e.g. (i) th ick n ess
o f the crust near the eq u ato r and p o les and (ii)
uneven distrib u tio n o f rad io activ e elem en ts in the
crust. A scending co n v ectiv e cu rren ts o rig in ate un­
der the crust near the eq u ato r b eca u se o f greater
thickness o f crust w hereas d e scen d in g convection
currents are o rig in ated u n d er the p o la r c ru st because
o f its shallow depth. T he risin g c o n v ec tiv e currents
originating from below the continental crust are more
pow erful than the convective currents originating from
below the oceanic crust because o f g reater concentra­
tion o f radioactive elem ents in the continental crust.
(4) THERMAL CONVECTION CURRENT THEORY OF
HOLMES
Objectives
A rth u r H olm es p o stu lated his therm al con­
v ectio n cu rren t theory in the year 1928-29 to explain
the in tricate problem s o f the origin o f m ajor relief
featu res o f the earth's surface. H o lm es’ m ajor objec­
tiv es w ere not co nfin ed to search the m echanism o f
m o u n tain building b ased on sound scientific b ack ­
g round b u t w ere also directed to w ard s finding scien ­
tific explanation for the origin o f the continents and
ocean basins in term s o f co n tin en tal d rift as he was
opposed to the co n cep t o f perm anency o f the co n ti­
nents and ocean basins as en v isag ed by the ad v o ­
cates o f therm al contractio n o f the earth. W ooldridge
and M organ have aptly rem arked, “T he only unify­
ing theory w hich show s a hopeful sings o f reconcil­
ing certain o f the d iv erg en t h ypotheses o f m ountain
b uilding and co n tin en tal d rift is that due to H olm es'
(S.W . W ooldridge and R .S. M organ, 1959).
Orogenetic Force
T he driving force o f m ou n tain building im ­
plied by A rthur H o lm es is p ro v id ed by therm al
convection currents originating deep w ithin the earth.
T he m ain source o f the origin o f co n v ectiv e currents
is e x c e s s iv e h e a t in th e s u b s tr a tu m w h e re in
d isin te g ra tio n o f rad io a ctiv e elem en ts generates
h eat regularly. In fact, the w hole theory depends
exclusively on the m ech an ism o f co n v ectiv e c u r­
rents.
Mechanism of the Theory
Base of the Theory
C o n v ectiv e cu rren ts, th u s, are generated at
som e p laces in th e su b stratu m . B ecau se o f differ­
ence o f tem perature gradient from the e q u a t o r (greater)
to w ard s the p o les (lo w ) risin g c o n v ec tiv e currents
are fo rm ed u n d er the eq u ato rial cru st w hile dow n­
w ard m o v in g (d escen d in g ) co n v ec tiv e currents are
g en erated u n d er the p o lar cru st. T he convective
cu rren ts o rig in atin g u n d er the co n tin en ta l crust are
m ore p o w erfu l th an the c o n v ec tiv e cu rren ts origi­
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A ccording to H olm es the earth co n sists o f 3
zones or layers e.g. (i) u p p er lay er o f g ran o d io rite
(10 to 12 km ), (ii) interm ediate lay er (20 to 25 km )
of am phibolite and (iii) low er layer o f eclo g ite. He
has further grouped these three layers into tw o zones
e.g. (i) crust consisting o f u pper and m iddle or
interm ediate layers and cry stallin e u p p er p art o f
lower layer and (ii) su b stra tu m rep resen tin g m olten
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235
m o u n t a in b u il d in g
nating u n d e r th e o c e a n ic cru st. I t m a y b e p o in te d o u t
that th e c u rre n ts o rig in a tin g u n d e r th e e q u a to ria l
crust m o v e to w a rd s th e p o le s i.e. to w a rd s n o rth an d
south a n d th u s th e c ru s ts a r e c a rrie d aw ay w ith th e
co n v ec tiv e c u rre n ts .
risin g c o n v e c tiv e c u rren ts d iv e rg e in o p p o s ite d ire c ­
tio n s, is stre tc h e d an d th in n e d d u e to te n sio n a l fo rces
an d u ltim a te ly th e c ru s t is ru p tu re d an d b ro k e n in to
tw o b lo c k s w h ich are c a rrie d aw ay by la te ra l d iv e r­
g en t c o n v e c tiv e c u rre n ts a n d th e o p e n in g b etw ee n
tw o b lo ck s b eco m es seas. T h u s, d iv e rg e n t c o n v e c ­
tiv e c u rren ts c a u se c o n tin e n ta l d rift, (ii) W h ere tw o
lateral c o n v e c tiv e c u rre n ts o rig in a tin g u n d e r th e
c o n tin en ta l and o c ea n ic c ru sts c o n v e rg e (fig . 13.11).
T h e re a re tw o s itu a tio n s o f ris in g c o n v e c tiv e
cu rrents w h e n th e y r e a c h th e lo w e r lim it o f th e
cru stal m a s s e s , (i) T h e c ru s ta l m a ss, w h e re tw o
C o n tin e n t
C o n tin e n ta l S h e lf
C o n t in e n t
Sea
tt
tt
A scending Current
Fig. 13.11 : Illustration o f rising and descending convective currents under the crust. A-D ivergent convective currents
cause opening o f the landm ass and creation o f oceans. B-convergent convective currents cause closin g o f la n d
m asses a n d oceans a n d create mountains.
c o m p re ss iv e fo rc e is g e n e r a te d w h ic h c a u s e s s u b sid ­
ence in th e c r u s ta l z o n e s g iv in g b irth to g e o s y n c lin e s
and c lo s in g o f s e a . It is a p p a r e n t th a t d iv e rg e n t
c o n n e c tiv e c u r r e n ts m o v e th e c ru s ta l b lo c k s aw ay in
o p p o site d ir e c tio n s a n d th u s c re a te s e a s a n d o cea n s
w hile c o n v e r g e n t c o n v e c tiv e c u rre n ts b rin g c ru sta l
to H o lm e s th e e q u a to ria l c r u s t w a s s tr e tc h e d a n d
ru p tu re d d u e to d iv e rg e n c e o f r is in g c o n v e c tiv e
c u rre n ts w h ic h c a rrie d th e r u p tu r e d c r u s ta l b lo c k s
to w a rd s th e n o rth a n d s o u th a n d T e th y s S e a w a s
fo rm ed . T h is p h a s e is c a lle d ‘O p en in g o f T eth y s’ .
A g ain tw o sets o f c o n v e rg e n t o r d o w n w a rd m o v in g
(d e s c e n d in g ) c u rre n ts b r o u g h t L a u r a s ia a n d
G o n d w a n a la n d to g e th e r a n d th u s T e th y s w a s c o m ­
p re s s e d an d fo ld e d in to A lp in e m o u n ta in s . T h is
p h a se is c a lle d ‘C losing o f T eth y s’.
blocks to g e th e r a n d th u s fo rm m o u n ta in s .
T h e c o n v e c tiv e c u r r e n ts a re d iv id e d in to tw o
g roups o n th e b a s is o f th e ir lo c a tio n a l a s p e c t e.g . (i)
convective cu rren ts o f risin g colum ns an d (ii)
convective cu rren ts o f fa llin g colum ns. T h e ris in g
c o n v e c tiv e c u r r e n ts a f te r r e a c h in g th e lo w e r lim it o f
the c ru s t d iv e rg e in o p p o s ite d ire c tio n s . T h is o u t­
w ard o r d iv e rg e n t m o v e m e n t in tro d u c e s te n sio n a l
‘T h e c o n v e c tiv e m e c h a n is m is n o t a s te a d y
p ro c e s s b u t a p e rio d ic o n e , w h ic h w a x e s a n d w a n e s
a n d th e n b e g in s a g a in w ith a d if f e r e n t a r r a n g e m e n t
o f c e n tr e ’ (A . H o lm e s 1 9 5 2 ). I t m e a n s th a t th e
c o n v e c tiv e c u rre n ts o r ig in a te a t s e v e r a l c e n tr e s
w h ic h a re n o t p e rm a n e n t. G e o s y n c lin e s a re f o r m e d
d u e to s u b s id e n c e o f c ru s ta l b lo c k s m a in ly c o n t in e n ­
tal s h e lv e s d u e to c o m p r e s s iv e f o r c e g e n e r a te d by
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force d u e to w h ic h th e c r u s t is s tr e tc h e d , th in n e d an d
u ltim a tely b r o k e n a n d th e b ro k e n c ru s ta l b lo c k s are
m o v ed a p p a rt. T h e w id e o p e n a re a b e tw e e n tw o
d riftin g c ru s ta l b lo c k s in o p p o s ite d ir e c tio n s is fille d
w ith w a te r a n d th u s a n o c e a n is fo rm e d . A c c o rd in g
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236
GEOMORPHOLOGY
convergent convectiv e cu rren ts m oving laterally to ­
g ether under continental and o ceanic crusts. In other
w ords, w hen the co n tin en tal and oceanic crusts
m ove together and co n v erg e u nder the continental
shelves, they descen d d o w n w ard and thus cause
im m ense com pression due to w hich the crust is
subjected to subsid en ce to form geosyncline. The
F irs t S tag e
£
Geosyncline
S eco n d S tag e
g eosynclines w hich are alw ays located above the
convective currents o f rising colum ns under conti­
nental and oceanic crusts bring m aterials in the
d escending convective currents o f falling column.
C ontinuous com pression and sedim entation causes
gradual subsidence o f geosynclines. H olm es has
described a cyclic pattern o f therm al convective
currents which includes the origin o f convective
currents, form ation o f g eo sy n clin es, sedim entation
and orogenesis and further rise in the m ountains.
A ccording to H olm es the cyclic pattern o f convec­
tive currents and related m ountain b uilding pass
through three phases or stages (fig. 13. 12).
£.
F ir s t S tag e : The first stage is o f the longest
duration during w hich convective currents are o rig i­
nated in the substratum . The rising con vective c u r­
rents o f two centres converge under the continental
shelves and thus form geosynclines due to c o m p re s­
sion com ing from the co n vergence o f tw o sets o f
lateral currents. G eosynclines are su b jected to co n ­
tinuous sed im en tatio n and s u b sid e n c e . A s the
sedim ents are pressed d o w nw ard into g eosynclines,
these go further dow nw ard and are in tensely heated
and m etam orphosed. M etam o rp h ism o f sedim ents
causes rise in their density w hich fu rth er causes
dow nw ard m ovem ent o f the m etam o rp h o sed m ate­
rials. Thus, the falling colum n o f d o w n w ard m oving
convective currents is the co lu m n o f increasing
den sity . A m p h ib o lite s are m e ta m o rp h o se d into
eclogites. A portion o f heat is sp en t during the
process o f m etam orphism and h en ce the heat does
not accum ulate to g reater ex ten t. T h e first stage,
characterized by high velo city co n v ec tiv e currents,
is in fact the prep arato ry stage o f m o u n tain building
w hich is m arked by the creatio n o f geosynclines,
sedim entation and su b sid en ce o f m aterials partly,
caused by co m p ressio n resu ltin g from convergence
o f co n v ectiv e cu rren ts and partly by increase in the
density o f m aterials due to m etam o rp h ism .
£
t:
"S eco n d S ta g e : T he seco n d stage is marked
by phenom enal in crease in the velo city o f convec­
tive currents but this stage is relativ ely o f short
duration. T he m ain cause fo r the phenom enal in­
crease in the velocity o f co n v ec tiv e currents is the
dow n w ard m o v em en t o f cold m aterials in the falling
colum n and upw ard m o v em en t (rise) o f hot materi­
als in the risin g co lu m n o f co n v ectiv e currents.
In c r e a s e d p r e s s u r e d u e to m e ta m o rp h is m o f
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Fig. 13.12 : Illustration o f successive stages o f thermal
convective currents under the crust and moun­
tain building.
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m o u n t a in b u il d in g
g eo m aterials in th e fa llin g co lu m n o f d esc e n d in g
cu rrents in c re a s e s th e v e lo c ity o f d o w n w a rd m o v in g
co n v ec tiv e c u rre n ts . T h e h ig h v elo c ity c o n v e rg e n t
co n v ec tiv e c u rre n ts b u c k le g e o sy n c lin a l sed im en ts
and thus in itia te th e p ro c e s s o f m o u n ta in b u ild in g
(fig. 13.12). T h is s ta g e , th u s, is c a lle d th e sta g e o f
o ro g en esis.
on th erm al co n v ec tiv e cu rren ts. T h e theory w as
criticised , at the tim e o f its p o stu latio n in 1928-29,
on the fo llo w in g grounds.
(1) C o n v ectiv e cu rren t th eo ry , no doubt, is a
lead in g th eo ry in a new d irectio n b u t w hole o f the
th eory d ep en d s on such facto rs ab o u t w hich very
little is know n. R ising and fallin g co lu m n s are doubtful
p h en o m en a and th e re fo re d o u b tfu l stag e can nev er
be taken fo r the ex p lan atio n o f n atu ral p h en o m en a.
T h ir d S t a g e : T h e t h i r d s t a g e is c h a r a c te r iz e d
by w a n i n g p h a s e o f t h e r m a l c o n v e c t i v e c u r r e n ts d u e
to i n c o m i n g h o t m a t e r i a l s in t h e f a l l in g c o l u m n a n d
u p w a r d m o v e m e n t ( r i s e ) o f c o l d e r m a t e r i a l s in the
risin g c o l u m n . G r a d u a l l y , t h e r is i n g c o l u m n b e ­
(2) T he w h o le m e ch an ism o f c o n v e c tiv e c u r­
rents d ep en d s on the h eat g e n e ra te d b y ra d io a c tiv e
elem en ts in the su b stratu m (now m a n tle ) b u t sev eral
scien tists have raised d o u b t ab o u t th e a v a ila b ility o f
req u ired am o u n t o f h eat g e n e ra te d by ra d io a c tiv e
elem en ts. If h e a t5 thus, is in su ffic ie n t, c o n v e c tiv e
cu rren ts m ay not be g en era ted an d , th e re fo re , th e
w hole m ech an ism and w o rk in g o f th e th e o ry w o u ld
not be p ossible. It m ay be fu rth e r p o in te d o u t th a t th e
rising cu rren ts p ass on th e ir h e a t in to th e c ru s t
through co n d u ctio n . T h is p ro c e ss a lso c a u s e s lo s s o f
heat w hich m ay w eak en th e c u rre n ts.
c o m e s a c o l d c o l u m n i.e. c o l d m a t e r i a l s a re a c c u m u ­
lated a t t h e c e n t r e o f t h e o r i g i n o f r is in g ( u p w a r d
m o v in g ) c o n v e c t i v e c u r r e n t s d u e to w h i c h th e s e
c u r r e n ts c e a s e t o o p e r a t e a n d th e w h o l e m e c h a n i s m
o f c o n v e c t i v e c u r r e n t s c o m e s to a n e n d . T h e t e r m i ­
n a tio n o f t h e m e c h a n i s m o f c o n v e c t i v e c u rr e n ts
y ie ld s s e v e r a l r e s u l t s e .g . (i) T h e m a t e r i a l s o f the
f a llin g c o l u m n s t a r t r i s i n g b e c a u s e o f d e c r e a s e in the
p r e s s u r e a t t h e t o p o f t h e f a l l in g c o l u m n d u e to the
e n d o f d e p o s i t i o n o f m a t e r i a l s . T h i s m e c h a n is m
c a u s e s f u r t h e r r i s e in th e m o u n t a i n s , (ii) T h e d e ­
p r e s s e d a n d s u b s i d e d h e a v i e r m a t e r i a l s in the fallin g
c o l u m n o f d e s c e n d i n g c o n v e c t i v e c u r r e n ts start r is ­
ing d u e to d e c r e a s e in th e w e i g h t a n d p r e s s u r e at the
(3) T he h o rizo n tal flo w o f th e rm a l c o n v e c tiv e
cu rren ts u nder the co n tin e n ta l an d o c e a n ic c ru s ts is
also a d o u b tfu l p h e n o m e n a b e c a u se o f la c k o f r e ­
q uired am o u n t o f h e a t to d riv e th e se c u rre n ts . I f
h o rizontal flow o f c o n v e rg e n t m o v e m e n t o f c o n v e c ­
tive cu rren ts is n o t p o ssib le , th e n th e fa llin g c o lu m n
w ould not exist and hen ce m o u n tain c a n n o t b e fo rm e d .
top o f t h e f a l l i n g c o l u m n , (iii) E c l o g it e , w h i c h w as
d e p r e s s e d d o w n w a r d , g e t s m e l t e d d u e to i m m e n s e
h e a t a n d t h u s it e x p a n d s . T h i s e x p a n s i o n in the
v o l u m e o f m o l t e n e c l o g i t e c a u s e s f u r t h e r ris e in the
(4) T h e m e ta m o rp h ism o f a m p h ib o lite s in to
eclo g ites and re su lta n t d o w n w a rd m o v e m e n t o f r e la ­
tively d en ser e c lo g ite s is a lso a d o u b tfu l p h e n o m ­
e n o n . E v e n w e a c c e p t th e m e t a m o r p h i s m o f
am p h ib o lites in to e c lo g ite s b u t th e r e s u lta n t in c re a s e
in d en sity from 3.0 to 3.4 w o u ld n o t b e e n o u g h to
m o u n t a i n s . T h i s s t a g e is k n o w n as the s t a g e o f
g lip to g e n e s is . I t is, t h u s , a p p a r e n t th a t th e th e r m a l
c o n v e c t i v e c u r r e n t s o f H o l m e s e x p l a i n s all th e th re e
s ta g e s o f m o u n t a i n
b u i l d i n g e .g . l i t h o g e n e s i s ,
o r o g e n e s is a n d g l i p t o g e n e s i s .
G r i g g s t h r o g h h i s e x p e r i m e n t s h a s v a lid a te d
d ep ress and sin k e c lo g ite s in th e fa llin g c o lu m n . I f
d esired sin k in g o f e c lo g ite s is n o t p o s s ib le , th e re
w o u ld not be p ro p e r a c c o m m o d a tio n o f m e te ria ls
b ro u g h t by the h o riz o n ta l c o n v e rg e n t c o n v e c tiv e
c u rre n ts in to th e fa llin g c o lu m n . I f th is is so , th e
w hole fallin g c o lu m n w o u ld be fille d w ith e c lo g ite s
and the n ext stag e o f the m e c h a n is m o f th e c o n v e c ­
tive cu rre n ts w o u ld n ot w o rk . It is, th u s , a rg u e d th a t
the m e c h a n i s m o f c o n v e c t i v e c u r r e n t s a n d c o n s e quent m o u n ta in b u ild in g .
Evaluation of the T h e o ry
C o m m e n t i n g o n H o l m e s 1t h e r m a l c o n v e c u v c
c u r r e n t t h e o r y J .A . S t e e r s ( 1 9 3 2 ) h a s r e m a r k e d
The
t h e o r y is i n t e r e s t i n g , b u t it d e p e n d s u p o n s u c h f a c ­
to rs a b o u t w h i c h lit tl e is k n o w n ' . It m a y b e p o i n te d
the th e o ry d o e s not m a k e p ro p e r p ro v is io n f o r th e
a c c o m m o d a tio n o f a d d itio n a l m a te ria ls .
o u t t h a t t h i s c o m m e n t o f S t e e r s a b o u t 6 8 y e a r s a g o is
n ot v alid to d a y as th e re are am p le c o n v in c in g s e e n •r*a
v v h irh v a l i d a t e th e m e c h a n i s m o t
(5 ) A c c o rd in g to th is th e o ry c o n v e c tiv e c u r ­
ren ts are o rig in a te d a t few c e n tr e s o n ly u n d e r th e
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oolwTCtive c u rre n ts o rig in atin g from w ithin the m antle.
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238
GEOMORPHOLOGY
traction o f the substratum o f the earth resulting into
transgressional and regressional phases o f the seas
(geosynclines). T he expansion and contraction of
the substratum are based on the m echanism o f heat
generated by radioactive elem ents o f the rocks. It
m ay be pointed out that the theory o f A. H olm es and
Joly are based on radioactive elem ents but they
sought their help differen tly e.g. H olm es used radio­
active elem ents to explain the origin o f therm al
convective currents in the sub stratu m w hile Joly
used them to explain the m elting and resolidification
o f the substratum . H e also im plied tid al fo rce and
fric tio n to explain continental drift.
continental and oceanic crusts but question arises,
why are they not originated at all places ? If this so
happens, the horizontal m ovem ent o f these currents
w ould not be possible. The w hole o f the continents
w ould be divided into several blocks as the rising
convection currents originating from num erous cen ­
tres w ould break the crusts and w ould give birth to
volcanic eruptions o f various sorts. This observation
has been now validated on the basis o f plate tectonics
as rising convective currents diverge under the midoceanic ridges and thus the plate is ruptured and two
plates m ove in opposite directions due to divergent
convective currents and fissure flow s o f lavas occur
along the m id-ocean ic ridges representing the rup­
ture zone. It m ay be concluded that the idea o f
therm al convective currents conceived by A. H olm es
about 71 years ago (in 1928-29) proved its w orth in
1960s w hen the scientists w ere looking forw ard to
search such a force w hich can explain the m ovem ent
o f plates. N ow , the process o f m o u n tain ’building can
be very satisfactorily explained on the basis of co n ­
vective currents though not in the way as conceived
by A. H olm es in 1928-29 but on the lines o f plate
tectonics.
B a se of the Theory
The w hole m echanism o f Jo ly 's theory is
based on the presence o f radioactive elem ents o f the
rocks o f the earth. In ord er to explain various aspects
o f the m echanism o f rad io activ e elem en ts Joly has
described first the structure o f the earth. A ccording
to him continents are m ade o f lig h ter sialic m aterials
the density o f w hich is 2.67 w h ile the o cean ic beds
are form ed o f heav ier m aterials o f sim a h aving
average density o f 3.0. T hus, the cru st has been
assum ed to have been co m p o sed o f sial and s u b stra ­
tum o f basalt (sim a). B esides a few ex ce p tio n s, sial
is not found in oceanic beds.
(5) RADIOACTIVITY THEORY OF JOLY
O bjectives
Joly postulated his theory based on rad io ac­
tivity o f certain rad io activ e m inerals in the year 1925
in his book, ‘S urface H istory o f the E a rth ’ to account
fo r the origin and evo lu tio n o f su rface features o f the
earth. H is theory is also know n as ‘th e r m a l cycle
th e o r y ’ or ‘th e o ry o f th e s u r f a c e o f th e e a r t h ’.
T h o u g h the m ain o b jectiv e o f Joly's theory w as to
p resen t a d e ta ile d acc o u n t o f the therm al history o f
the earth and m ath em atical ex p lan atio n o f the stru c­
ture o f the in te rio r o f the earth but he also attem p ted
to e x p la in the p ro b lem s o f m o u n tain b u ild in g and the
co n tin en tal drift. In fact, ‘J o ly ’s view s on the earth 's
surface history are based on such reasonable prem ises,
and are so sim p le in th e ir c o n ce p tio n , that they have
m et w ith a g reat deal o f fa v o u r’ (J.A . S teers, 1932).
W hile co m m e n tin g on Jo ly 's th eo ry o f rad io a ctiv ity
he has rem a rk ed , ‘It sh o u ld not be acc ep ted as
p ro v ed , but reta in ed as an h y p o th e sis w h ich p ro b ­
ab ly co n ta in s a certain ele m e n ts o f tru th ’.
A ccording to Joly the ro ck s o f th e ea rth c o n ­
tain rad io activ e e lem en ts b u t th e ir d istrib u tio n is not
uniform in all zo n es o f the earth . R ad io a c tiv e e le­
m ents are found in ab u n d an c e in sialic zo n e or the
co n tin en tal ro ck s b u t the ro ck s o f sim a fo rm in g the
oceanic crusts are less rad io activ e. C o n tin u o u s break­
dow n o f certain ra d io a ctiv e ele m e n ts lik e uranium ,
thorium etc. g en era tes heat. It m ay b e p o in ted out
that the actual rate o f heat p ro d u c tio n by rad io activ e
elem en ts is e x ce ed in g ly sm all b u t it b e c o m e s suffi­
cien t en o u g h to p ro d u ce a p p re c ia b le re su lt after
long p erio d o f ac c u m u la tio n . T h o u g h th e production
o f heat is c o m p a ra tiv e ly h ig h e r in th e continental
c ru st b eca u se o f m o re ra d io a c tiv e e le m e n ts than the
o cea n ic b ed s b u t th e re is no la rg e -sc a le accu m u la­
tion o f heat in th e c o n tin e n ta l c ru s t d u e to continuous
lo ss o f h eat th ro u g h ra d ia tio n .
M echanism of the Theory
A c c o rd in g to Jo ly th e d isin te g ra tio n o f rad io ­
activ e e le m e n ts o f sialic o r c o n tin e n ta l ro ck s pro­
d u c e s h e a t b u t it d o e s n o t a c c u m u la te in the contir
Orogenetic Force
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T h e d riv in g force o f m o u n ta in b u ild in g as
in v o k e d by Jo ly is p ro v id e d by e x p a n sio n and c o n ­
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m ou ntain b u il d in g
239
nfftts o r sial b e c a u se th e to tal loss o f h eat th ro u g h
radiation fro m th e sia lic c ru s t is m o re th an the total
heat p ro d u c e d by th e ra d io a c tiv e e le m e n ts. H e has
further p o in te d o u t th at te m p e ra tu re in c re ases w ith
increasing d e p th . A fte r d e ta ile d m a th e m a tic a l c a lc u ­
lation Jo ly e s tim a te d th e a m o u n t o f te m p e ra tu re at
the depth o f 3 0 k m to b e 1050°C. H e estim a te d the
m axim um th ic k n e s s o f sial to b e 3 0 km . A cco rd in g
to him th e re is n o tra n s fe r o f h eat fro m sim a to
overlying sia l. H e h a s a lso e stim a te d th e am o u n t o f
te m p eratu re a t th e o u te r lim it o f sim a u n d er the
co n tin en ts to b e 1050°C . T h e c o n d itio n s u n d er the
oceans a te la th e r d ilte r e n t. S in c e th e re is no sial in
the o c e a n ic b e d s , so th e h e a t p ro d u c e d by rad io activ e
elem en ts, th o u g h v e iy sm a ll, is lost to the oceanic
w ater th ro u g h c o n d u c tio n but su ch situ atio n does
not e x ist a t g re a te r d e p th s in th e su b stratu m (sim a)
u n d er the o c e a n s . T e m p e ra tu re in c re ases w ith in­
crea sin g d e p th in th e s u b stra tu m (sim a) u nder the
o cean s b e c a u s e o f a c c u m u la tio n o f h e a t p ro d u ced by
ra d io a c tiv e e le m e n ts . T h is m e c h a n ism cau ses te m ­
p eratu re g r a d ie n t a t g re a t d ep th in sim a (su b stra­
tum ). T h e te m p e ra tu r e b e c o m e s eq u al to the m elting
p o in t o f b a s a lt. T h e re is no tra n s fe r o f h eat from the
lo w er p art o f s im a to the u p p e r part o f sim a so there
is a c c u m u la tio n o f h e a t in the lo w er lay ers o f sim a
b e n e a th th e o c e a n s . T h e m e ltin g p o in t is 1150°C
w h ereas th e te m p e ra tu re at th e top ot su b stratu m
(sim a ) is 1050°C . I f th e te m p e ra tu re o f the su b stra­
tum rise s to 1150°C it w o u ld attain its m eltin g point
but the s u b s tr a tu m w o u ld still rem a in in solid state
u n less re q u ire d a m o u n t o f l a te n t h e a t o f fu s io n is
pro v id ed . J o ly h as c a lc u la te d that the req u ired am ount
o f h eat to liq u e fy th e s u b stra tu m w o u ld be available
in 33 0 0 0 0 0 0 to 5 6 .0 0 0 .0 0 0 y ears. If su ch condi-
level o f ocean ic w ater rises d u e to sin k in g o f sialic or
con tin en tal m asses into liq u id sim a. T his m e ch a­
nism cau ses exten sio n o f ocean ic w ater o v e r the
con tin en tal m argins. T h is p ro cess o f ex p an sio n o f
o ceanic w aters and th eir en cro a ch m en t on co n tin en ­
tal m argins is called transgression of sea and th e
c o n c e r n e d s ta g e is k n o w n as th e p h a se o f
transgressional sea. (5) T ran sg ressio n o f se a re ­
sults in sed im en tatio n on the su b m e rg e d c o n tin en ta l
m argins. T hus, this th eory o f rad io a c tiv ity a cc o u n ts
fo r the origin o f g eo sy n clin es d ue to s u b m e rg e n c e o f
continental m arg in s d u rin g tra n sg re ssio n a l p h a s e o f
“sea. (6) The co n d itio n s u n d er th e o c e a n s are d iffe re n t
b ecause there is ab sen ce o f sial. T h e in c re a s e in th e
radius and the circ u m feren c e o f th e g lo b e d u e to
m elting o f sim a p ro d u ces ten sio n in th e o c e a n ic b e d s
w hich cau ses cracks and fau lts. M o lte n m a te ria ls o r
m olten basalts co m e u p w ard th ro u g h th e se c ra c k s
and faults. T h ese m o lten b asalts are th e n s o lid ifie d
and thus oceanic islan d s are fo rm ed . T h e r a d io a c tiv ­
ity theory, thus, ex p lain s th e isla n d s o f th e P a c ific
and other oceans. (7) C o n tin e n ta l m a ss e s e a s ily flo a t
over m olten sim a, c o n s e q u e n tly th e y are m o re a f ­
fected by tidal force w h ich c a u se s w e s tw a rd m o v e ­
m ent o f the co n tin en ts. It is in th is w ay th a t th e
radio activ ity th eo ry also d e s c rib e s th e p ro c e s s o f
con tin en tal drift. (8) C o n tin e n ta l d rift c h a n g e s th e
position o f the c o n tin e n ts an d th e o c e a n s a s th e
fo rm er occupy th e p o sitio n s o f th e la tte r. T h is p r o c ­
e s s a llo w s th e e s c a p e o f h e a t a n d t h u s th e
tran sg ressio n al p h ase co m es to an e n d .
P e r io d o f R e g r e s s io n a l S e a - T h e p h a s e o f
reg ressio n al sea is c h a ra c te riz e d by th e f o llo w in g
ev en ts - (1) T h e te m p e ra tu re o f th e s u b s tra tu m d e ­
creases b ecau se ot lo ss o f h e a t d u e to c o n tin e n ta l
drift. T h u s, the c o o lin g o f the s u b s tra tu m re s u lts in
the re so lid ific a tio n o f m o lte n s u b s tra tu m . T h e c o o l­
ing o f the su b stra tu m b e g in s fro m its u p p e r la y e r a n d
co n tin u es d o w n w a rd and u ltim a te ly th e w h o le o f t h e
su b stratu m b e c o m e s so lid o n c o o lin g . (2 ) T h e d e n ­
sity o f the s u b stra tu m , w h ic h w a s re la tiv e ly d e ­
crea sed d u rin g its m o lten sta g e , a g a i n i n c r e a s e s t o
reg ain its p re v io u s v alu e. (3 ) T h e r a d i u s a n d t h e
circ u m fe re n c e o f the g lo b e , w h ic h w e r e i n c r e a s e d
due to m e ltin g o f th e s u b stra tu m , a r e a g a i n s h o r t e n e d
to th e ir p re v io u s p o sitio n , w i t h t h e r e s u l t t h e c o n t i ­
nents, w h ich w ere ra ise d re la tiv e t o t h e c e n t r e o f t h e
g lobe, are a g ain b ro u g h t to th e ir p re v io u s p o s i t i o n s
tions b e c o m e p o s s ib le i.e. if th e su b stra tu m reach es
the m o lten c o n d itio n , s e v e ra l ch a n g e s take p lace in
the e a rth ’s s tru c tu re .
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te re stin g e v e n ts ta k e p la c e w h en the su b stratu m
reac h es the m o lte n c o n d itio n d u e to acc u m u ation ot
g reater a m o u n t o f h e a t p ro d u c e d by the b reak ow n
o f ra d io a c tiv e elem en ts. (1 ) T h e e x p a n sio n o f sim a
d u e to m e ltin g c a u se s in c re ase in the rad iu s ot the
g lo b e. (2 ) C o n tin e n ta l m asses or sialic m asses are
ra ise d r e la tiv e to the cen tre o f th e g lo b e. (3) I he
d e n sity o f s im a d e c re a se s d u e to m e ltin g and hence
sia lic m a ss e s b e g in to sin k in m o lten sim a. (4) The
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240
geom orphology
JJ
(5) R elative increase in th e den sity o f the sub stratu m
due to reso lid ificatio n cau ses co n tractio n o f the
oceanic beds w hich resu lts in the w ithdraw al o f
oceanic w aters from the co n tin en tal m argins. T his is
called th e p h a se o f reg r essio n a l sea. B ecause o f the
w ithdraw al o f ocean ic w ater p rev io u sly su b m erg ed
c o n t in e n ta l m a r g in s ( d u r in g th e
phase o f
tran sg ressio n al sea) rise up w ard and the d ep o sited
sed im en ts are ex p o sed above the w ater level. (6) It
m ay be rem e m b ered that the o ceanic beds w ere
su b jected to m ax im u m ex p an sio n d u rin g the p erio d
o f tran sg ressio n al ph ase d u e to m eltin g o f the su b ­
stratu m . S im ilarly , the o cean ic beds are also su b ­
je c te d to m a x im u m co n tractio n d u rin g the p erio d o f
reg ressio n al sea due to reso lid ificatio n o f m olten
su b stratu m . T h u s, co n tra ctin g beds o f tw o oceans
e x e rt lateral co m p ressio n on the sed im en ts d e p o s­
ited on the co n tin en tal m arg in s (g eo sy n clin es), c o n ­
seq u en tly the sed im en ts d e p o sited d u rin g the perio d
o f tran sg ressio n al sea are sq u eezed , b uckled and
fo ld ed and th u s m o u n tain s are form ed.
tain bu ild in g is altern ated by the p eriod o f quies­
cence.
Evaluation of the Theory
T h o u g h the rad io a ctiv ity th eo ry o f Joly based
on scien tific facts and m a th em a tic al calcu latio n was
w idely ap p reciated by sev eral scie n tists b u t sim ulta­
n eously it w as also sev erely criticized . A few critics
o f the theory do n o t g ran t th e o re tic al statu s to the
v iew s o f Joly rath e r they ta k e h is v iew s as m erely
d escrip tiv e acco u n ts o f th e e a rth 's in terio r. In fact,
the theory o f Jo ly is a w ell d ev e lo p e d geom orphic
story o f the earth rath e r than a th eo ry . J.A . S teers
(19 3 2 ) has rem ark ed that ‘th e th e o ry is, a t first sight,
co n v in cin g and it certain ly d o es g iv e a d e q u a te ex ­
plan atio n s o f m any featu res o f the e a rth 's s u rfa c e ’.
T h e fo llo w in g sh o rtco m in g s h av e been p o in te d ou t
by the critics o f the theo ry .
(1) T he theory is b a se d on ra d io a c tiv e e le ­
m ents o f the ro ck s o f th e e a rth at d iffe re n t d ep th s
about w hich very little is k n o w n . T h u s, th e fo rc e o f
ex p an sio n and co n tra ctio n o f th e s u b stra tu m (sim a)
du e to m eltin g an d c o o lin g re sp e c tiv e ly b a s e d on
rad io activ e elem en ts is d o u b tfu l an d p e rh a p s is no t
en o u g h to form m o u n tain s.
Joly has d e sc rib e d tw o p arallel p ro cesses o f
m o u n tain b u ild in g , (i) T h e sed im en ts d ep o sited in
the sh allo w seas o f the co n tin en tal m arg in s are
sq u eeze d and fo ld ed d u e to lateral co m p ressio n
cau sed by tw o co n tra c tin g o cean ic beds, (ii) V ertical
force is produced durin g the process o f resolidification
o f th e su b stratu m . T h is v ertical force raises the
w h o le m o u n ta in sy stem fo rm ed d u rin g the first p ro c ­
ess. It is o b v io u s th a t a cc o rd in g to this theory m o u n ­
ta in s are alw a y s fo rm ed alo n g the m arg in s o f the
c o n tin e n ts fa c in g o cea n s. T h e in ten sity o f lateral
p re s s u re an d c o n s e q u e n t m a g n itu d e o f fo ld in g d e ­
p e n d on the a m o u n t o f c o n tra c tio n o f o cea n ic b eds.
It m ay be arg u e d th a t la rg e o cea n s w o u ld p ro d u ce
m o re p o w e rfu l lateral c o m p re ssio n and h en ce g re a t­
est m o u n ta in w o u ld face la rg e st o cean . T o som e
e x te n t th is s ta te m e n t is tru e as the R o ck ies and the
A n d es m o u n ta in s face th e P ac ific O cean .
(2) Jeffrey s d id n o t a g re e w ith th e 3 0 -k m
th ick n ess o f the co n tin e n ta l m a ss e s as e n v isa g e d by
Joly. A cco rd in g to Je ffre y s th e th ic k n e s s o f the
co n tin en ta l c ru st m ay n o t b e m o re th a n 16 k m . I f the
th ick n ess o f th e c o n tin e n ta l c ru s t is a c c e p te d to b e 16
km then the w h o le m e c h a n is m o f J o ly 's th e o ry w ould
co m e to a g rin d in g h alt as re q u ire d a m o u n t o f h e a t of
1 150°C w o u ld not be p o s sib le at th e d e p th o f 16km.
(3) Jo ly 's c o n c e p t o f c y c lic n a tu re o f m oun­
tain b u ild in g has b een d is p u te d by s o m e c ritic s. The
th eo ry e n v isa g e s u n ifo rm p e rio d s o f q u ie sc e n c e be­
tw een tw o p e rio d s o f m o u n ta in b u ild in g bu t this
c o n c e p t h as also b e e n d is p u te d . J.A . S tee rs has
c o m m e n te d th a t “ In s h o rt, th e v ery e s se n c e o f the
th e o ry , th e a p ro x im a te ly e q u a lly s p a c e d recurrence
o t s im ila r c o n d itio n s , s e e m s to be o n e o f its m ain
d ra w b a c k s .” H e h a s fu rth e r re m a rk e d th at “there
se e m s to b e little d o u b t th a t m o u n ta in building
p e rio d s h a v e b e e n re c u r r e n t to s o m e e x te n t, b u t it is
v ery d o u b tfu l if th ey h a v e b e e n so re g u la r as Joly's
th e o ry w o u ld m a k e th e m ” (J.A . S te e rs , 1932).
Jo ly a lso e x p la in s th e p erio d o f q u ie sc e n c e
b etw ee n tw o p e rio d s o f m o u n ta in b u ild in g . T h e total
p erio d o f tw o so lid p h a se s o f th e s u b stra tu m (so lid
p h ase, m olten p h ase an d re s o lid ific a tio n p h ase o f th e
su b stra tu m ) is c alled o n e r e v o lu tio n w h erein th e
m e ltin g o f su b stra tu m (s im a ) la k es to tal tim e p erio d
o f 3 3 ,0 0 0 ,0 0 0 to 5 6 ,0 0 0 ,0 0 0 y ears. It m ay be, th u s,
in ferre d th at th e p ro c e ss o f m o u n tin g b u ild in g o c ­
c u rs in cy clic m a n n e r w h e re in th e p e rio d o f m o u n https://telegram.me/UPSC_CivilServiceBooks
(4 ) T h is th e o ry e n v is a g e s tw o fa c ts about
m o u n ta in b u ild in g , (i) ‘T h e g r e a te st m o u n ta in s m ust
https://telegram.me/UPSC_CivilServiceBooks
https://telegram.me/UPSC_CivilServiceBooks
https://telegram.me/UPSC_CivilServiceBooks
m o u n t a in b u i l d i n g
241
face the greatest ocean ic beds. ’ (ii) B oth the m argins
0f the continent m u st hav e m o u n tain s o f the sam e
period and both th e m arg in s should be regular. The
first fact is v alid ated to som e ex ten t but the second
fact is not validated .
Base of the Theory
T he rigid lithospheric slabs or rigid and solid
land m asses having a thickness o f about 100 km
com posed o f earth ’s cru st and som e portion o f upper
m antle are technically called ‘p la tes’ . T he term
‘p la te ’ was first used by C anadian g eologist J.T .
(5)
This th e o ry p resen ts erro n eo u s concept
W ilson in 1965. T he w hole m ech an ism o f the ev o ­
about g eosynclines. A s p er this theory g eosynclines
lution, nature and m otion and resu ltan t reactions o f
are alw ays fo rm ed d u e to su b m erg en ce o f co n tin en ­
plates is called ‘plate tecton ics’. Plate tectonic theory,
tal m argins due to tra n sg re ssio n o f seas. It m eans that
a great scientific ach iev em en t o f the d ecad e o f
geosynclines sh o u ld alw ay s be lo cated around the
1960s, is based on tw o m ajo r scien tific ev id en ces
continents. O n th e o th e r h an d , it has been generally
e.g. (i) ev idences o f p alaeo m ag n etism an d (ii) e v i­
accepted that g e o sy n c lin e s are long, narrow and
dences o f sea-floor spreading. S ix m a jo r and 20
shallow w ater b o d ie s w h ich are ch aracterized by
m inor plates have been id en tified so far (e.g. E u ra ­
continuous se d im e n ta tio n and su b sid en ce but Joly's
sian plate, In d ian -A u stralian p late, A m eric an p late,
geosynclines re c e iv e sed im en ts but do not undergo
Pacific plate, A frican plate an d A n ta rc tic p late).
the process o f su b sid en ce. W ith o u t subsidence the
M cK enzie and P ark er d isc u sse d in d etail the
enorm ous th ic k n e ss o f sed im en ts o f the present
m echanism o f plate m o tio n s on the b asis o f E u le r s
A lpine m o u n ta in s c a n n o t be exp lain ed .
geom etrical theorem in 1967. H ary H ess (1 9 6 0 )
elaborated the m ech an ism o f p la te m o v e m e n t on th e
basis o f the ev id en ces o f sea -flo o r sp re a d in g . W .J.
M organ and Le P ichon e la b o ra te d th e v a rio u s a s ­
pects o f p late tecto n ics in 1968.
(6) PLATE TECTONIC THEORY
Objectives
P late te cto n ic th eo ry is a com prehensive theory
w hich o ffers e x p la n a tio n s fo r v arious re lie f features
and te cto n ic e v e n ts viz. m o u n tain building, folding
and fau ltin g , c o n tin e n ta l drift, vu lcan icity , seism ic
events (e a rth q u a k e s ) etc. T h e theory belongs to a
host o f s c ie n tists o f d iffe re n t d iscip lin es. Plate tec­
tonic th e o ry is, in fact, the o u tco m e o f com bined
efforts o f m a n y sc ie n tists o f d iffere n t co untries w ork­
ing to g e th e r and se p a ra te ly . T h e th eo ry cam e into
light in th e 1960s. It e n v isa g e s the fo rm atio n o f
T hree types o f p la te b o u n d a rie s (see fig . 6.7 in
chapter 6 on the o rig in o f c o n tin e n ts a n d o c e a n
b a sin s) have been id e n tified e.g. (i) d e s tru c tiv e p la te
boundaries, (ii) c o n stru ctiv e p la te b o u n d a rie s a n d
(iii) co n serv ativ e plate b o u n d a rie s.
(1) C on stru ctive p la te b o u n d a r ie s also c a lle d
as ‘d iv er g en t p la te b o u n d a r y ’ o r ‘a c c r e tin g p la te
b o u n d a ry ’ rep resen t zo n es o f d iv e rg e n c e a lo n g the
m id -o cean ic rid g es and are c h a ra c te riz e d b y c o n ­
m o u n tain s d u e to c o llisio n o f plate bo u n d aries.
tin u o u s ad d itio n (a c c re tio n ) o f m a te ria ls as there is
co n stan t u p w ellin g o f m o lte n m a te ria ls (b a s a ltic
Orogenetic force
T h e o ro g e n e tic fo rce to form m o u n tain s is
lavas) from b elo w the m id -o c e a n ic rid g e s . T h ese
p ro v id ed by th e c o m p re s s iv e fo rces cau sed by the
b asaltic lav as are c o o le d an d s o lid ifie d a n d a re a d d e d
collision o f tw o c o n v e rg e n t p la tes alo n g the d e stru c ­
to the trailin g m a rg in s o t th e d iv e rg e n t p la te s an d
tive p la te b o u n d a rie s. T h e rm a l co n v e c tiv e cu rren ts
thus new o ce a n ic c ru s t is c o n tin u o u s ly fo rm e d . In
o rig in atin g in th e m a n tle h av e been acc ep ted as the
co m p eten t fo rce fo r th e m o v e m e n t o f p lates. The
plates m o v e in d iffe re n t d ire c tio n s rela tiv e to each
fact, o cean ic p la te s sp lit a p p a rt a lo n g the m id -o c e ­
anic rid g es an