https://telegram.me/UPSC_CivilServiceBooks ■ fl - ' ' ■ ' I::: mt 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 9 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. * : 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) https://telegram.me/UPSC_CivilServiceBooks CHAPTER 17 https://telegram.me/UPSC_CivilServiceBooks 308-333 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 : 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. ( iii) https://telegram.me/UPSC_CivilServiceBooks CHAPTER 23 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks : 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. : 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. ■ /' * CHAPTER 29 : 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 ) https://telegram.me/UPSC_CivilServiceBooks # CHAPTER 31 ANTHROPOGENIC GEOMORPHOLOGY https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 1 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks n ature o f g e o m o r p h o l o g y https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks wards applied aspect o f the subject. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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.) https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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) https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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* https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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- https://telegram.me/UPSC_CivilServiceBooks 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’. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks fTf/Mtmnunsxn 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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— https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 15 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 2 https://telegram.me/UPSC_CivilServiceBooks 24-56 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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) https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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- https://telegram.me/UPSC_CivilServiceBooks The processes (mainly endogenetic) which affect the earth's crust act in a cyclic manner. Hutton https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 2.2 : Lava-capped mesa and butte. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 29 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 31 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 https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks Fig. 2.10 Development o f trellis drainage and cuesta on uniclinal strata of coastal plain, after Von Engeln. 1948. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 2.11 : Development ofstripped and structural plains on horizontal structure, after W.M. Davis and C.A. Cotton (in Chorley et. al, 1985). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks h y d r a u l i c https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (6) The mechanism o f periglacial processes is quite different to other processes i.e. congelifrac- https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks > 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— https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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’ https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 2.13 : Timcscales-(A) cyclic tune, (/ij graded tii and (C) steady time. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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) https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks It may be mentioned that spatial scale has much significance in controlling the rate and mecha­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 49 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), https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 50 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 51 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 https://telegram.me/UPSC_CivilServiceBooks It may be mentioned that landscape mosaic of any physiographic region or morphogenetic region https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks 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.). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 3 https://telegram.me/UPSC_CivilServiceBooks 57-88 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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- https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 ­ https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks The base of historical geomorphic theories has been the historical succession of individual or https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks Fig. 1 1 : Graphical presentation of geographical cycle presented by W.M Davis. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (vii) Long profiles of the rivers are character­ ized by rapids and water falls which gradu­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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. ‘ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks The cycle o f pediplanation is performed by twin processes viz. scarp retreat and pedimentation. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks If wc com pare the geom orphic models o f W.M. Davis (geographical cycle) and L.C. King J i https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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’. https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks In the case o f stable base level o f erosion he maintains that if any landmass is uplifted and then https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks 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) https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks °f friction. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 80 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- https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks A https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks precipitous escarpment from 200m to 260 or 280m and is highly c ren u laied and indented having https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 4 https://telegram.me/UPSC_CivilServiceBooks 89-104 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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), https://telegram.me/UPSC_CivilServiceBooks (4) Besides, ‘not only do different levels of magnitude and frequency of processes have differ­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks Fig. 4.1 A https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks Fig. 4.2: https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 4.3 :A : Nature ofmassmovement, B : pluvial erosion and C : wind action in different climatic conditions (after L C . Peltier, 1950). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks The changes in thermal conditions above freez­ ing point influence and control mechanisms of weath­ ering and erosion by different geomorphological https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks It may be m entioned that interception o f rain­ water by vegetation varies from season to season and https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks Fig. 4.4 : Morphogenetic regions according to L.C. Peltier, J950. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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; https://telegram.me/UPSC_CivilServiceBooks Glacial https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 1. Glacial https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 4. Trophical Wet-Dry 101 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (a) Savanna m orphogenetic region is char­ acterized by dry and humid seasons which effectively influence morphogenetic processes. Ground surface https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 5 https://telegram.me/UPSC_CivilServiceBooks 105-113 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 1. Artificial Sources https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks .tf-.v. https://telegram.me/UPSC_CivilServiceBooks s iM https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 ­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks ^ ^ outer and lower The average density o crust is 2.8 and 3.0 respectively. It may uc pointed ^ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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’ https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 6 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 115 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 . https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 11 6 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks o b j e c t i v e https://telegram.me/UPSC_CivilServiceBooks I j https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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 | https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks and ocean basins. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks and chronological sequence of the displacement of the continents. He did not describe the situations of https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 6.8:Diagramatic presentation o f different types o f plate margins. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Though the sequence o f events o f continental displacem ent based on the evidences o f palaeomag- https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks Fig. 6.13 Atlantic https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks : 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 7 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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.’ https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks • 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 . https://telegram.me/UPSC_CivilServiceBooks CHAPTER 8 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Others, less than https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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- https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 8.1 : D iagram atic presentation o f a granitic batholith. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks regions o f folded m ountains (fig. 8.3). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks g r a i n e d https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 8.8: https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Organically Formed Sedimentary Rocks 151 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 . https://telegram.me/UPSC_CivilServiceBooks ‘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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 154 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) https://telegram.me/UPSC_CivilServiceBooks (i) As stated above, the rocks surrounding the igneous intrusions are altered due to intense heat o f https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 8.11 : Example ofdynamic regional metamorphism. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 9 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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- https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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 ­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks GEOMORPHOLOGY https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 9.2 : Different components o f a fold. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks Fig. 9.4 : D ip a n d strike. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 9.7 : (A) Closed fo ld s and (B) open folds. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks It may be m entioned that the rift valleys are not only con fin ed to the continental crustal surfaces https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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" https://telegram.me/UPSC_CivilServiceBooks Fig. 9.14 . Formation o f a rift valley according to E C Bullard. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks EARTH’S MOVEMENT K9 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks : 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 10 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 172 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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- https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 ­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 1 970). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (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- https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks w hich is erosional w h ile the lo w er part is faulted (fig . 10.6(4). https://telegram.me/UPSC_CivilServiceBooks STRUCTURAL g e o m o r p h o l o g y https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks GEOMORPHOLOGY H https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks O n e o f th e resultant features o f p rolon ged ero sio n o f https://telegram.me/UPSC_CivilServiceBooks s I https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 10.11: D evelopment o f flu via l cycle o f erosion on fo ld e d structure (after Von Engeln). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. . https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks f https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 ). https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks : 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 . https://telegram.me/UPSC_CivilServiceBooks CHAPTER 11 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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). https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks GEOMORPHOLOGY https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 ). https://telegram.me/UPSC_CivilServiceBooks PLATE TECTONICS https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks GEOMORPHOLOGY 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 https://telegram.me/UPSC_CivilServiceBooks 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 . https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 191 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 https://telegram.me/UPSC_CivilServiceBooks (after A. Cox, 1969). https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks ‘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). https://telegram.me/UPSC_CivilServiceBooks plate te c to n ic s https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 * https://telegram.me/UPSC_CivilServiceBooks 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 ­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks : 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 . https://telegram.me/UPSC_CivilServiceBooks CHAPTER 12 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (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 ' https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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" https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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) https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 12.3: World distribution o f volcanoes. https://telegram.me/UPSC_CivilServiceBooks p la te a u https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks Fig. 12.4 : Volcanic-ridge-chain on Pacific plate. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks % https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 — https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks nroduced during volcanic activities. fig. 12.6: Different types o ,f .landforms proauc https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Elevated Forms https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks % 12,7: Different types o f volcanic cones. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 , https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks p h u r fu m a r o le s. https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks : 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. https://telegram.me/UPSC_CivilServiceBooks CHAPTER 13 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks m ountain ch ain s and m ountain system s. The m oun­ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks 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, https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 ^ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Fig. 13-3 ; distribution o f rigid masses and geosynclines during Mesozoic era as depicted by £ H aug https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 222 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 https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks -/ "'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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 - https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks ■ • 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. https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks (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 https://telegram.me/UPSC_CivilServiceBooks b ecau se https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks Mechanism of the Theory https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 230 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 https://telegram.me/UPSC_CivilServiceBooks b u ild in g .’ https://telegram.me/UPSC_CivilServiceBooks https://telegram.me/UPSC_CivilServiceBooks 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 https://telegram.me/UPSC_CivilServiceBooks 1926 to e x p la in the o rig in an d e v o lu tio n o f d iffe re