Our Planet
I. Internal Structure of the Earth:
From the results of seismology, we can obtain the picture of the broad structure of the
earth as a whole given in the figure below. The three main units are the crust, the mantle
and the core.
1. The Crust:
The outermost and
thinnest layer of the
lithosphere is called the
crust. The thickness of the
crust varies greatly between
the ocean basin (as little as
13 km in places) and
continents (possibly 30-50
km thick under certain
mountains). The specific
gravity of the crust ranges from 2.5 to 3.4. The following figure and table indicate the
variation of density with depth in the earth.
Depth
Discontinuity
Layer
Composition
State
Density
(oC)
(km)
Upper Crust
Granitic
Solid
2.7
(SIAL)
17-25
400
Conrad
Lower Crust
Basaltic
Solid
3.0
(SIMA)
32-38
Temperature
600-
Mohorovicic
1000
Mantle
Olivine
Solid
3.3
5.3-8.0
2900
Gutenberg
1500Outer Core
NIFE
Liquid
9.0-10.
5000
11.5
11.7
5120
Lehmann
Inner Core
NIFE
Solid
10-18
1900-6000
Crustal rocks vary not only in thickness and density, but also in composition.
Those beneath the ocean basins are heavier than those which underlie the continents.
They have been called SIMA because they are rich in iron, silicon, and magnesium.
Therefore, they have a greater specific gravity/density of about 2.9.
The material comprising the continental crust appears to occur in two distinct
layers. The upper layer is essentially granitic in nature. Since these rocks contain a
high percentage of silicon and aluminum, they are often referred to as SIAL. This
layer is 16-24 km thick and has a density of about 2.65 to 2.7.
The lower layer, also about 16-24 km thick, appears to be composed of simatic
rocks similar to those underlying the ocean basins. They are often referred to as SIMA.
The boundary of SIAL and SIMA is called "Conrad Discontinuity". Figure below
shows the relationship between surface features and crustal structure.
The base of the crust is marked by a rather clearly defined break called the
"Mohorovicic Discontinuity" or the MOHO. This sharp boundary lies 32-48 beneath
the surface. Here the velocities of P and S Waves are somewhat accelerated (from
about 5 to 8 km per second), implying change in density in the rocks below the
HOMO.
2. The Mantle:
Beneath the "Mohorovicic Discontinuity" there is an 2880 km thick
intermediate zone called the Mantle. The velocities of P and S waves increase
gradually upon entering this zone. Their behaviour suggests that the mantle increase
in density with depth. The specific gravity of the rocks in this zone ranges from 3.3
(in the upper part of the mantle) to as much as 8.0 at the bottom.
The Upper Mantle is a "Low-Velocity Zone (LVZ)". When P and S waves
pass through this layer, the velocity is reduced markedly. Magma is found in this layer.
"LVZ" is also called "Asthenosphere".
3. The Core:
The earth's core is the region extending downwards from the core-mantle
boundary at 2900 km. The boundary is called the "Gutenberg Discontinuity".
Generally, core is only 15% of the total volume of the earth, but it is 32% of the total
mass of the earth. The core with a diameter of about 3475 km, is very hot (about
3700oC), dense and under tremendous pressure. It has been divided into two parts: an
exterior "Outer Core" (Liquid); and an "Inner Core" which is believed to be solid.
The outer core is believed to be fluid because the material in this zone will not
transmit S waves, and P waves travel at reduced velocity. The outer core is believed
to be about 2220 km thick; the materials composing this zone have specific gravities
of 12.0 or more.
The inner core, with a diameter of approximately 1255 km, is believed to be
solid. This is suggested by the fact that there is an abrupt increase in the speed of P
waves deep within the core. It has also been suggested that the inner core may be
composed largely of nickel and iron (NIFE). These rocks are presumed to be quite
heavy; some may have a density of more than 17.0.
II. Theory of Continental Drift:
1. Development:
In the 17th and 18th centuries, when the outlines of the continents had become
sufficiently well known, many scientists, such as Francis Bacon (1620) commented
on the similarity of the shaped of the coastlines on either side of the Atlantic. By the
end of the 19th century, the geology of the southern continents was sufficiently well
known for an Austrian geologist, Eduard Suess, to suggest that Africa, South America,
Australia and India were once part of a super-continent, Gondwanaland (after a region
in India called Gondwana).
In 1910, a German meteorologist, Alfred Wegener, proposed that at the
beginning of the Mesozic era (200 million years ago) Gondwanaland was united with
Eurasia and North America to form a single super-continent, which he called Pangaea
which was surrounded by a primeval sima-floor ocean (Panthalassa). Wegener
proposed that this vast continent began to break up at the beginning of the Mesozoic
era by a process of lateral crustal movement, forming a northern continent (Laurasia)
and a southern continent (Gondwanaland) separated by a long narrow ocean known as
Tethys.
2. Evidences
To support his theory, Wegener marshalled considerable evidences:
i) The fitness of the edges of continents: The real edge of a continent lies somewhere
on the continental slope,
where the sea bottom falls
comparatively rapidly from
somewhere near sea level
down to the deep ocean
floor, about 4000 metres
below the surface. The
similarity in shape of a
contour (500-fathoms)
drawn on the continental
slope of eastern South
America and the same
contour drawn on the west
African slope is quite
striking.
ii) Glaciation: All four southern continents reveal signs of a period of large-scale
glaciation in Carboniferous-Permian times (250 million years ago) when they
were closer together and nearer the South Pole.
iii) Evidences from Geology: Similar fauna and flora fossils and geological structures
were found in the corresponding continents. For example, coalfields in
Appalachians and Europe are of the same carboniferous period. In northern
America, Appalachian Mts extends to Newfoundland, stop at Atlantic Ocean, but
the extension can be found at the East Atlantic edge in Ireland.
iv) The matching of rock layers: Similarity of age, time and sequence of rock layers,
eg. the Cretaceous and Jurassic sedimentary basins in West Africa and Brazil. In
Lower Cretaceous time (about 135 to 100 m.y.a.) several sedimentary basins
formed along the continental margin of Brazil and West Africa.
v) Evidence from Paleontology: A plant called Glossopteris was widely grown in
South America, South Africa, Australia, India and Antarctica at
Carboniferous-Permian. The seeds of Glossopteris was too big to transport by
wind from continent to continent. On the other hand, a ancient animal called
Lystrosaurus was found in South Africa, South America and Antarctica at
Permian. Lystrosaurus could not swim. For these reasons, continents may be
joined together at the period of Carboniferous and Permian.
vi) Evidence from Climatlogy: Coalfields are found in Antarctica. It is believed that
the ancient climate of Antarctica was very warm and humid for vegetation growth.
However, A huge ice-cap has been covering Antarctica now. So, the location of
Antarctica is not fixed but moved.
vii) Palaeomagnetism: Igneous rocks when cooled retain a certain magnetisation, the
result of the presence of grains of substances containing iron which were aligned
and permanently magnetized in accordance with the direction of the earth's
magnetic field at that time. This 'fossil magnetism' thus provides a record by
which the position of the magnetic poles at various times may be located. The
figure below shows the polar wandering curves for South America and Africa. It
shows that when plotted on the best-fit reconstruction, the Carboniferous, Permian
and Triassic poles lie within overlapping circles of error. None of them have
overlapping circles of error in their present-day positions. The palaeo-latitudes of
Permian and Carboniferous time place the southern parts of both continents within
the Antarctic circle of that time, that is within 23.5 degrees of the pole. The
Triassic poles from Africa and South America place both continents forty degrees
closer to the equator. There are as yet no reliable data for the Jurassic pole of
South America, but the Cretaceous poles are first pole on the best-fit
reconstruction that do not have overlapping circles of error. Such a separation
suggests the continents moved apart in the Triassic to Cretaceous time interval.
IV. The Hypothesis of Sea-Floor Spreading:
While palaeomagnetic directions have been instrumental in 'proving'
continental drift, they also form the critical evidence in favour of spreading. Just over
twenty years ago, the ocean floors were still regarded as the oldest and most dormant
parts of the Earth. By 1960, however, improved techniques had enabled scientists to
examine, for the first time, the detailed topography of large areas of ocean floors. For
the ocean floors were revealed not entirely as dull, flat regions but as possessing
"mountain ranges" comparable with the most impressive mountainous regions on the
continents. It later emerged that these ranges, or "ridges" were not isolated features
but formed part of a world-wide and almost continuous network, and that, furthermore,
they were seismically active. From this quickly developed the idea that the ocean
floors, far from being dormant and old, were, in parts, the most active and youngest
regions of the Earth.
The hypothesis was first formulated by Harry Hess in 1960. Hess suggested
that, despite their great age and apparent permanency, continents have been, and are
being, passively drifted apart and together on the backs of mantle-wide convection
cells. In contrast the ocean floors are young and ephemeral features of the Earth's
surface, constantly being
regenerated at ridge crests
and destroyed in the trench
systems.
From consideration
of the earlier ideas
regarding the age of the
initiation of drift in the
Atlantic area (based on the
geologic record of the surrounding continental margins) Hess suggested that the
sea-floor might be spreading at a rate of approximately 1 cm per year per ridge flank.
The occurrence of earthquakes along the crest of the mid-ocean ridge system,
the dearth of sediments at ridge crests and the active volcanic islands associated with
the crest of the Mid-Atlantic Ridge are all readily explained by Hess's model.
Moreover, the ocean basins as a whole contain a remarkably thin veneer of sediments
and small number of sea mounts if recent rates of accumulation and formation are
extrapolated over the whole of geologic time. In 1960 Hess was also able to state that
no material greater than about 100 million years in age had ever been recovered from
the deep ocean floor or truly oceanic islands.
The flow of heat through the ocean floor from the Earth's interior was first
determined in the 1950's. Results obtained revealed that the heat flow through the
ocean floor is in general comparable to that determined previously for the continents.
Over the mid-ocean ridges, however, it is, in places, several times the emplacement of
hot mantle-derived material in the vicinity of ridge crests.
Moreover, for most latitudes and orientations of ridge crests the magnetic
anomalies (disturbances) are roughly symmetrical about the ridge axis.
V. The Theory of Convection Currents:
Subcrustal convection within the earth was suggested by William Hopkins in
1839. Osmond Fisher in his "Physics of the Earth's Crust" of 1881 pointed out that the
frictional drag on the underside of the crust would be expected to promote mountain
building along the margins of the continents, where two systems of currents
approached one another and turned downwards. He also recognized that "The
existence of convection currents beneath the cooled crust of the earth at once
furnishes a means of obtaining those local increments of temperature which in some
form or another appear to be needful in order to explain volcanic phenomena.
The theory was later developed by A. Holmes which provides a support the
theory of continental drift, and helps in the development of the theory of plate
tectonics in the 1970's.
The suggestion has been made that radioactive heat is responsible for the
energy required, which sets up convection currents moving (at an estimated rate of 1
cm per year) in a series of "cell" throughout the upper part of the mantle and the
simatic layer.
These currents rise under the oceans, then move out horizontally in either
direction, taking with them the "continental rafts" of the lighter sialic material in
opposite directions, and causing them to drift apart at present rates of about 2.5 cm a
year. The table below shows the estimated rate of crustal movements. These lateral
crustal movements would result in tensional cracks in the ocean floor midway
between the continents, such as the Mid-Atlantic Ridge, the East African Rift Valley
and the San Andreas Fault.
Areas
Movements
Continental Drift (average over 200 m.y.)
2-4 cm/year
Renewal of the Pacific floor
3-4 cm/year
Elongation of Hawaiian Chain (3000 km in 100 m.y.)
3 cm/year
Maximum transport of Atlantic Islands form Ridge
3 cm/year
San Andreas fault (present displacement rate)
5 cm/year
Great Alpine fault, N.Z. (present displacement rate)
2.5 cm/year
Magna Fossa fault, Japan (160 km in 4-8 m.y.)
2-4 cm/year
Algerian faults (180 km in 6-12 m.y.)
1.5-3 cm/year
VI. The Theory of Plate Tectonics:
This great, unifying concept draws sea-floor spreading, continental drift,
crustal structures and world patterns of seismic and volcanic activity together as
aspects of one coherent picture. Theory of Plate Tectonics was developed.
It is proposed that the entire surface of the Earth comprises a series of
internally rigid, but relatively thin plates. Although the size of the plates is variable,
much of the earth's present surface is occupied by half a dozen or so large plates.
These plates are continuously in motion both with respect to each other and to
the Earth's axis of rotation. Virtually all seismicity, vulcanicity and tectonic activity is
associated with different motion between adjacent plates.
These plates consist of the earth crust and upper mantle forming a strong, rigid
earth shell called the lithosphere. Beneath the lithosphere lies the soft, weak layer
(Asthenosphere).
There are three types of plate margin:
1. Constructive Margin:
During the spreading process, which occurs at oceanic ridges, new crust is
created and moves away from the ridge along with the underlying, uppermost mantle.
The newly generated crust and its upper mantle is effectively welded to the plate's
trailing edge. Thus a ridge represents a zone along which two plates are in motion
away from each other; yet they do not separate, because new material is continuously
added to the rear of each. (eg. Mid-ocean Ridges)
Constructive Plate Margin and Destructive Plate Margin
2. Destructive Margin:
It occurs at the deep ocean
trenches or fold mountain belts where
two plates approach each other and one
slips down under the margin of the
other at an angle of about 45o. This
zone is known as the subduction zone.
(eg. Himalayas, Java Trench)
3. Conservative Margin:
These are margins at which the plates neither gain nor lose surface area, but
simply past each other. However, due to the friction arising from the lateral
movement of the plates, earthquakes are usually caused. (eg. San Andreas Fault,
Rockies Mts.)
The mechanism for the plate movement is very complex and not yet very clear.
Nevertheless, it is generally thought to be related to the circulation of convection
current which is driven by the heat derived from the radioactive elements in the
asthenosphere. Ascending current may locate at the constructive plate margin and
drags the plates apart while the descending current may located at the destructive
plate margin and pulls the plates to be plunged into the mantle.
Folding and Fold Mountains
All major mountain chains are 'fold-belts', known also as 'orogenic belts'. They are
characterized by very complex folding, plutonic and volcanic rocks of various types and very
thick sequences of sedimentary rocks, often 10000 m thick or more.
1. Features and Characteristics of Fold Mountain Areas:
A study of the present fold mountain belts reveals the following characteristics:
a. There are parallel belts of fold mountains separated by intermountain plateau where
the sedimentary strata are much less intensely folded. For example, between the
Rockies and other fold mountains of North America is found the Colorado and other
plateaux; between the Himalayas and the Kunlun is the Qinzang Gaoyuan.
b. They are composed of enormous thickness of sedimentary rock strata (8000 to 10000
m) of materials that are today found in continental shelf. But when the core of fold
mountains has been exposed, igneous and metamorphic rocks are also found.
c. Young fold mountain zones represent lines of weakness of the earth's crust where
most of the world's recently active volcanoes lie and where the greater majority of the
earthquakes originate.
d. The sedimentary strata have been compressed into various kinds of folded structures.
The degree of folding and the types of folded structure formed depends on the nature
and intensity of the compression.
Folds are structures in which the primary surfaces of reference are bent or
distorted without loss of essential continuity. Three geometrical varieties of folds can
be distinguished, anticlines, synclines and monoclines. An anticline is an arch in
which the two sides or limbs dip outwards away from one another. A syncline is a
fold in which the limbs dip towards one another. A monocline is a simple step-like
flexure in which more or less horizontal beds locally assume a dip in one direction
and then flatten out again.
The elements making up a fold are illustrated in the figure below. In a single
bed, the two sides or limbs meet at the fold-axis or hinge-line which is the of
maximum curvature. The axial plane is the locus of the fold-axis of successive layers,
it is the place at which the two limbs meet. The crest of an anticline is the highest
point of the fold with reference to any particular bed; it is parallel to the fold-axis.
Similarly the trough of a syncline is the bottom of the fold with reference to a given
bed. The fold-axis may be horizontal or it may be inclines, in which case the axis and
the fold are said to plunge or to pitch. The plunge is measured in degrees from the
horizontal in a vertical plane, while the pitch is given by the angle between the
fold-axis and the strike of the axial plane.
The profile of a fold is its form as seen in a plane perpendicular to the axis.
Simple/ Symmetrical Fold: The fold is upright and symmetrical when the axial plane
is vertical and the two limbs dip at similar angles.
Asymmetrical Fold: Where on limb dips more steeply than the other, the axial plane is
inclined and the fold is asymmetrical.
Overturned/ Over Fold: With increasing asymmetry. The middle limb connecting an
anticline with its adjacent syncline becomes inverted and the fold is said to be
overturned.
Recumbent Fold: Where the axial plane is roughly horizontal, the fold is said to be
recumbent . In structures of recumbent fold most of the middle limb may be
sheared out altogether.
Overthrust Fold: Further development of the structure then results in forward
movement of the rocks of the upper limb along the plane of shearing. The
latter has become a thrust plane and the structure an Overthrust Fold. The
sheet of rocks that has moved forward along the thrust plane is referred to as a
nappe.
Anticlinorium & Synclinorium: Moreover, a complex of folds of different orders is
called an anticlinorium, if the whole appears to have been arched upwards; or
a synclinorium if the generalized form is that of a depression.
2. Formation of Fold Mountains:
According to the plate tectonics theory, fold mountain originates where plates
of crust converge. Orogeny occurs where a block of continental crust is carried into
collision with another crustal plate. The full development of an orogenic belt requires
collision between two plates, with subduction of one under the one and a continental
block at or near the edge of at least one of the plates.
Firstly, the land surface is being actively eroded supplying a large amount of
sediments. There is an accumulation of great depths of sediments in a large depression
called a geosyncline under an ocean.
Later the plates on each side of the sediment zone move towards each other
producing a great compressional force. The movement of the plates towards each
other crumples the sedimentary deposits. As the sedimentary rocks are folded up, fold
mountains are formed. While this is happening, the hot magma of the mantle may also
flow out to the earth's crust forming volcanoes. Besides, this is sometimes
accompanied by faulting. A good example is the Himalayas along the boundary
between the Eurasian Plate and the Australian Plate.
A special case is that of continent-continent collision, eg. the closing of the
Tethys Sea and the formation of the Himalayas by the collision of India with the
Asian continent in early Tertiary time. The figure below shows the formation of the
Himalayas by the collision of plates.
Faulting and Its Associated Landforms
1. Faults:
A fault is a fracture of dislocation in the earth's crust along which there has
been displacement of the rocks on one side relative to those on the other. The
movement of the rocks on a fault may have been in any direction, vertical or
horizontal or some combination of these.
The surface of the break along which the movement has taken place is the
fault-plane; a fault-zone comprises a group of such surfaces. The fault-plane may be
vertical, inclined or gently undulating and its inclination is recorded as a dip, the angle
between the fault-plane and the horizontal. Hade is the angle between the fault-plane
and the vertical. The horizontal trend of a fault is its strike. If the fault-plane is
inclined, the upper side and the rocks which lie above it are referred to as the
hanging-wall; those below it are the foot-wall. The intersection of a fault-plane with
the ground surface is known as the fault-line.
The total movement on one side of a fault relative to that on the other, in the
fault-plane, is termed the slip or displacement; the vertical component of the slip is
the throw; the horizontal component in the strike direction of the fault-plane is usually
referred to as the shift and the other horizontal component of the slip, at right angles
to the strike, is the heave.
The walls of a clean-cut fault may be polished by friction between the moving
blocks, and traversed by grooves or striations . Such surfaces are called slickensides.
Major faults are generally accompanied by a great deal of grinding and crushing of
the wall rocks, and there may be a more or less broad fracture belt, known as a fault
zone, consisting of lenticles and sheets of shattered rock. The latter is called a fault
breccia when it is largely made up of angular fragments.
Faults may be divided into several categories in relation to the movements that
have taken place on them.
A normal fault is generally inclined at an angle between 45o and the vertical.
The rocks abutting against the fault on its upper face or hanging wall are displaced
downwards relative to those against the lower face or footwall. The terms 'downthrow'
and 'upthrow' for the two sides are purely relative. Normal faults involve an extension
of the faulted beds. Regions that are divided by faults in to relatively elevated or
depressed blocks are said to be block faulted. The face of the earth today is diversified
by many boldly preserved topographic features which are primarily due to vertical
movements involving normal faulting. Such movements have been unusually active
during late geological time, and they still are, right down to the present time. fault
scarps exposed at the surface are being gradually worn back by erosion, but long ages
must elapse before such features as the block mountains illustrated in the figure below
are levelled out.
Reverse or thrust fault is inlined at an angle between 45o and the horizontal.
The beds on the upper side are displaced up the fault plane relative to those below.
Shortening of the faulted area is thus involved and the operation of compression
seems to the implied. The celebrated Moine and associated thrusts of the North-west
Highlands, where they mark the north-western boundary of the Caledonian orogenic
belt in Scotland, are the classic examples of low-angle thrust faults.
Tear faults or wrench faults are
formed where the movement was
dominantly horizontal and are often called
transcurrent or strike-slip faults. The best
known of all wrench faults is the great San
Andreas 'Rift' of California, which has a
known length of over 1350 km. Table 5
indicates that the average rate of
movement during the last 140 million years has been 4 km per million years. But
Pleistocene and present-day rates are much higher.
2. Block Fault mountains:
Regions which have been divided by faulting into relatively elevated or
depressed blocks are said to be block faulted. The upstanding fault blocks, which may
be small plateaux or long ridge-like block mountains, are called horsts (block
mountains).
A long fault trough,
forming a tectonic valley
bordered by parallel fault scarps,
is known as a rift valley.
Between the horsts of the
Vosges and the Black Forest the
Rhine flows through a rift valley.
The river occupies the valley,
but did not excavate it. The most
renowned system of rift valleys,
however, is that which traverses
the East African plateaux from
River Zambezi to the Red Sea
and beyond.
Cross-section of East African Rift Valley
Volcanism and the Earth's Crust
Vulcanicity is the process by which matter is transferred from the earth's interior
and erupted on to its surface. The oldest rock exposed on the surface of the earth were
produced by volcanic activity. Volcanoes are undoubtedly one of the greatest natural
hazards to life on this planet.
Below the crust, there is material at times becomes liquid and thus may be able to
rise to the surface of the earth through conveniently placed fissures and pipes or vents.
Despite the high temperatures at depth this material is usually solid because of the great
pressure exerted by the superior masses of rock. At times, a local increase of heat,
combined almost certainly with a reduction in pressure causes this material together, in all
probability, with the basal layers of the crustal SIMA to become molten rock or magma.
As the magma rises the decreasing pressure releases some of the included gases and when
the surface of the earth is reached much of the original gas has been lost. The molten rock
is the lava which unlike the original magma is relatively poorly charged with gas.
Whilst access of magma to the continental surfaces and ocean floors results in
volcanic activity the rising magma may penetrate only to a limited extent into the earth's
crust, chilling, after engulfing some of the crustal rock, when its movement is arrested.
All such activity, whether taking place within the crust or on its surface, is known as
vulcanicity. There are two types of vulcanicity: Intrusive and Extrusive vulcanicity.
1. Intrusive Forms of Vulcanicity:
The results of the forcing into the earth's crust of molten magma depend on firstly
its degree of fluidity, and secondly the character of the planes of weakness, such as joints
and faults, or cracks and fissures in the crests of anticlines along which it can penetrate. A
mobile magma flows farther, as a thin sheet, while a more viscous magma solidifies
rapidly in a dome-shaped or lenticular mass. The main intrusive forms are shown in the
figure :
a. Dykes:
These are formed when the magma has risen through near-vertical fissures,
solidifying to form 'walls' of rock cutting discordantly across the bedding planes of
the country rock. Where affected by denudation, the dyke may either stand up as a
wall (where the dyke-rock is harder than the country rock), or it may be worn away to
form a long narrow ditch-lick depression.
b. Sills:
Horizontal sheets of rock solidify from magma which has been ejected
concordantly between bedding planes; they may be of any thickness and extend for
many square kms. In North America the Palisades extend for over 80 km along the
western bank of the Hudson River near New York; this sill is probably 300 m thick.
c. Laccoliths:
These features are produced where tongue-like lateral intrusions of viscous
magma have forced the overlying strata into a dome. In its simplest form the magma
solidifies as a cake-like mass, but often there are subsidiary laccoliths around the main
one, or several laccoliths may be formed one above another, resulting in a 'cedar-tree'
laccolith.
d. Batholiths:
Large masses of rock occur in the heart of mountain ranges, formed by
deep-seated movements on an enormous scale, so that the masses of magma cooled
slowly to form large-crystalled rocks such as granite, later exposed by prolonged
denudation as massive upland areas. The edges of a batholith descend steeply to
unknown depths, while the country rocks with which the intrusions have come into
contact are often metamorphosed by thermal contact.
Intrusive rocks can be classified into Hypabyssal (Intermediate) and Plutonic.
Dykes, Sills and Laccoliths are hypabyssal, but Batholiths is plutonic.
2. Extrusive Forms of Vulcanicity:
A volcano consists of vent or opening at the surface of the crust through which
material is forced in an eruption. This may accumulate around the vent to form a hill, or it
may flow widely over the country rock as an extensive level sheet. The resultant
landforms depend largely on the nature of the material ejected, which varies in different
cases and different stages in an individual eruption. There are three types of extrusive
materials.
a. Gases:
Gases emitted during the course of an eruption include gaseous compounds of
sulphur and hydrogen with carbon dioxide, but most are dissipated directly into the
atmosphere. Stream (about 50-70%) is possibly derived from surface water such as in
a crater-lake or from the sea, but most probably originates from the water to be
found in magma. Stream is the most important factor affecting the eruption.
b. Solid:
When an eruption is accompanied by a series of explosions, solid materials are
ejected, known generally as Pyroclastic Debris/ Pyroclasts or Tephra. These may
include fragments of the country rock disrupted when the pipe was blown through the
crust, angular fragments of solidified lava from a previous eruption that had cooled in
the pipe, and finer materials such as scoria, pumice, cinders (lapilli), dust and ash. The
coarser fragments form breccia, while the finer materials may be loosely cemented as
tuffs. Sometimes small amounts of liquid magma are thrown into the air and solidify
before reaching the ground in globular masses known as 'volcanic bombs'. Other
masses remain liquid until they hit the ground, when they spatter and congeal,
forming small spatter cones, 3-6 m high, as around Sunset Crater in Arizona.
c. Liquid:
Usually the most important product of an eruption is lava, the molten magma
which reaches the surface. The form of a volcanic cone depends to a large extent upon
the nature of this lava, and so to some extent does the nature of the eruption.
Some lavas contain much silica (SiO2), i.e. acid lavas, with a high
melting-point; they are very viscous, solidify rapidly and so do not flow far. Acid
lavas build high, steep-sided cones, and they may solidify in the vent and cause
recurrent explosive eruption.
Where, on the other hand, the lava is relatively poor in silica and rich in iron
and magnesium minerals (basic lava), it has a lower melting-point and will flow
readily for a considerable distance before solidifying. Such a lava tends to produce a
much flatter cone of great diameter, and as the flow from the vent is unchecked and
widespread, the eruption is quiet, without much explosive activity. The mobility of
the lava also varies with the amount of gas dissolved within its mass; in Hawaii lavas
containing gas have remained mobile at a temperature of 850oC, while others without
much gas content have solidified at 120oC.
3. Types of Volcanoes:
Volcanoes are built by the eruption of molten rock and heated gases under
pressure from a relatively small pipe, or vent, leading from a magma reservoir at
depth. Both explosive and quiet types of eruption occur, the forms built differing for
the two types.
Volcanoes of explosive eruption are cinder cones and composite cones; those
formed by relatively quiet outflow of lava are lava domes. Quiet eruption of lava, if
issuing from extensive cracks, of fissures, in sufficient quantities may make great
plains or plateaux of lava.
a. Cinder Cones:
Smallest of the volcanoes are the cinder cones, built entirely of pieces of
solidified lava thrown from a central vent. They form where a high proportion of gas
in the molten rock causes it to froth into a bubbly mass and to be ejected from a vent
with great violence.
The froth breaks up into small fragments which solidify as they are ejected
and fall as solid particles near the vent. Large pieces up to several tons in weight are
volcanic bombs; they may be somewhat plastic when ejected. Smaller pieces, a
fraction of an inch up to an inch or two in size, are cinders; these make up the bulk of
the cinder cone. Still finer particles are termed ash and volcanic dust. The ash falls
like snow upon the ground within a few miles of the eruption. Finer dust is carried by
winds to distant regions and may settle out only after years of drifting in the
atmosphere.
Cinder cones rarely grow to more than 150 to 300 m in height. Growth is rapid.
Monte Nuovo, Italy, grew to a height of 120 m in the first week of its existence. The
angle of slope of a recently formed cinder cone ranges between 26o and 30o. Cinder
cones usually occur in groups, often many dozens in an area of a few tens of square
miles.
b. Basic Lava Cones/ Shield Volcanoes:
A very important type of volcano is the lava dome or shield volcano. The best
examples are from the Hawaiian Islands, which consist entirely of lava domes. Lava
domes are characterized by gently rising, smooth slopes which tend to flatten near the
top, producing a broad-topped volcano. The Hawaiian domes range to elevations up to
4000 m above sea level, but including the basal portion lying below sea level they are
more than twice that high. In width they range from 16 to 80 km at sea level and up to
160 km wide at the submerged base.
Lava domes are built by repeated outpourings of lava. Explosive behaviour
and emission of fragments are not important. The lava, which in the Hawaiian lava
domes is of a dark basaltic type, is highly fluid and travels far down the low slopes,
which do not usually exceed 4o or 5o.
c. Acid Lava Cones:
When explosive activity is absent, and the lava flows quickly from the vent,
building a volcano. Viscous acid lava does not flow far and it produces a steep dome,
with convex sides, eg. Puys d' Auvergne. Some of the more highly viscous lavas may
be squeezed out slowly to form great 'spines', called volcanic plug. The vent of the
volcano was solidified by lava, and the country rock were removed. The exposed vent
is also called volcanic plug.
d. Composite Volcanoes:
Most of the world's great volcanoes are composite cones. They are built of
layers of cinder and ash alternating with layers of lava, and for this reason have been
called strato-volcanoes. The steep-sided form is governed by the angle at which the
cinder and ash stands, whereas the lava layers provide strength and bulk to the
volcano. Among the outstanding examples of recently formed composite volcanoes
are Fujiyama in Japan, Mayon in the Philippines. Height of several thousand feet and
slopes of 20o to 30o are characteristic.
Many composite volcanoes lie in a great belt, the circum-Pacific ring,
extending from the Andes in South America, through the Cascades and the Aleutians,
into Japan; thence south into the East Indies and New Zealand. There is also an
important Mediterranean group, which includes active volcanoes of Italy and Sicily.
The eruption of large composite volcanoes is usually accompanied by
explosive issue of steam, cinders, bombs and ash, and by lava flows. The crater may
change form rapidly, both from demolition of the upper part and from new
accumulation.
e. Calderas:
One of the most catastrophic of natural phenomena is a volcanic explosive so
violent as to destory the entire central portion of the volcano. There remains only a
great depression, a caldera. A portion of the upper part of the volcano is blown
outward in fragments, whereas most of the mass subsides into the ground beneath the
volcano.
Krakatoa, a volcanic island in Indonesia, exploded in 1883, leaving a great
caldera. It is estimated that 75 cubic km of rock disappeared during the explosion.
Great seismic sea waves, or tsunamis, generated by the explosion killed many
thousands of persons living on low coastal areas of Java and Sumatra.
4. Dormant and Extinct Volcanoes:
An active volcano is one which is definitely known to have eruption
periodically in historic times.
However, some volcanoes may be described as dormant when a renewal of
eruption activity is possible. Vesuvius had been dormant so long before its eruption of
A.D. 79 that it was thought to be extinct.
Other volcanoes may definitely be regarded as extinct, since they were formed
in long-past geological times, and are situated in areas with no sign of any volcanic
activity. They may retain their original forms, as the miniature cones of Auvergne, but
for the most part many ancient cones have been destroyed beyond recognition through
long continued denudation.
5. Distribution of Volcanoes:
A large part of the world has not experienced volcanic activity during the most
recent periods of geological time; there are some 520 known active volcanoes, but many
thousands of extinct ones.
The main volcanic peaks are indicated on the figure above; it is evident that there
is a close relationship between their location and the major lines of weakness in the
earth's crust. The most striking development is around the basin of the Pacific Ocean - the
'Pacific Ring of Fire', where two-thirds of the world's volcanoes occur. Most are
associated with fold mountain ranges or faulted blocks. A chain of volcanoes nearly 3,200
km in length can be traced through southern Alaska, the Alaska Peninsula and the
Aleutian Islands, one of the most volcanically active parts of the world. There are 80
active volcanoes, ten calderas over 1.6 km in diameter, and since 1760, 225 major
eruptions have been recorded.
In South America most of the highest peaks are volcanoes rising from the folded
ranges, including Aconcagua (7021 m), which though extinct, is the world's loftiest
volcano, and Guayatiri (6060 m), which erupted in 1959.
On the Asiatic margins of the Pacific, volcanoes are strung along the island arcs;
the most famous is snow-capped Fujiyama, rising to 3776 m with 24 km of the sea.
Another line runs through the East Indies towards New Zealand; the island arcs of the
western Pacific, which are often on the edge of submarine trenches, have hundreds of
volcanoes, while many of the more remote islands (Hawaii, Tonga, Samoa) are volcanic
cones rising from the ocean floor.
In Africa volcanoes are to be found along the line of the East African rift-valley;
Mount Kenya (5195 m) and Kilimanjaro (5889 m) are probably only recently extinct. The
Alpine-Himalayan belt of folding is not associated to the same extent with active
volcanoes, except in the central Mediterranean, where they are probably related to
geologically recent movements of subsidence. Farther east, in Asia Minor and to the
south of the Caspian, are many extinct cones. The Himalayas form the most striking
exception to the general occurrence of volcanoes along zones of recent folding, for none
is to be found there. Conversely, volcanic activity is widespread in Iceland, where there
has been no recent folding of the crust.
Several of the Atlantic islands, notably along the line of the Mid-Atlantic Ridge,
have volcanoes; a peak on Tristan da Cunha erupted in October 1961, and the Azores and
Canaries have experienced eruptions within historic times. The West Indian arcs have
many indications of past vulcanicity, as well as a few active cones.
6. Minor Volcanic Forms:
A variety of minor volcanic forms can be distinguished, usually though not
necessarily associated with volcanoes approaching extinction.
a. Solfatara, Fumarole and Mofette:
A solfatara is a volcano which only emits steam and gas. The floor of this
crater is comparatively cool, and one can walk about on it, although here and there are
pools of boiling water, while jets of water vapour or puffs of sulphurous gas are
emitted periodically.
The term solfatara is commonly limited to cases where the gas emitted are
sulphurous, while the name fumarole is applied to emissions of steam and other gases.
Perhaps the most famous example of fumarole is in the valley of Ten Thousand
Smokes in Alaska. The special name of mofette distinguishes vents which emit
carbon dioxide; examples are to be found in the Phlegraean Fields, in Auvergne and
in Java.
b. Hot Springs and Geysers:
In some areas associated with past or present volcanic activity, the chief
product is hot water. It may flow out continuously as a thermal spring, containing
mineral substances in solution or suspension, which may be deposited round the edge
of the surface pool in the form of crusts of travertine (calcium carbonate) or of
siliceous sinter (geysertie). Several thousand hot springs are known in Iceland.
Where hot water is ejected with considerable force accompanied by steam, an
intermittent paroxysmal fountain occurs, known as a geyser, as in Iceland,
Yellowstone National Park, and the North Island of New Zealand. Some erupt
periodically at regular intervals, others more spasmodically .
The cause is complex, but it is due to superheating far down in the pipe of the
geyser; the temperature of the water at depth increases, but convection cannot easily
take place up the column of water because of its length. The water at depth is heated
beyond 100oC because of the pressure of the column above, and its ultimate sudden
conversion into superheated steam causes the water in the upper part of the pipe to be
violently emitted. Cooler water flows into the pipe, and the heat increase begins again.
One of the most famous geysers is 'Old Faithful', in Yellowstone, which has an
average eruption interval of about 65 minutes, ranging between 33 and 95 minutes. It
shows 50-100 cubic metres of near-boiling water, with steam, 40-60 m into the air,
lasting from 2 to 5 minutes.
Ocean Ridges, Ocean Trenches and Island Arcs
1. Ocean Ridges:
These are the main features rising above the abyssal plains. They connect through
all the oceans to form a worldwide feature nearly 60,000 km long. Each ocean ridge has
its own distinctive features, but all are composed solely of the basaltic lavas typical of
ocean-floors; there is no mixture of sedimentary, igneous and metamorphic rocks
characteristic of the continental areas.
The ocean ridges are formed by fissure eruptions with a fairly uniform rate of
molten rock emission. In combination with fracturing along the central rift valley sides
this leads to the formation of the parallel ridge system.
Iceland is a particular interesting place in this context, since it is built astride the
Mid-Atlantic Ridge system. It is formed mostly of lava flows, and volcanic activity,
including hot springs, is a constant feature of the island's environment. Great fissures
cross the island and basaltic material wells up along these at temperatures of 1200oC; it
cools and blocks the fissure, forming wall-like intrusions, or dykes. At times volcanic
cones are formed after explosive activity.
Other features of ocean ridges indicate that there is a flow of energy along their
length. Shallow earthquakes are located commonly beneath the centres of ridges, with
their foci at 25-35 km depth. Since earthquakes take lace where stresses are relieved
inside the earth, and are known to occur where movements of molten rock take lace
beneath a volcano prior to eruption, it can be inferred that similar movements are taking
lace beneath the ridges. This is confirmed by the subsurface temperature pattern (the
figure below), which suggests a strong heat flow concentrated at the ridge and giving rise
to the melting of rock material beneath the crust.
2. Ocean Trenches:
The deepest parts of the oceans are elongated troughs descending to depths of
over 10,000 metres. They
are hundreds of
kilometres across and
thousands of kilometres
long. Sediments
accumulating on the
trench floors are
relatively thin today.
Nearly all the trenches
occur around the margins
of the Pacific Ocean, and
an arc of volcanic islands
is commonly present on
the continental side of the
trenches. The trenches
are interpreted as the
lines along which the ocean-floor, manufactured at the ocean ridges and spreading out
towards the continents, plunges beneath lower density continental or ocean-floor rocks at
the subduction zone to a depth where the rock materials are fused, resulting in volcanic
and earthquake activity.
3. Island Arcs:
Island arcs are often related closely to fold mountain ranges. They are found today
mostly around the western margin of the Pacific Ocean and in the northeast of the Indian
Ocean. They form where a section of the ocean-floor is subducted in the ocean basin
away from a continent, i.e. where ocean-floor crust is on either side of the convergent
plate boundary.
Japan is the largest area of land formed in this way, and is a mountainous country.
Mount Fuji reaches nearly 4,000 m, and several other peaks on Honshu top 3,000 m.
They are all volcanic in origin. The island of Honshu is largely a pile of basalt and
andesite lying between the Japan Trench and the Sea of Japan. The Pacific Ocean floor is
subducted at the trench, a hypothesis which is supported by the greatest volume of
volcanic rocks being erupted over western Japan during the last 2 million years. It seems
that the subducted ocean-floor becomes mobilized as magma when it reaches 120 km
depth, and this is why the east of the island experiences little volcanic activity compared
with the west. The evidence of the double metamorphic belts suggests that Japan has been
formed by the driging together of two island arc masses at different times, and just to the
south of Japan today are the two separated arcs of the Philippines and Marianas. The Sea
of Japan now forms an effective sediment trap for material eroded from Asia as well as
from western Japan. On the Pacific Ocean side a wedge of flysch-type sediment is
accumulating. Thus the volcanic rocks form the first land, but the atmospheric processes
act on these rocks and produce sedimentary rocks.
It would seem that the trench develops as plate descent begins. Ocean-floor crust
together with any covering sediment is thrust beneath more ocean-floor crust, giving rise
to rocks metamorphosed under high pressure conditions on the continental side of the
trench. When the descending plate reaches over 100 km in depth partial melting takes
place, magma rises to form a pile of volcanic rocks and the island arc begins to rise. The
heat generated in this way results in further metamorphism as a result of combined heat
and pressure. The greater the depth reached by the plates, the more intense the volcanic
activity and the farther the magma has to ascend. This affects the type of lava produced:
basalts are poured out first, followed by andesites which become increasingly alkaline
and are erupted farther from the trench.
Earthquakes
Earthquakes, natural vibrations within the earth's crust, provide indisputable evidence
that crustal movements are still taking place today. Some of these earth tremors are
quite violent and are responsible for large-scale death and destruction. Most, however,
are too small to be felt by man and must be detected by means of delicate recording
instruments called seismographs.
1. Seismic Waves:
The investigation of earthquakes and the transmission of earthquake waves
through the earth is known as Seismology. An earthquake shock generates elastic
vibrations or 'waves'. which move out in all directions from the point of origin, or
focus, of the earthquake.
There are four kinds of seismic waves:
a. Primary or compressional waves (P waves) consisting of longitudinal vibrations
which give an oscillatory movement to particles in the direction in which the waves
are propagated. These waves propagate very rapidly through both solids and liquids
and are usually the first indication that an earthquake has occurred. The speed of
P-waves varies and is in the region of 5.5 km/s in the crust, 8 km/s just below the
crust and about 13.5 km/s in the lower portion of the mantle.
b. Secondary, Shear- or Distortional waves (S waves) which are transverse vibrations
with an oscillatory movement at right angles to their path. The P waves and S waves
are body waves travelling through the earth. The S waves travel at only about 60% of
the speed of P waves. However, their amplitude and period are almost double those of
P waves. S waves require a rigid medium for propagation and cannot propagate
through the liquid core.
P waves and S waves are body waves.
c. Love waves (L waves) are waves of long period which travel around the periphery of
the earth. These are surface waves.
d. Rayleigh waves consist of a retrograde elliptical motions similar to wind-driven ocean
waves. Like L waves, Rayleigh waves are also surface waves. However, L waves
propagate at about 4.5 km/s while Rayleigh waves are about 10% slower. A large
proportion of the earthquake energy is carried by the L waves and the Rayleigh
waves.
The P waves have the greatest velocity and are the first to arrive at a recording
station at a given distance from the focus; the S waves are a little slower, and the L
waves the slowest of the three groups. But the L waves have the greatest amplitude
and are those that do the most damage.
2. Distribution:
Although earthquakes may occur at any place over the entire earth, most of
them originate in areas of crustal unrest and are associated with mountain-building
movements. Earthquakes, like volcanoes, occur in rather well-defined seismic belts.
About 80% of the world's earthquakes originate in the Circum-Pacific belt of young
mountain ranges and chains of volcanic mountains. This belt extends from Chile
along the western borders of South and North America, northward to the Aleutians,
Alaska, Japan, the Philippines, Indonesia, New Zealand, and certain Pacific islands.
The second major seismic belts, the Mediterranean and Trans-Asiatic belt, extends
from the Caribbean area through the Himalayas and Alps and includes Spain, Italy,
Greece, and northern India. Approximately 15% of the earth's seismic energy is
released in the Mediterranean and Trans-Asiatic zone; the remaining 5% is released in
other parts of the world.
3. Earthquake and Plate Tectonic:
The occurrence of earthquake is considered to be highly related with the
location of plate margins. The greatest intensity of earthquake is found along the
destructive plate margins where oceanic plates undergo subduction such as the
volcanic arcs and trenches of the Pacific Ocean Basin. Intense pressure is built up
along the zone of collision and stress and strain relieved by sudden fault slippages
generate devastating earthquake.
Constructive plate margins and conservative plate margins also experience
seismic activities though the intensity is generally much smaller in comparison with
those along the destructive plate margins. Typical examples are the San Andreas Fault
which locates along the Pacific Plate and the America Plate in California and the
Mid-Atlantic Oceanic Ridge.
4. Effects of Earthquake:
The destructive effects of earthquakes are familiar to almost everyone :
shattered buildings, displaced roads and railways, collapsed bridges, great cracks in
the ground and changes in sea level.
The loss of life accompanying a major earthquake may be staggering. For
example, an earthquake that occurred in north central China in 1556 is said to have
killed 830,000 people. Because earthquakes normally occur rapidly and unexpectedly,
there is little time for precautionary measures. The death toll may also be raised by
complicating factors such as disease, flood, fire and famine.
In addition, an earthquake will usually produce numerous geologic changes in
an area. These include landslides, avalanche and mudflows, disruption of
ground-water circulation, and sunken and fissured ground.
Earthquakes that occur beneath the ocean often generate great waves of water
called tsunamis. These waves, which have been known to be as much as 60 m high
and to travel at speeds of up to 800 km/hr, are capable of producing tremendous
destruction. One such wave, associated with the Lisbon earthquake of 1755, attained
an estimated height of 15 m and washed inland for more than 1 km. Another,
occurring north of Tokyo along the Pacific coast of Japan in 1896, reaching heights of
up to 30 m and was responsible for the deaths of more than 27,000 people.