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IASBABA’S
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GEOGRAPHY
HANDOUTS

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GEOGRAPHY-4
TOPICS: Geomorphology - Theory of Plate Tectonics, Folding, Faulting, Volcanism

CONTINENTAL DRIFT THEORY

 This theory was suggested by Alfred Wegener in 1920’s.
 According to Wegener’s Continental Drift Theory, there existed one big land mass
which he called Pangaea which was covered by one big ocean called Panthalassa.
 A sea called Tethys divided the Pangaea into two huge landmasses: Laurentia
(Laurasia) to the north and Gondwanaland to the south of Tethys.
 Drift started around 200 million years ago (Mesozoic Era), and the continents began
to break up and drift away from one another.

Forces for Continental Drift - The drift was in two directions,

 Equator wards due to the interaction of forces of gravity, pole-fleeing force (due to
centrifugal force caused by earth’s rotation) and buoyancy
 Westwards due to tidal currents because of the earth’s motion. Tidal force is due to
the attraction of the moon and the sun that develops tides in oceanic waters.

Evidence in support of Continental Drift

 The Matching of Continents (Jig-Saw-Fit) - The coastlines of South America and Africa
fronting each other have a remarkable and unique match.
 Rocks of the Same Age across the Oceans - The radiometric dating methods have
helped in correlating the formation of rocks present in different continents across
the ocean.
 Tillite - It is the sedimentary rock made from glacier deposits. The Gondwana system
of sediments from India is recognized as having its counterparts in 6 different
landmasses in the Southern Hemisphere like Madagascar, Africa, Antarctica, Falkland
Island, and Australia, which indicates common origin
 Placer Deposits - The presence of abundant placer deposits of gold along the Ghana
coast and the complete lack of its source rocks in the area is a phenomenal fact. The
gold-bearing veins are present in Brazil and it is evident that the gold deposits of
Ghana in Africa are obtained from the Brazil plateau from the time when the two
continents were beside each other
 Distribution of Fossils - The interpretations that Lemurs occur in India, Africa
and Madagascar led to the theory of a landmass named “Lemuria” connecting these
3 landmasses. Mesosaurus was a tiny reptile adapted to shallow brackish water. The
skeletons of these creatures are found in the Traver formations of Brazil and
Southern Cape Province of South Africa.

Drawbacks of Continental Drift Theory

 Wegener failed to explain why the drift began only in Mesozoic era and not before.
 Forces like buoyancy, tidal currents and gravity are too weak to be able to move
continents.
 Modern theories (Plate Tectonics) accept the existence of Pangaea and related
landmasses but give a very different explanation to the causes of drift.
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CONVECTION CURRENT THEORY

 Convection Current Theory is the soul of Seafloor Spreading Theory.
 Arthur Holmes in 1930s discussed the possibility of convection currents in the
mantle.
 According to this theory, the intense heat generated by radioactive substances in the
mantle (100-2900 km below the earth surface) seeks a path to escape and gives rise
to the formation of convection currents in the mantle.
 Wherever rising limbs of these currents meet, oceanic ridges are formed on the
seafloor due to the divergence of the lithospheric/ tectonic plates, and wherever
the failing limbs meet, trenches are formed due to the convergence of the
lithospheric plates (tectonic plates).
 The movement of the lithospheric plates is caused by the movement of the magma
in the mantle.
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SEA FLOOR SPREADING

 The idea that the seafloor itself moves as it expands from a central axis was
proposed by Harry Hess in 1960
 Seafloor spreading is a process that occurs at midocean ridges, where new oceanic
crust is formed through volcanic activity and then gradually moves away from the
ridge.
 Seafloor spreading helps explain continental drift in the theory of plate tectonics.
 When oceanic plates diverge, tensional stress causes fractures to occur in the
lithosphere, basaltic magma rises up the fractures and cools on the ocean floor to
form new sea floor.
 Older rocks will be found farther away from the spreading zone while younger rocks
will be found nearer to the spreading zone.

PLATE TECTONICS THEORY

 In 1967, McKenzie and Parker suggested the theory of plate tectonics.
 According to the theory of plate tectonics, the earth’s lithosphere is broken into
distinct plates which are floating on asthenosphere (upper mantle). Plates move
horizontally over the asthenosphere as rigid units.
 The lithosphere includes the crust and top mantle with its thickness range varying
between 5-100 km in oceanic parts and about 200 km in the continental areas.
 Lithospheric plates (crustal plates, tectonic plates) vary from minor plates to major
plates, continental plates (Arabian plate) to oceanic plates (Pacific plate), sometime
a combination of both continental and oceanic plates (Indo-Australian plate).
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Types of Plate boundaries –

BOUNDARY FEATURES
 A divergent boundary occurs when two tectonic plates
move away from each other.
 Along these boundaries, earthquakes are common and
magma (molten rock) rises from the Earth’s mantle to the
surface, solidifying to create new oceanic crust
 Diverging currents produce tension at the contact-zone of
crust leading to fracture.
 Magamatic material penetrates into the fractures and gets
solidified.
 This continuous process pushes the blocks in opposite
direction and creates a new zone, known as “zone of
construction”.
 Found mostly at mid-oceanic ridges. Eg – Mid Atlantic
ridge
 When two plates come together, it is known as
a convergent boundary.
 A chain of volcanoes often forms parallel to convergent
plate boundaries and powerful earthquakes are common
along these boundaries.
 When both sides are of continental nature, a mountain
formation is evident.
 When one of the two is continental and the other
maritime again mountain comes into being along the
boundary. In this case, continental plate overrides the
maritime plate. Oceanic crust is often forced down into
the mantle where it begins to melt
 When both plates are of maritime, both of them break,
subduct and penetrate below and, hence, trenches are
formed. Along this boundary earthquakes and volcanic
activities are prominent.
 In all these three situations, surface area is reduced,
therefore, this is also known as “zone of destruction”.
 Transform fault is the one when two adjacent plates slide
past each other.
 Direction of movement may be along or against but they
move parallel to each other.
 Therefore, neither there is any construction of fresh area
nor it has any destruction. Hence, it is known as “zone of
preservation”.
 Plates are not permanent features but they vary in size
and shape. Plates can split or get welded with adjoining
plate.
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FOLDING
 When a body of rock, especially sedimentary rock, is squeezed from the sides by
tectonic forces, it is likely to fracture and/or become faulted if it is cold and brittle, or
become folded if it is warm enough to behave in a plastic manner.
 If the bending is upwards forming a crest it is called anticline. On contrary if it results
into a trough it is called a syncline.
 It should be noted that folding is found in sedimentary and igneous rocks and not in
metamorphic rocks as they crumble under compressive force

 A plane drawn through the crest of a fold in a series of beds is called the axial plane
of the fold.
 The sloping beds on either side of an axial plane are limbs.
 An anticline or syncline is described as symmetrical if the angles between each of
limb and the axial plane are generally similar, and asymmetrical if they are not.
 If the axial plane is sufficiently tilted that the beds on one side have been tilted past
vertical, the fold is known as an overturned anticline or syncline.

 A symmetrical fold is one in which the axial plane is vertical.

 An asymmetrical fold is one in which the axial plane is inclined.
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 An overturned fold has a highly inclined axial plane such that the strata on one limb
are overturned.
 A recumbent fold has an essentially horizontal axial plane.
 An isoclinal fold has limbs that are essentially parallel to each other and thus
approximately parallel to the axial plane.
 Fold mountains are created where two or more of Earth’s tectonic plates are
pushed together. At these colliding, compressing boundaries, rocks and debris
are warped and folded into rocky outcrops, hills, mountains, and
entire mountain ranges.

FAULTS
A fault is a crack across which the rocks have been offset. They range in size from
micrometers to thousands of kilometers in length and tens of kilometers in depth, but they
are generally much thinner than they are long or deep.
In other words, a fault is a fracture in the earth’s crust due to tension force. It can also occur
due to compression in hard and brittle rocks.
 When there is tension the crust ruptures. One block is thrown upwards and the
other downwards. The upthrown block is called Horst while the downthrown block is
called Graben. The line along which the fault occurs is called strike. This fault is
called normal fault and is most common. In case of a normal fault, new surface is
generated in the form of scarp.
 When there is compression, in case of hard rocks instead of folding, the faulting
occurs. The block with hanging wall is thrown upwards while the one with footwall is
thrown downwards. This is called a reverse fault. In case of a reverse fault there is
net destruction of the surface.
 When the forces are acting parallel to each other, along the line of fault the blocks
move past each other without being upthrown or downthrown. This is called lateral
fault.
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SUMMARY OF TYPES OF MOUNTAINS

TYPE FEATURES
Fold Mountains  Mountain ranges mainly consisting of uplifted folded
sedimentary rocks are called Fold Mountains.
 They are formed due to the force of compression arising
from the endogenic or internal forces.
 Synclines (trough) and anticlines (crest) are part of Fold
Mountains.
 The Himalayas in Asia, the Alps in Europe, the Rockies in
North America, and the Andes in South America are the
most prominent fold mountains of the world.
 Since these were formed during the most recent mountain
building period, they are also known as Young Fold
Mountains.
Block Mountains  Block Mountains are also formed by the internal or
endogenic earth movements which cause the force of
tension and faulting.
 The down-lifting or uplifting of land in between two parallel
faults results in the formation of Block Mountains.
 A block mountain is also called as Horst and the rift valley
formed as a result of faulting is called Graben.
 Examples: The Sierra Nevada in North America, Black Forest
Mountains in Germany etc are typical examples of Block
Mountains.
Volcanic Mountains  The mountains formed by the accumulation of volcanic
materials are called as Volcanic Mountains or Mountains of
accumulation.
 Examples: Mount Mauna Loa in Hawaii Island, Mount Popa
in Myanmar, Fuji Yama in Japan etc are some examples.
Residual or Relict  The process of wearing down depends on the shape and
Mountains structure of the rocks upon which it acts.
 Some portions of an elevated area escape from the process
of weathering due to the hardness of the materials it is
made of.
 These portions remain unweathered while its surrounding
area gets eroded constantly. This results in the formation
of Residual or Relict Mountains.
 Examples: Hills like Nilgiri, Palkonda, Parasnath and
Rajmahal and Mountains like the Aravalli, the Vindhya, and
the Satpura are some of the examples of Relict Mountains
in India.
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Volcanism
 A volcano is a place where gases, ashes and/or molten rock material – lava – escape
to the ground.
o Magma is molten rock below the surface of the Earth’s crust, when this
molten rock reaches the surface of the earth is then called lava.
 The process is called Volcanism and has been ongoing on Earth since the initial
stages of its evolution over 4 billion years ago.
 Volcanism occurs because of Earth’s internal heat, and is associated with tectonic
processes and a part of the rock cycle.
 The mantle contains a weaker zone called asthenosphere. It is from this that the
molten rock materials find their way to the surface.
 The material in the upper mantle portion is called magma. Once it starts moving
towards the crust or it reaches the surface, it is referred to as lava.
 The material that reaches the ground includes lava flows, pyroclastic debris, volcanic
bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds
and minor amounts of chlorene, hydrogen and argon.
 How violent the eruption is depends on the amount of silica present in the magma.
Silica produces a thicker magma that is better at trapping gasses. The more gas
present the greater the pressure. Therefore the more silica present in the magma
the more violent the eruption will be.

Types of lava and their effect on the volcano structure

There are two types of lava, acidic and basic.
Acidic lava has a high silica content and this makes it thicker. This thick lava doesn’t travel
far and due to the high level of dissolved gas it has violent eruptions. Combined these cause
the volcano to have a steep sided cone. These are known as Cone Volcanoes.

Basic lava contains less silica, this allows the gasses to escape and gives a runny lava.
Eruptions of this type of lava a gentler and this along with it being runny allows the lava to
flow further. Volcanoes of this type of lava will have gently sloping sides. They are known as
Shield volcanoes.

Structure of Volcano
It is made up of a magma chamber, a vent, a
crater and a cone shaped mountain made of
layers of ash and lava.
 Magma chamber → Magma from the
Earth’s mantle collects in a large
underground pool. The magma in a
magma chamber is under great
pressure trying to force its way upward
to the surface.
 Vent → The magma forces its way up through the vent which is like a chimney for
the volcano. There is the main vent but there can also be secondary vents on the
side or flank of the volcano. These secondary vents produce secondary cones on the
flank of the volcano.
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 Crater → The crater or caldera is the bowl shaped feature on top of the volcano that
the magma from the vent erupts form.
 Cone shaped mountain → The majority of volcanoes are cone shaped mountains.
They are formed of alternating layers of lava and ash from multiple eruptions. As the
volcano erupts a layer of lava forms, the ash cloud formed during the eruption later
cools and falls, this is known as pyroclastic flow. This forms a layer of ash on top of
the lava. This process is repeated each time the volcano erupts.

Where do Volcanoes occur?

The majority of volcanoes occur at plate boundaries. They can occur where plates separate,
an example of this is Iceland. Here volcanoes are formed by the North American and
Eurasian plates pulling apart. (see divergent plate boundaries)

They can also occur where plates collide. Mount Etna is formed by the subduction of the
African plate under the Eurasian plate. The melting of the subducted plate causes an
increase in pressure which leads to the formation of a magma chamber and in turn a
volcano.

Image credit: National Geographic

Volcanoes can also form at areas known as Hot-Spots. Hotspots are caused by magma of
increased temperature from a mantle plume. This hot magma melts through the rock of the
Earth’s crust and rises through the cracks to form a volcano.

452 of the world’s volcanoes can be found in what is known as the Pacific Ring of Fire. this is
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an area of intense volcanic activity due to plate tectonics. 75% of the world active and
dormant volcanoes can be found here.

Does Volcanism increase or decrease temperature?

Volcanism can both increase and decrease temperature.
 Volcanism can cause long term increases in average temperatures by releasing
greenhouse gases, but at a very slow rate over millions of years.
 Volcanic events can also cause short term cooling by increasing the amount of
airborne particles that reflect sunlight in the atmosphere.

How can we forecast Volcanoes?

Eruptions can be predicted in a number of ways:
 Tiltmeters are very sensitive devices that are used to identify any bulging of the
sides of a volcano. Increased pressure that causes the volcano’s sides to bulge out
indicating an eruption may be about to happen.
 Gases or steam coming out of vents in the volcano or the appearance of geysers
could suggest an eruption will soon follow.
 Seismometers are used to detect vibrations in the rock. These could be caused by
the movement of the magma or the cracking of rocks due to increased hea both of
these would indicate an eruption being imminent.

Classification – 

Based on Activity:
 Active Volcano are those volcanoes where the materials mentioned are being
released or have been released out in the recent past.
 Dormant volcanoes are volcanoes that have not erupted in a long time but are
expected to erupt again in the future. Examples of dormant volcanoes are Mount
Kilimanjaro, Tanzania, Africa and Mount Fuji in Japan.
 Extinct → Extinct volcanoes are those which have not erupted in human history.
Examples of extinct volcanoes are Mount Thielsen in Oregon in the US

Shield Volcano  These volcanoes are mostly made up of basalt, a type of lava
that is very fluid when erupted. For this reason, these volcanoes
are not steep.
 They become explosive if somehow water gets into the vent;
otherwise, they are characterised by low-explosivity.
 The upcoming lava moves in the form of a fountain and throws
out the cone at the top of the vent and develops into cinder
cone.
 Barring the basalt flows, the shield volcanoes are the largest of
all the volcanoes on the earth.
 The Hawaiian volcanoes are the most famous examples.
Composite  These volcanoes are characterised by eruptions of cooler and
Volcanoes more viscous lavas than basalt.
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 These volcanoes often result in explosive eruptions.

 Along with lava, large quantities of pyroclastic material and
ashes find their way to the ground.
 This material accumulates in the vicinity of the vent openings
leading to formation of layers, and this makes the mounts
appear as composite volcanoes.
Caldera  These are the most explosive of the earth’s volcanoes.
 They are usually so explosive that when they erupt they tend to
collapse on themselves rather than building any tall structure.
 The collapsed depressions are called calderas.
 Their explosiveness indicates that the magma chamber supplying
the lava is not only huge but is also in close vicinity.
Flood Basalt  These volcanoes outpour highly fluid lava that flows for long
Provinces distances.
 Some parts of the world are covered by thousands of sq. km of
thick basalt lava flows.
 There can be a series of flows with some flows attaining
thickness of more than 50 m. Individual flows may extend for
hundreds of km.
 The Deccan Traps from India, presently covering most of the
Maharashtra plateau, are a much larger flood basalt province.
Mid-Oceanic  These volcanoes occur in the oceanic areas.
Ridge Volcanoes  There is a system of mid-ocean ridges more than 70,000 km long
that stretches through all the ocean basins.
 The central portion of this ridge experiences frequent eruptions.
We shall

Volcanic Landforms
Intrusive Forms
 The lava that is released during volcanic eruptions on cooling develops into igneous
rocks. The cooling may take place either on reaching the surface or also while the
lava is still in the crustal portion.
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 Depending on the location of the cooling of the lava, igneous rocks are classified as
volcanic rocks (cooling at the surface) and plutonic rocks (cooling in the crust). The
lava that cools within the crustal portions assumes different forms. These forms are
called intrusive forms.

Batholiths  A large body of magmatic material that cools in the deeper

depth of the crust develops in the form of large domes.
 They appear on the surface only after the denudational
processes remove the overlying materials.
 They cover large areas, and at times, assume depth that may be
several km.
 These are granitic bodies.
 Batholiths are the cooled portion of magma chambers
Lacoliths  These are large dome-shaped intrusive bodies with a level base
and connected by a pipe-like conduit from below.
 It resembles the surface volcanic domes of composite volcano,
only these are located at deeper depths. It can be regarded as
the localised source of lava that finds its way to the surface.
 The Karnataka plateau is spotted with domal hills of granite
rocks. Most of these, now exfoliated, are examples of lacoliths
or batholiths.
Lapolith, As and when the lava moves upwards, a portion of the same may tend
Phacolith and Sills to move in a horizontal direction wherever it finds a weak plane. It may
get rested in different forms.
 In case it develops into a saucer shape, concave to the sky body,
it is called lapolith.
 A wavy mass of intrusive rocks, at times, is found at the base of
synclines or at the top of anticline in folded igneous country.
Such wavy materials have a definite conduit to source beneath
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in the form of magma chambers (subsequently developed as

batholiths). These are called the phacoliths.
 The horizontal bodies of the intrusive igneous rocks are called
sill or sheet, depending on the thickness of the material. The
thinner ones are called sheets while the thick horizontal
deposits are called sills.
Dykes  When the lava makes its way through cracks and the fissures
developed in the land, it solidifies almost perpendicular to the
ground.
 It gets cooled in the same position to develop a wall-like
structure. Such structures are called dykes.
 These are the most commonly found intrusive forms in the
western Maharashtra area. These are considered the feeders
for the eruptions that led to the development of the Deccan
traps.

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