NeoStencil Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 … · 2020-05-25 · Pseudo Volcanic features 144 18. Hotspot Volcanism 148 Landforms 19. Fluvial Depositional Landforms
Post on 13-Aug-2020
7 Views
Preview:
Transcript
1
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
2
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
3
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Table of Content Topic Sub-Topic Page No
Movements of Earth
1. Interior of Earth 5
2. Endogenetic Movements 11
3. Exogenetic Earth Movements 17
4. Classification of Mountains 31
5. Fold Mountains 37
6. Block Mountains 51
7. Rocks and types of Rock 55
Plate Movement Theories
8. Continental Drift Theory 68
9. See Floor Spreading – Paleomagnetism 75
10. Plate Tectonics 82
11. Divergent Boundary 89
12. Earthquakes 93
13. Tsunami 105
Volcanism
14. Volcanism 116
15. Volcanic Landforms – Extrusive, Intrusive 128
16. Volcanism Types Based on Out Flow of Lava 136
17. Pseudo Volcanic features 144
18. Hotspot Volcanism 148
Landforms
19. Fluvial Depositional Landforms 157
20. Fluvial Erosional Landforms 170
21. Glacial Landforms 186
22. Karst Landforms 199
23. Marine Landforms 208
24. Arid Landforms 220
25. Lake – Classification of Lakes 241
4
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
5
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
1. Interior of Earth
Structure and Composition of the Earth
The interior structure of the Earth is made up of three main shells: the very thin and brittle crust, the mantle, and the core. Furthermore, the mantle and core are each divided into two parts. The core and mantle are equal in thickness but, the core of the earth only occupies 15 percent of Earth’s volume whereas the mantel occupies 84 percent and the crust occupies remaining 1 percent.
Sources of Information about Interior of the Earth
The earth’s radius is 6,370 km. It is rather difficult to make observations or collect samples of the material from inside of the earth due to its huge size and the changing nature of its internal composition. Only a part of the information is obtained through direct observations and analysis of materials. Most of our understanding about the interior of the earth is based on estimates and inferences.
6
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Direct Sources of Information
Surface rock – Surface rocks are the most readily available solid earth material to make direct observations. Laboratory experiments on surface rocks and minerals provide important information about the interior of the earth.
Mining – rocks that we get from mining areas are another source that gives us information about Earth’s interior. Through mining and drilling operations we have been able to observe the earth’s interior directly only up to a depth of few kilometres. World’s deepest mining is limited only to the depth of fewer than 5 kilometres. Going beyond this depth is not possible due to excessive heat at this depth.
Deep Ocean Drilling Projects – Scientists have undertaken some major projects to penetrate the surface of oceans to assess the conditions in crustal portions. The deepest drill at Kola, in the Arctic Ocean, has so far reached a depth of 12 km. This and many deep drilling projects have provided a large volume of information through the analysis of materials collected at different depths.
Volcanic Eruptions – Volcanic eruption forms an important source of obtaining direct information through laboratory analysis of the molten material (magma) that is thrown onto the surface of the earth, during a volcanic eruption. However, it is difficult to find out about the depth of the source of such magma.
Indirect Sources of Information
Meteors – Meteors are bits of interplanetary material falling through Earth’s atmosphere and heated to incandescence by friction. Meteors that at times reach the earth are an important source of information about the interior structure of the Earth. Although the material that becomes available for analysis from meteors do not from the part of the interior of the earth, they provide valuable information as the structure observed in meteors are similar to that of the earth.
Gravitation – The reading of the gravity at different places is influenced by many factors viz. distribution of mass, distance from the centre of the Earth. Such a difference is called gravity anomaly. Gravity anomaly gives us information about the distribution of mass of the material in the crust of the earth.
7
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Magnetic Field – Magnetic surveys provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
Seismic Activity – Seismic activity is one of the most important sources of information about the interior of the earth. Body waves, generated by an earthquake, especially S-waves, which travel only through solid material, have helped in understanding the interior structure of the Earth.
The Layers of the Earth
Earth’s interior is divided into basically three layers – Crust, Mantle and Core, which we shall discuss in detail as below:
The Crust
Due to its accessibility, its geology has been widely studied So, we have good understanding of the structure and composition of the
crust. The crust is the outermost layer of the earth. It is brittle in nature.
8
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The crust of the Earth has two distinct types: continental crust and oceanic crust.
These two types have different chemical compositions and physical properties and were formed by different geological processes.
The thickness and density of the crust vary under the oceanic and continental areas.
Oceanic crust is thinner as compared to the continental crust. The mean thickness of oceanic crust is 5 km. The mean thickness of the continental crust is around 30 km. It is much
thicker in the areas of major mountain ranges, extending up to 70 km in the Himalayan region.
Oceanic crust is denser as compared to the continental crust. Continental crust has the mean density of 2.7 g/cm3. It is mainly composed
of silicon and aluminium. Therefore, it is often termed as “sial.” The mean density of material in oceanic crust is 2.9 g/cm3. It is mainly
composed of basaltic rocks. The crust makes up about 1% of Earth’s volume.
The Mantle
Our knowledge of the upper mantle, including the tectonic plates, is derived from analyses of earthquake waves, heat flow, magnetic, gravity studies and laboratory experiments on rocks and minerals.
The portion of the interior beyond the crust is called the mantle. The mantle extends from Moho’s discontinuity to a depth of 2,900 km. It has an average density higher than that of the crust (3.4 g/cm3). The mantle is divided into upper and lower mantle. Asthenosphere – The upper portion of the mantle is called asthenosphere,
extending up to 400 km. The word “astheno” means weak. It is the main source of magma that finds its way to the surface during volcanic eruptions. It lies below the lithosphere.
Lithosphere – The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km. The lithosphere is subdivided into tectonic plates.
The lower mantle extends beyond the asthenosphere. It is in the solid state. Major constituent elements of the mantle are magnesium and silicon. Hence,
this layer is termed as “sima”. The mantle makes up about 84% of Earth’s volume
9
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The Core
The earthquake wave velocities have helped in understanding the existence of the core of the earth.
The innermost layer surrounding the earth’s centre is called core, which is about 3500 km in radius.
The core-mantle boundary is located at a depth of 2,900 km. The core consists of two sub-layers. The outer core is in the liquid state while
the inner core is in the solid state. The core is the densest layer of the earth. The density of material at the
mantle-core boundary is around 5 g/cm3, and at the centre of the earth at 6,300 km, the density value is around 13 g/cm3.
The core is made up of very heavy material mostly constituted by nickel and iron.
It is sometimes referred to as the “nife” layer. The core makes up about 15% of Earth’s volume.
Seismic Discontinuities
Seismic discontinuities aid in distinguishing divisions of the Earth into the inner core, outer core, lower mantle, upper mantle, and the crust
Conorad discontinuity – it refers to the zone between upper crust and lower crust.
Mohorivic discontinuity – also called as moho discontinuity is the zone that separates the Earth’s crust from the upper mantle. It can be detected by a sharp increase downward in the speed of earthquake waves there.
Repiti discontinuity – it refers to the zone between upper mantle and lower mantle.
Gutenberg discontinuity – It refers to the zone separating the lower mantle from the core. It is located at a depth of about 2,900 km.
Lehmann discontinuity – it refers to the zone separating solid inner core from the liquid outer core.
10
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Temperature, Pressure and Density of the Earth’s interior
The temperature increases towards the centre of the earth. However, the rate of increase of temperature is not uniform from the surface towards the earth’s centre. It is faster at some places than at others.
The temperature at the centre is estimated to lie somewhere between 3000°C and 50000C.
Such a high temperature inside the earth may be due to chemical reactions under high-pressure conditions and disintegration of radioactive elements.
The pressure also increases from the surface towards the centre of the earth due to huge weight of the overlying rocks.
Due to increase in pressure and presence of heavier materials towards the earth’s centres, the density of earth’s layers also goes on increasing. The materials of the innermost part of the earth are very dense.
11
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
2. Endogenetic Movements
Though the surface of earth appear to be static the interior of the earth is in a
dynamic state and this dynamism of the earth results in endogenetic movements.
The Movements of the earth
The surface of the earth is dynamic. It has been moving vertically and
horizontally.
Since the origin of the earth, there have been major changes in the distribution
of continents and oceans.
The earth has experienced innumerable earth movements which have brought
about vast changes in its surface.
The lithosphere is broken into a number of plates known as the Lithospheric
plates.
The movement and interaction of these plates cause changes on the surface of
the earth.
The forces which act in the interior of the earth are called as Endogenetic forces
and the forces that work on the surface of the earth are called as Exogenetic
forces.
In general terms, the endogenetic forces are mainly land building forces and the
exogenetic processes are mainly land wearing forces.
The actions of exogenetic forces result in wearing down (degradation) of relief.
But, the endogenetic forces continuously build up parts of the earth’s surface and
hence the exogenetic processes fail to even out the relief variations of the surface
of the earth.
So, variations remain as long as the opposing actions of exogenetic and
endogenetic forces continue.
12
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Geomorphic Processes
The endogenetic and exogenetic forces that bring about changes in the
configuration of the surface of the earth through physical and chemical actions
on earth materials are known as geomorphic processes.
Diastrophism and volcanism are endogenetic geomorphic processes.
Weathering, mass wasting, erosion and deposition are exogenetic geomorphic
processes.
Endogenetic geomorphic processes
The energy emanating from within the earth is the main force behind
endogenetic geomorphic processes.
This energy is mostly generated by radioactivity, rotational and tidal friction and
primordial heat from the origin of the earth.
This energy due to geothermal gradients and heat flow from within induces
diastrophism and volcanism in the lithosphere.
13
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Due to variations in geothermal gradients and heat flow from within, the action
of endogenetic forces are not uniform and hence the tectonically controlled
original crustal surface is uneven.
As explained above, diastrophism, volcanism and earthquake are the
endogenetic geomorphic processes
Endogenetic Movements
Endogenetic forces sometimes produce sudden movements and at the other
times produce slow movements.
Sudden movements like earthquakes and volcanoes cause mass destruction over
the surface of the earth.
While diastrophic movements are rather slow. Diastrophism refers to
deformation of the Earth’s crust, and more especially to folding and faulting.
Slow Movements
The movement which bring about changes in the Earth’s crust gradually taking
hundreds or thousands of years and which cover a period much longer than a
human life span are called slow movements. These movements act on the earth’s
crust either vertically or horizontally.
Diastrophism
Diastrophism is included within slow movements. All processes that move, elevate
or build up portions of the earth’s crust come under diastrophism. They include:
1. Orogenic processes
2. Epeirogenic processes
Through the processes of orogeny, epeirogeny, there can be folding, faulting and
fracturing of the crust. All these processes cause pressure, volume and
temperature (PVT) changes which in turn induce metamorphism of rocks.
14
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Epeirogenic Processes
Epeirogeny is a continental building process.
Due to epeirogeny, there may be mild deformations of the surface of the Earth.
Epeirogenic processes involve uplift or warping of large parts of the earth’s crust.
Epeirogenic or continent forming movements act along the radius of the earth;
therefore, they are also called radial movements.
Their direction may be towards (subsidence) or away (uplift) from the centre.
The results of such movements may be clearly defined in the relief.
Orogenic Processes
Orogeny is a mountain building process.
Orogenic processes involve severe folding and affecting long and narrow belts of
the earth’s crust.
In contrast to epeirogenic movement, orogenic movement is a more complicated
deformation of the Earth’s crust
Orogenic processes are associated with crustal thickening, notably associated
with the convergence of tectonic plates.
Orogenic processes may push deeply buried rocks to the surface.
If the orogeny is due to the colliding of the two continental plates, very high
mountains can result. E.g. Himalayas.
Sudden Movements
Contrary to the slow movements, there are certain movements which bring about
abrupt changes in the crust. The examples of such movements are volcanic
eruptions and earthquakes.
15
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Earthquake
Earthquakes refer to trembling of the earth surface due to a sudden release of
energy from within the earth’s interior along a fault line or weak zone.
Earthquake can occur at any time of the year, day or night. Its impact is very
sudden.
The place in the crust where the movement starts is called the focus. The place
on the surface above the focus is called the epicentre.
A sudden release of energy creates a vibration of the earth surface
Vibrations travel outwards from the epicentre as earthquake waves.
These earthquake waves create mass destruction on the earth surface.
The greatest damage is usually closest to the epicentre and the strength of the
earthquake decreases away from the centre.
Volcanism
A volcano is a vent in the earth’s crust through which molten material erupts
suddenly toward the earth’s surface.
Volcanism is responsible for formation of many intrusive and extrusive volcanic
forms.
Volcanoes are classified on the basis of nature of eruption and the form
developed at the surface. Major types of volcanoes include – Shield Volcanoes,
Composite Volcanoes, Caldera, Flood Basalt Provinces and Mid-Ocean Ridge
Volcanoes
Volcanic landforms
The solidification of the lava forms volcanic landforms either inside the surface of
the earth or outside its surface. When the lava is not able to reach the surface of
the earth it enters in the fissures of the earth and depending on the shape of the
fissure and its position with respect to the surface of the earth different types of
16
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
intrusive landform develop. Major intrusive forms include – Batholiths, Laccoliths,
Lopolith, Phacolith, Sills and Dykes.
Volcanic Landforms
Batholiths – A large body of magmatic material that cools in the deeper depth of
the crust develops in the form of large domes.
Laccoliths – These are large dome-shaped intrusive bodies with a level base and
connected by a pipe-like conduit from below. These are located at deeper depths.
Lopolith is a saucer shape intrusive form, concave to the sky body.
Phacolith – These are lense shaped intrusive mass formed by the solidification of
lava either on the crest of anticline or trough of the syncline.
Sills or Sheets – These structures are formed by the solidification of lava in a
surface parallel to the earth’s surface. The thinner ones are called sheets whereas
the thick ones are called sills.
Dykes – These are a wall-like structure formed by the solidification of lava
perpendicular to the surface of the earth.
17
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
3. Exogenetic Earth Movements
The natural forces which help in levelling of the earth’s surface by the process of
degradation, transport and aggradation are called exogenetic agents.
Exogenetic Movements
The surface of the earth is ever changing by the endogenetic and exogenetic forces.
The exogenetic movements on the earth’s surface gradually try to level the uneven
surface of the earth. The agents of weathering and erosion are continuously
involved in undoing the changes created by the endogenetic movements of the
earth.
Geomorphic Processes
The endogenetic and exogenetic forces that bring about changes in the
configuration of the surface of the earth through physical and chemical actions
on earth materials are known as geomorphic processes.
Diastrophism and volcanism are endogenetic geomorphic processes.
18
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Weathering, mass wasting, erosion and deposition are exogenetic geomorphic
processes.
Geomorphic agents
Any Exogenetic element of nature (like water, ice, wind, etc.,) capable of
removing, transporting and depositing earth materials can be called a
geomorphic agent.
Running water, groundwater, glaciers, wind, waves and currents, etc., can be
called geomorphic agents.
Exogenetic Geomorphic Processes
The exogenetic processes derive their energy from atmosphere determined by
the ultimate energy from the sun and also the gradients created by tectonic
factors.
As explained above, Weathering, mass wasting, erosion and deposition are
exogenetic geomorphic processes.
All the exogenetic geomorphic processes are covered under a general term,
denudation. The word ‘denude’ means to strip off or to uncover.
Weathering, mass wasting/movements, erosion and transportation are included
in denudation.
The basic reason that leads to weathering, mass movements and erosion is the
development of stresses in the body of the earth materials.
Force applied per unit area is called stress. Stress is produced in a solid by pushing
or pulling which induces deformations in the body.
Different kinds of stress are produced in the earth materials viz. sheer stress,
gravitational stress, molecular stress etc.
Exogenetic geomorphic processes are greatly influenced by climatic elements
such as temperature and precipitation. Hence, the exogenetic geomorphic
processes vary from region to region.
19
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Exogenetic geomorphic processes also depend upon type and structure of rocks.
As different types of rocks offer varying resistance to various geomorphic
processes.
Weathering
Weathering is the action of elements of weather and climate over earth
materials.
Weathering is defined as mechanical disintegration and chemical decomposition
of rocks through the actions of various elements of weather and climate.
Weathering processes are responsible for breaking down the rocks into smaller
fragments
Erosion cannot be significant if the rocks are not weathered. That means,
weathering aids mass wasting and erosion.
Weathering is a static process as very little, or no motion of materials takes place
in weathering, it is an in-situ or on-site process.
Weathering processes are conditioned by many complex geological, climatic,
topographic and vegetative factors.
20
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Types of the weathering process
There are three major groups of weathering processes. However, very rarely does
any one of these processes ever operate completely by itself. In reality, the
weathering process involves combinations of all three types of weathering
processes. These processes are:-
1. Chemical weathering processes
2. Physical or Mechanical weathering processes
3. Biological weathering processes
Chemical weathering processes
Chemical weathering causes rocks to decompose or dissolve and reduce them to
a fine clastic state through chemical reactions by oxygen, water or acids.
The mineral contained in the rocks undergo chemical changes when they get in
contact with atmospheric air and water.
Presence of Water, air (oxygen and carbon dioxide) and high-temperature help
in speeding up the weathering process.
Types of the chemical weathering process
There are different weathering process related to chemical action viz. hydration,
carbonation and oxidation. These weathering processes are interrelated and go
hand in hand and hasten the weathering process.
Solution
When something is dissolved in water or acids, the water or acid with dissolved
contents is called solution.
This process involves removal of solids in solution
The process of weathering through solution depends upon the solubility of a
mineral in water or weak acids.
21
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Soluble rock-forming minerals like nitrates, sulphates, and potassium etc. are
easily leached out without leaving any residue in the rainy climate.
Carbonation
Carbonation is the reaction of carbonate and bicarbonate with minerals
It is a common process helping the breaking down of feldspars and carbonate
minerals.
It takes place in rocks containing carbonates of calcium, sodium, magnesium,
potassium etc. when they come in touch with rainwater which contains dissolved
carbon dioxide.
Hydration
Hydration is the chemical addition of water.
Many rock minerals swell and contract during wetting and drying and a repetition
of this process results in their disintegration.
Salts in pore spaces undergo rapid and repeated hydration and help in rock
fracturing.
Oxidation
In weathering, oxidation means a combination of a mineral with oxygen to form
oxides or hydroxides.
Oxidation occurs where there is ready access to the atmosphere and oxygenated
waters.
In the process of oxidation, rock breakdown occurs due to the disturbance caused
by the addition of oxygen
The minerals most commonly involved in this process are iron, manganese,
sulphur etc.
22
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Reduction
When oxidised minerals are placed in an environment where oxygen is absent,
reduction takes place.
Such conditions usually exist below the water table, in areas of stagnant water
and waterlogged ground.
Physical Weathering Processes
The disintegration of rocks by some applied forces is called physical or mechanical
weathering.
These applied forces could be due to the action of gravity, heat and water.
Many of these forces are applied both at the surface and within different earth
materials leading to rock fracture
Most of the physical weathering processes are caused by thermal expansion and
pressure release.
These processes are small and slow but can cause great damage to the rocks
because of continued fatigue the rocks suffer due to the repetition of contraction
and expansion.
Types of the physical weathering process
Unloading
The process of unloading involves removal of overlying rock load because of
continued erosion
Unloading causes a release of vertical pressure on the rock resulting in expansion
of upper layers which further results in disintegration of rock masses
Due to disintegration fractures are developed in the rock mass, roughly parallel
to ground surface
In areas of a curved ground surface, rock fractures tend to produce Large, and
smooth rounded domes called exfoliation domes
23
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
An exfoliation dome
Temperature Changes
With the rise in temperature, every mineral expands, and as the temperature
falls, a corresponding contraction takes place.
Because of diurnal changes in the temperatures, there is a regular internal
movement among the mineral grains
These regular movements make the rocks weak due to continued fatigue and
cause fracture and further disintegration of rock masses
This process is most effective in dry climates and high elevations where diurnal
temperature changes are drastic.
Frost Weathering
Frost weathering occurs due to the growth of ice within pores and cracks of rocks
during repeated cycles of freezing and melting.
This process is most effective at high elevations in mid-latitudes where freezing
and melting is often repeated.
Rapid freezing of water causes its sudden expansion which causes joints, cracks
and small inter granular fractures to become wider and wider till the rock breaks
apart.
24
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Salt Weathering
Many salts in rocks like calcium, sodium, magnesium, potassium expand due to
thermal action, hydration and crystallisation.
Salt weathering causes splitting of individual grains within rocks, which eventually
fall off.
This process of falling off of individual grains may result in granular disintegration
or foliation.
Salt weathering is common in desert areas due to high-temperature ranges
Biological Weathering Processes
This refers to disintegration and decomposition of rock masses due to growth or
movement of organisms.
Burrowing and wedging by organisms like earthworms, termites, rodents etc.,
help in exposing the new surfaces to moisture and chemical attack causing their
decomposition.
Human activities like ploughing and cultivating cause mixing of air, water in the
minerals, thereby aiding in weathering process.
Decaying plant and animal matter help in the production of humic, carbonic and
other acids which enhance decay and solubility of some rocks.
Plant roots penetrate the cracks in the rocks and exert tremendous pressure on
the earth materials mechanically breaking them apart.
Mass Movements
These movements transfer the mass of rock debris down the slopes under the
direct influence of gravity.
The debris may carry with it air, water or ice.
The process of weathering aids in mass movements. Mass movements are very
active over weathered slopes rather than over unweathered materials.
25
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
No geomorphic agent like running water, glaciers, wind, waves and currents
participate in the process of mass movements.
Mass movements are aided by gravity
Mass movement is also aided by weak unconsolidated materials, thinly bedded
rocks, faults, steep slopes, abundant precipitation and torrential rains and
scarcity of vegetation etc.
Classification of Mass Movements
Heave (heaving up of soils due to frost growth and other causes), flow and slide are
the three forms of movements. The figure, given below shows the relationships
among different types of mass movements, their relative rates of movement and
moisture limits.
Types of Mass Movements
Mass movements can be grouped into two major classes:
1. Slow movements
2. Rapid Movements
26
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Slow movements
Creep
Creep refers to the movement of materials which is extremely slow and
imperceptible in normal conditions
Creep, generally occur on moderately steep, soil-covered slopes.
Depending upon the type of material involved, several types of creep viz., soil
creep, talus creep, rock creep, rock-glacier creep etc., can be identified.
Creep
Solifluction
Solifluction refers to slow downslope flowing soil mass or fine-grained rock debris
saturated or lubricated with water.
This process is quite common in moist temperate areas
Rapid Movements
Earthflow
Earthflow refers to the movement of water-saturated clayey or silty earth
materials down steep slopes
27
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
These movements are most prevalent in humid climatic regions and occur over
gentle to steep slopes.
Mudflow
In the region of sparse vegetation and heavy rainfall, thick layers of weathered
materials get saturated with water and flow down along definite channels.
It looks like a stream of mud within a valley.
Mudflows frequently occur on the slopes of erupting or recently erupted
volcanoes.
Mudflows can cause great destruction to human habitations
Avalanche
This is also a type of debris flow.
28
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Debris avalanche can be much faster than the mudflow.
Debris avalanche is similar to snow avalanche.
It is more characteristic of humid regions with or without vegetation cover
It occurs in narrow tracks on steep slopes.
Debris Avalanche
Landslide
Landslide involves relatively rapid and perceptible movements of the rock mass.
The materials involved are relatively dry.
The size and shape of the detached mass in the landslide depends on the nature
of discontinuities in the rock, the degree of weathering and the steepness of the
slope
Depending upon the type of movement, a landslide can take place either
by slump involving back rotation with respect to the slope or by rapid rolling or
sliding of earth debris without backward rotation, referred to as debris slide.
Similarly, sliding down of individual rock masses is referred to as the rock slide.
29
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Landslide
Erosion
Erosion involves acquisition and transportation of rock debris.
Erosion results in degradation of the surface relief i.e. wearing down of the
landscape.
It is erosion that is largely responsible for continuous changes that the earth’s
surface is undergoing.
When massive rocks break into smaller fragments through weathering and any
other process, erosional geomorphic agents like running water, groundwater,
glaciers, wind and waves remove and transport it to other places
Abrasion by rock debris carried by these geomorphic agents also aids greatly in
erosion.
Thus, weathering aids erosion, but it is not a pre-condition for erosion to take
place.
30
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Deposition
Deposition is a consequence of erosion.
Gradually, the erosional agents lose their velocity and hence, the materials
carried by them start to settle themselves.
The coarser materials get deposited first and finer ones later.
By deposition, depressions get filled up.
The same erosional agents, viz. running water, glaciers, wind, waves and
groundwater act as aggradational or depositional agents also.
31
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
4. Classification of Mountains
Mountains- An uplifted portion of the earth’s surface is called a hill or a mountain.
Difference between Hills and Mountains
When the summit or top rises to more than 900 m above the base, then it is termed
as Mountain while those with less than this elevation are called Hills.
Classification of Mountains
On the basis of their origin or mode of formation, the mountains are classified as
Structural or Tectonic
Residual or Dissected And
Volcanic
Structural Mountains- These mountain systems are hundreds of kilometres wide
and thousands of kilometres long. Many of them lie near or parallel to continental
coastlines. All great mountain systems of the earth are of this type. Both the fold
and the block mountains are included in this type.
Fold Mountains
Fold mountains are formed due to the folding of the crustal rocks by the
compressive forces which in turn are generated by endogenetic forces.
Folded mountains which have originated before tertiary period are called old fold
mountains. – Caledonian, Hercynian, Vindhyachal and Aravalis. These are also
called relict fold mountain because of denudation
Some new fold mountains are the Alps in Europe, the Rockies of North America,
the Andes of South America, the Himalayas of Asia and Atlas of North Africa.
These young fold mountains are still rising under the influence of the earth’s
tectonic forces. They are known for a variety of rock structures, deep gorges and
the high pyramidal peaks.
32
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The granitic core of such mountains is surrounded by metamorphic rocks,
merging with sedimentary layers along the margins.
The phenomenon of folding and faulting is most complex in the central areas of
these mountains.
The Urals, the Appalachians, the Tien Shan . and the Nan Shan were formed
during an earlier mountain-building period.
The highlands of Scotland and Norway and the Sayan and Stanovoy mountains in
Russia are of still earlier period.
Fold Mountains
Block Mountains
These mountains are formed when great blocks of the earth’s crust may be raised
or lowered during the late stages of mountain-building
During the uplift of structural mountains, sometimes magma flows upward into
the crust.
33
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
On its cooling and hardening beneath the surface, it contracts and the overlying
rock may crack into large blocks moving up or down.
An intense folding of rocks is generally followed by faulting of strata due to the
horizontal force of tension.
The land between the two parallel faults either rises forming block mountains or
horsts or subsides into a depression termed as a rift valley or
An old fold mountain may also be left as block mountains due to continuous
denudation. These mountains have flat tops, steep fault scarps and the subsided
portions between parallel fault are flat-bottomed.
The Vosges in France, Black Forest mountains in Germany and the Salt Range in
Pakistan are cited as typical examples of block mountains. Sierra Nevada of
California (USA); Wasatch range in the Utah province are also examples of Block
mountains.
River Rhine in Europe flows through a rift valley.
The Great Rift Valley of the world runs for about 6,000 kilometres from East Africa
to Syria through the Red Sea.
34
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Block Mountain
Volcanic Mountains
As these are formed by the accumulation of volcanic material, they are also
known as mountains of accumulation.
The matter is thrown out and deposited around the crater to form a mountain. If
the lava is thin and basic in its composition, it spreads a long distance forming a
flatter cone of gentler slope and of low elevation. If it is thick and of acid
composition, a small volcanic cone sharply pointing out is the result.
Sometimes lava is thrown out along with ash and cinders. Such a volcanic cone is
termed as ash and cinder cone.
Mount Mauna Loa in Hawai islands is an example of the former type.
Fuji Yoma of Japan and Mt Popa in Central Myanmar are examples of the latter
one.
35
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Volcanic Mountain
Residual or Dissected Mountains
They owe their present form due to erosion by different agencies.
That is why they are also known as relict mountains or mountains of
circumdenudation.
They have been worn down from previously existing elevated regions.
Hills like the Nilgiris, the Parasnath, the Girnar and Rajmahal in India are examples
of this type.
But Nilgiris got their present height as a result of subsequent uplift.
36
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
All mountains of the Peninsula with the exception of the Aravallis are relict
mountains
37
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
5. Fold Mountains
Fold mountains are the result of folding of the Earth’s crustal rocks by compressive
forces. They are considered as the “true mountains” and the term Orogenesis or
mountain building is commonly used for Fold mountains. Examples – Rockies
(North America), Andes (South America), Alps (Europe), Atlas(Africa), Himalayas
(Asia) etc.
Fold Mountain
In this article, we will discuss in detail about the various types of fold mountains,
their characteristics, location and touch upon the theories of fold mountain
formation.
Fig.- Compressive Forces leading to the folding of Earth’s crust. (Source –
Certificate Physical and Human Geography by G. C. Leong)
A Brief on Mountains in general
Mountains are natural elevated second order relief features (refer to Table 1) on
the Earth’s surface.
The Penguin Dictionary of Geography defines a mountain as “any natural elevation
on earth’s surface with a summit small in proportion to its base, rising more or less
abruptly from the surrounding level.”
38
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Based on their mode of formation, mountains can be classified into four main types
1. Fold Mountain
2. Block Mountain
3. Volcanic Mountain
4. Residual Mountain
Table 1: Classification of features on Earth’s surface
First order
relief Oceans and Continents Eg - Asia, Atlantic
Second
order
relief
Features on the oceans and continents due to
Endogenous Processes (caused by forces
from within the Earth)
Eg: Fold mountains,
Volcanic mountains, Rift
valley, Trenches
Third
order
relief
Features on the ocean and continents due to
Exogenous Processes (caused by forces on or
above the Earth's surface like wind erosion).
Eg: River valley,
waterfalls, Gorges,
Canyons etc.
Types of Fold Mountains
On the basis of Nature of Fold
1. Simple folded mountains – folds are arranged in waves like pattern with a well-
developed system of anticline and synclines.
2. Complex folded mountains – folds are complex in nature due to extreme
compressional forces like overfold, recumbent fold and nappe.
39
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Fig.- Types of Folding (Source – Certificate Physical and Human Geography by G.
C. Leong)
On the basis of the Period of Origin
1. Old fold mountains – Those mountains which originated before the Tertiary
period. These mountains have been so greatly denuded (or eroded) that they have
become residual fold mountains. For example, Aravalis, Appalachians etc.
40
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Table 2: Major mountain building phases
Era Mountain
Building phase
Age/Years
before present Examples
Pre-
Cambrian Archean
2500-3800
Million years Aravali Range in India
Pre-
Cambrian Proterozoic
570-2500 Million
years
Wopmay Orogen in
northwest Canada
Paleozoic Caledonian 320 Million years Scandinavian Highlands,
Scotland mountains
Paleozoic Hercynian 240 Million years Ural, Pennines,
Appalachians
Cenozoic Tertiary 65 Million years Alps, Rockies, Andes,
Himalayas
2. Young or New fold mountains – These are the fold mountains of the Tertiary
period. They are further subdivided based on their location
(i) Andean type of fold mountains – At the ocean – continental convergent
boundaries (C-O). They are prone to both earthquakes and volcanic activities.
Example – the Rockies, Andes
(ii) Himalayan type of fold mountains – At the continental – continental
convergent boundaries (C-C). There is an absence of active volcanism here.
Example – Great Himalayas
41
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Characteristics of Fold Mountains
Rock Type – Formed due to the folding of sedimentary rocks by strong
compressive forces. Furthermore, these rocks are of marine origin i.e. formed
due to deposition and consolidation of sediments in water bodies.
Shallow water deposits – The marine fossils found in the sedimentary rocks
belong to such organisms which can survive only in shallow waters.
Size – They are the loftiest, most extensive and elongated mountain chains on
the Earth’s surface. Their length is far greater than their width. For example, the
Himalayas have an east-west length of 2400 km but their maximum width is only
400 km.
Volcanicity – They may or may not have active volcanism but volcanic rocks
of ancient times may be found there. For example, Himalayas don’t have active
volcanism but volcanic rocks are found in Pir Panjal, Dalhauji (Himachal Pradesh)
and Bhimtal (Kumaun).
Earthquake – Generally the region is prone to earthquakes due to the presence
of active plate boundaries.
They are one of the youngest mountains of the world.
Generally found in an arc shape with one side having a concave slope and the
other having convex slope.
42
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Where are the Fold Mountains Located?
Preliminary analysis of the above world map shows that
They aren’t located randomly on the Earth’s surface.
Young fold mountains are generally located on the margins of the continents.
They are present mostly in the northern and western direction of the continents,
like the Atlas in Africa, the Rockies and the Andes in North and south America.
If we consider the former Tethys sea, then the Himalayas were also once located
along the margins of the continent.
They mark some of the major plate boundaries.
Old fold mountains are present inside the current continents. They represent the
ancient plate boundaries and orogenic movements of those times.
Fold Mountains of India
1. Himalayan mountains – Young fold mountains, formed during the Tertiary period.
43
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
2. Aravali Range – Oldest fold mountain of India, formed during the Archean period
(2500 million years ago). Erosion over time has reduced their size
3. Vindhyan Range – formed during a Proterozoic period (500 million years ago),
erosion over time has left them to
4. Satpura Mountains – meaning “seven folds”, they are the fold mountains of the
Precambrian era and are highly deluded.
5. Eastern Ghat – they were once fold mountains but have been eroded by the east-
flowing peninsular rivers.
Fold Mountain formation
Various theories have been proposed to explain the formation of fold mountains.
A good theory should be able to explain various unique characteristics of fold
mountains and their location.
Thermal Contraction Model of Harold Jeffrey
He proposed mountains as the wrinkles on the Earth surface formed when Earth’s
crust cooled and contracted while differentiating from other parts
He used an analogy of layer of cream separating from a hot milk vessel cooling
slowly.
Limitation
Cannot explain the variation in age of various mountains on the Earth’s surface.
It proposes same types of rocks and random distribution of mountains.
44
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure – Illustration of Harold Jeffery model, Showing layer differentiation and
surface features formation
Horizontal Displacement Theory by F.B.Taylor
According to him mountains formed due to equator side movement and collision
of two ancient continents (Laurasia and Gondwanaland).
Force causing movement – Gravitational and tidal pull of Sun and Moon.
Limitation
Can best explain only Transeurasian mountains (the Alps + the Himalayas) but not
other mountains
Incorrect reason for the movement of continents
45
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Continental Drift Theory by Alfred Wegener
It was an extension of Taylor’s displacement hypothesis.
He proposed mountains as an accumulation of oceanic sediments (SIMA)
scrapped by floating continents (SIAL) along the leading edge.
This helped explain the marine origin of sediments and their general presence on
the western and northern side of the continents.
Limitation
Wrong concept of SIAL and SIMA.
The mechanism suggested for movement (Tidal pull of Sun and Moon and the
pole-fleeing force) was wrong
Modern Theory of Plate Tectonics
According to Plate Tectonics mountains were formed due to colliding lithospheric
plates along the convergent boundaries.
The colliding plates compress, accumulates and uplifts sediments between the
two plates.
46
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Arthur Holmes’ concept of Mantle Convection Currents explains the driving force
behind the lithospheric plate movements.
At Oceanic-Continental (O-C) convergent plate boundary, the denser oceanic
crust gets subducted under the relatively lighter continental crust.
Figure – Ocean-Continental Convergence
The subduction causes lateral compressive force which ultimately squeezes and
folds the sediments.
At Continental-Continental (C-C) convergent plate boundary, the amount of
sedimentation is maximum resulting in the formation of the highest mountains
of the world.
C-C convergence is also associated with the strongest compressive forces, hence
the fold mountains here develop complex folds and reverse faults.
47
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure – Continental – Continental Convergence
Geosynclinal Theory of Mountain Building
Geosynclines are the long and relatively narrow depressions on the Earth’s
surface.
They are characterised by the continuous sediment accumulation, which causes
gradual subsidence of the floor of Geosyncline.
Figure – Sedimentation causing subsidence of Geosyncline Floor (Source –
Physical Geography by Savindra Singh)
48
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Geosyncline as a concept helps in explaining the origin of an enormous amount
of marine sediments that were uplifted into fold mountains.
Tethys sea was one of the major Geosyncline, which was uplifted to form the
Himalayan system of mountains.
Figure – Mountain Formation as per Kober’s Geosynclinal Theory (Source
– Physical Geography by Savindra Singh)
49
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Fold Mountains and Human Life
Fold mountains impact the climate, vegetation, lifeforms and human activities in
the area. A sudden increase in altitude presents a large variation in the climate of
the region. For example, in the Andes mountain, equatorial rainforest exist just
miles away from its snow-covered peak Cotopaxi.
Similarly, the Himalayan fold mountains are responsible for the unique climate of
the Indian subcontinent. They block the cold Siberian winds, preventing people
from harsh winters. Also, they are responsible for the orographic rainfall from the
south-eastern monsoonal winds.
Fold mountains have significant economic importance as well. These areas house
major tourist spots of the world. They are a pleasant holiday destination in
summers and provide an opportunity for adventure sports.
Their steep slope and melting water from glaciers provide huge potential for Hydro
Electric Power, which is a cleaner energy source than the coal-based thermal
power.
50
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Apart from this forest produce like timber, fuel-wood etc, agriculture activities
(limited to sun-facing slope) and mining materials are some of the major economic
benefits from these mountains.
However, fold mountains are prone to disasters both natural and man-made.
Almost all of them are prone to Earthquakes and many are also vulnerable to
Volcanic eruptions as well. Soft soil of these mountains makes these areas prone
to landslides as well in the event of heavy rainfall or earthquakes. Human activities
have further destabilized the balance of nature in the area and increased the
vulnerability in the region as seen during the Nepal earthquake, 2015 and
Uttrakhand mountain Tsunami of 2103.
51
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
6. Block Mountains
Block faulted mountains
fault block mountains definition
block faulted mountains or simply Block mountains are the mountains formed as a
result of faulting caused by the tensile and compressive forces caused by the
endogenetic forces coming from inside the earth. The faults are the cracks formed
in the Earth’s crust due to the endogenic forces. The uplifted blocks are known as
horsts, while the lowered blocks are termed as graben.
Topography of block faulted mountains are generally very smooth and they have
steep slopes. The folded zones of these mountains gradually lose their plastic
properties. The surface of these mountains become smooth due to denundations
due to various external forces.
block mountains examples
1. Block mountains are found all over the world in all continents. In Europe Block
mountains are found in the Vosges and the Black Forest mountains around
faulted Rhine Rift Valley. In North America, Block mountains are found in
California in the Sierra Nevada mountains.
2. The block mountains examples include the great African Rift Valley in Africa.
3. The block mountains examples in India includes the Satpura and Vindhya ranges
in the central western part of India. These are the block faulted
mountains formed due to the development of cracks in the crust of the earth.
52
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
fault block mountains diagram
fault block mountains diagram – 01
fault block mountains diagram – 02
Formation of Block mountains
Different theories have been given for the formation of Block mountains. The
important ones are tension theory, compression theory, and erosion theory.
53
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Tension theory for the formation of block faulted mountains
Tension theory suggests the existence of a weak point on the Earth’s crust which
experiences the tremendous amount of tensile force. This tensile force pushes the
side rocky crust. The side rocky crust is pushed down, while the central block
remains stagnant at its position along the point of tension. The central block thus
becomes higher in elevation than the side blocks leading to the formation of block
faulted mountains.
Compression theory for the formation of Block mountains
Compression theory suggests that the compressional forces set up due to the
movement of earth can compress the middle block of rocks, as the force acts
inward to a point. Due to these compressional forces, the middle block rises
forming the Block mountains. The central block remains at a higher elevation than
the side blocks.
In some cases, the compressional forces may produce a reverse fault and the
Central block may be lowered in relation to the surrounding blocks. In this case, the
surrounding blocks are higher in elevation and form Block mountains.
Erosion theory for the formation of Block mountains
Some geologists like J.F. Spurr, on the basis of their research, have given the
erosional theory for the formation of Block mountains. The erosional theory has
been given after the research in the great basin range mountains of United States
of America. According to this theory, after the origin of these mountains in the
Mesozoic era, these mountains were subjected to intense erosion. Due to the
differential erosion, it led to the formation of the denuded Great Basin Range block
mountains of USA. However, the erosional theory for the formation of these
mountains remains debatable among the scientists.
Types of Block mountains
The Block mountains have been categorized as the tilted Block mountains and the
lifted Block mountains.
Lifted Block mountains: In the lifted Block mountains, there are two steep sides
represented by two boundary fault scraps. Examples of lifted type Block
mountains are the Sierra Nevada and Teton mountains of North America.
54
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Tilted Block mountains: The tilted block mountains have only one exposed fault
scarp and one gentle slope. The basin range mountains of USA and Rhine valley
of Europe etc. have the features of tilted Block mountains.
The Block mountains have diverse species of flora and fauna throughout the world
due to the diverse range of topographic features, temperature, and rainfall.
55
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
7. Rocks and types of Rock
Rocks are the most common material on the Earth; they ordinarily lie everywhere
on the ground. They constitute most of the landforms, and there is a close relation
between rocks and landforms. The earth’s crust is composed of rocks. A rock is an
aggregate of one or more minerals that have been fused together into a solid lump.
For example, granite, a common rock, is a combination of the minerals
quartz, feldspar and biotite.
Properties of rocks
The rocks may be made up entirely of one mineral or various minerals.
Rocks do not have a definite composition of mineral constituents. Feldspar and
quartz are the most common minerals found in rocks.
Rock may be hard or soft. For example, granite is hard, soapstone is soft.
Rocks may have varied colours. Some rocks are dark and some are light coloured.
For example, Gabbro is black and quartzite can be milky white.
56
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Types of rocks
Rocks are classified according to characteristics such as mineral and chemical
composition, permeability, the texture of the constituent particles, and particle
size. In the long run, all types of rocks can transform from one type into another,
as described by the rock cycle model. These transformations usually take
thousands or millions of years.
There are many different kinds of rocks which are grouped into three families by
their mode of formation. They are:
1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks
Igneous Rocks
Igneous rocks are called as primary rocks as they are formed out of magma and
lava from the interior of the earth.
The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools
and solidifies.
Igneous rocks are classified based on texture. The texture of these rocks depends
upon arrangement and size of grains and other physical properties or condition
of the materials.
Most igneous rocks are extremely hard and resistant. For this reason, they are
quarried for road construction and polished as monuments and gravestones.
Igneous rocks do not contain fossils as their forming material are super hot
magmatic materials.
Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the
examples of igneous rocks.
57
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Formation of Igneous rocks
During the volcanic eruptions, the molten rock materials – Magma and Lava, find
their way to the surface.
When magma in its upward movement cools and turns into a solid form, it is
called igneous rock.
This process of solidification can happen on the surface of the earth or inside the
earth’s crust.
The rate of cooling of the magma determines the size of its grain structure.
Magma cooled on the surface are fined grained whereas magma cooled slowly
inside the earth surface have coarse grain structure.
Formation of Igneous rocks
Classification of Igneous rocks
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. Depending on the location of the cooling of the
lava, igneous rocks are classified into two types:
58
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
1. Volcanic rocks (Extrusive rocks) – cooling at the surface – extrusive rocks are
formed when molten magma, erupted from volcano, cools on the surface. The
magma on the surface (lava) cools faster on the surface to form igneous rocks
that are fine grained. Pumice and basalt are examples of extrusive igneous rocks.
2. Plutonic rocks (Intrusive rocks) – cooling in the crust – Intrusive igneous rocks are
formed when the magma cools off slowly under the earth’s crust and hardens
into rocks. Intrusive rocks are very hard in nature and are often coarse-grained.
Gabbro and granite are examples of intrusive igneous rocks.
Sedimentary Rocks
The word ‘sedimentary’ is derived from the Latin word sedimentum, which
means settling.
Sedimentary rocks are those that are formed through deposition and lithification
(compaction and cementation) of sediment particles at the Earth’s surface, with
the assistance of running water, wind, ice, or living organisms.
They may be coarse or fine-grained, soft or hard.
Sediments are naturally occurring particles derived from weathering and erosion
of pre-existing rocks.
The particles that form sedimentary rocks may be brought by winds, streams,
glaciers and even animals.
They are non-crystalline and often contain fossils of animals, plants and other
micro-organisms.
Sedimentary rocks are distinguished from other rock types in their characteristic
layer formation and are termed stratified rocks. The strata may vary in thickness.
Layers may be distinguished by differences in colour, particle size, type of
cement, or internal arrangement.
Classification of sedimentary rocks is done according to their age and different
kinds of rocks formed during the same period are grouped together.
59
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Most of the solid surface of our planet (roughly 70%) is represented by
sedimentary rocks.
Sandstone, Limestone and Shale are some examples of sedimentary rocks.
Weathering, Erosion and Deposition (W.E.D.)
Formation of sedimentary rocks
Sedimentary rocks are formed from sediments accumulated over long periods,
usually under water.
60
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
All types of rocks (igneous, sedimentary and metamorphic) of the earth’s surface
are exposed to denudational agents (water, winds, ice and glacier etc.) and are
broken up into various sizes of fragments.
Such fragments are transported by different exogenous agents and deposited or
settled down in low lying areas or underwater, through the process of
sedimentation.
These deposits finally turn into rocks through compaction and compression by
weight of overlying material. This process is called lithification.
In many sedimentary rocks, the layers of deposits retain their characteristics even
after lithification. That is why we can see a number of layers of different thickness
in sedimentary rocks like shale, sandstone etc.
Formation of sedimentary rocks
Classification of sedimentary rocks
Depending upon the mode of formation, sedimentary rocks are classified into three
major groups:
61
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
1. Mechanically formed — they are known as Clastic sedimentary rocks because of
their formation by accumulations of clasts: little fragments of broken up rock
material which have been piled up and “lithified” by compaction and
cementation. Conglomerate, sandstone, shale, limestone, loess etc. are some of
the examples of mechanically formed sedimentary rocks.
2. Organically formed— these rocks are formed by an accumulation of living
organisms such as corals or shellfish. These rocks have significant amounts of
organic material. The most famous rocks formed in this way are of the calcareous
type such as Chalk. However, carbonaceous rocks are also organically formed but
from vegetative matter. Geyserite, chalk, limestone, coal etc. are some examples
of organically formed sedimentary rocks.
3. Chemically formed — many of these forms when standing water evaporates,
leaving dissolved minerals behind. These sedimentary rocks form when mineral
constituents in solution become supersaturated and inorganically precipitate.
Chert, limestone, halite, potash etc. are some examples of chemically formed
sedimentary rocks.
Metamorphic Rocks
The word metamorphic means ‘change of form’ whereby ‘meta’ means change
and ‘morph’ means ‘form.’
Metamorphic rocks are formed due to the transformation of a pre-existing rock.
They are predominantly sedimentary or igneous rocks that have undergone
physical and chemical changes under the action of extreme heat and pressure.
Formation of metamorphic rocks can take place under different physical
conditions i.e. in different temperatures (up to 200 °C) and pressures (up to 1500
bars)
Their natural characteristics undergo a massive transformation in such extreme
conditions of temperature and pressure.
Metamorphic rocks make up a large part of the Earth’s crust and form 12% of the
Earth’s land surface
62
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Gneissoid, granite, syenite, slate, schist, marble, quartzite etc. are some
examples of metamorphic rocks.
Formation of Metamorphic rocks
Metamorphic rocks can simply be formed when the sedimentary or igneous rocks
moves deep inside the earth and come under the influence of high temperature
and pressure of the overlying material.
Formation of metamorphic rocks can take place through tectonic processes
including continental collisions, which may cause changes in pressure and
temperature.
The intrusion of magma on the earth’s surface is also an important cause of
metamorphic transformation.
Formation of metamorphic rocksMetamorphism is a process by which already
consolidated rocks undergo re-crystallisation and re-organisation of materials
within original rocks.
63
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Dynamic Metamorphism – Dynamic metamorphism refers to mechanical
disruption and reorganization of the original minerals within rocks due to breaking
and crushing without any significant chemical changes.
Thermal Metamorphism – Due to thermal stress the material of rocks undergo
crystal transformation which alters the chemical properties of the rock. There are
two types of thermal metamorphism.
1. Contact Metamorphism – In contact metamorphism the rocks come in contact
with hot intruding magma and lava and the rock materials recrystallise under high
temperatures. Quite often new materials form out of magma or lava are added
to the rocks.
2. Regional Metamorphism – In regional metamorphism, recrystallisation of rocks
takes place as a result of deformations caused by tectonic shearing along with
high pressure and temperature.
Foliation or Lineation – In the process of metamorphism in some rocks grains or
minerals get arranged in layers or lines. This arrangement of minerals or grains in
metamorphic rocks is referred to as lineation or foliation.
Banding – Sometimes minerals or materials of different groups are arranged into
alternating thin to thick layers appearing in light and dark shades. Such
arrangement in metamorphic rocks is commonly referred to as banding. Rocks that
display banding are called banded rocks.
Classification of Metamorphic rocks
Types of metamorphic rocks depend upon original rocks that were subjected to
metamorphism. Metamorphic rocks are classified into two major groups —
1. Foliated rocks – foliated rocks are formed where pressure squeezes or elongates
the crystals due to differential stress, they have a clear preferential alignment.
Rocks that were subjected to uniform pressure from all sides, or those that lack
minerals with distinctive growth habits, will not be foliated. For example, the
slate is a foliated metamorphic rock, originating from shale.
64
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
2. Non-foliated rocks – they are formed where the crystals have no preferential
alignment. Some rocks, such as limestone are made of minerals that simply don’t
elongate, no matter how much stress you apply
Rock cycle
Rock Cycle
Like most Earth materials, rocks are created and destroyed in natural cycles. Rocks
do not remain in their original form for long but may undergo a transformation.
The rock cycle is a model that describes the formation, breakdown, and
reformation of a rock as a result of sedimentary, igneous, and metamorphic
processes. Rock cycle is a continuous process transforming old rocks into new ones.
65
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic)
are formed from these primary rocks. Igneous rocks can be changed into
metamorphic rocks. Sedimentary rocks are formed from the fragments derived out
of igneous and metamorphic rocks. Sedimentary rocks themselves can turn into
fragments and the fragments can be a source for the formation of sedimentary
rocks.
In the subduction zones in the earth’s crust. The crustal rocks can get inside the
mantel where they melt and become a source of the igneous rock, thereby
completing the rock cycle.
Related terms
Mineral – a mineral is an organic or inorganic substance that occurs naturally on
the earth surface. They have an orderly atomic structure and a definite chemical
composition and physical properties. A mineral consists of two or more elements.
However, sometimes single element minerals like copper, sulphur, gold, silver,
graphite etc. can be found.
Magma and Lava – 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
Petrology – Petrology refers to the science of rocks. Under petrology, rocks are
studied with respect to all their aspects viz., mineral composition, structure,
texture, occurrence, origin, alteration and relationship with other rocks.
Weathering – Weathering is an action of elements of weather and climate over
earth materials. Weathering includes mechanical disintegration and chemical
decomposition of rocks through the actions of various aspects of weather and
climate.
Erosion – Erosion involves acquisition and transportation of rock debris. When
massive rocks are broken into smaller pieces through the process of weathering or
any other process, erosional geomorphic agents like groundwater, running water,
wind, waves and glaciers remove and transport it to other places depending upon
the dynamics of each of these agents.
66
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Diagenesis – Diagenesis is a process which includes the formation of sedimentary
rocks by compaction and cementation of grains, or by crystallization from water or
solutions and recrystallization.
Cementation – Cementation is the process by which clastic sediment is lithified by
precipitation of mineral cement, such as calcite cement, among the grains of the
sediment.
Compaction – compaction is a process through which the porosity of a given
sedimentary material is reduced as a result of its mineral grains being squeezed
together by the weight of overlying sediment.
67
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
68
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
8. Continental Drift Theory
To explain the present distribution of oceans and continents, various theories have
been proposed. Continental Drift Theory is considered a pivotal theory which
provided various conclusive proof to establish the movement of the earth crust but
the theory failed to establish the mechanism behind the process.
People for a very long time, till the early 20th century, thought that the continents
were fixed land masses. But it was in 1912, Alfred Wegener, a geologist came up
with the theory of continental drift. He further expanded the idea in his book “The
Origin of Continents and Oceans” which was published in 1915. Though Abraham
Ortelius, a Dutch cartographer was the first one to work on ideas of symmetric
coastlines on the sides of Atlantic ocean.
Continental Drift Theory
Continental drift means the movement of the continents across the ocean bed. This
drifting happens very, very slowly, over hundreds of million years!
According to Alfred Wegener, all the continents formed a single continental mass
known as Pangea (Pan=all + Gea=earth).
Pangea was surrounded by a mega-ocean, Panthalassa (Pan=all +
Thalassa=ocean).
Wegener further argued that about 225 million years ago, Pangea began to split.
It first broke into two large continental masses- Laurasia (the northern
component) and Gondwanaland (the southern component).
The intertwining part between Laurasia and Gondwanaland was known as Tethys
Sea, a shallow and meandering waterbody.
Subsequently, Laurasia and Gondwanaland continued to break into smaller
continents that we see today.
69
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing different stages of Continental Drift
Stages of Continental Drift
1. First stage – During the Carboniferous period in which a supercontinent Pangea
was surrounded by mega-ocean, Panthalassa.
2. In the second stage – Around 200 million years ago, Flight of continents took
place, continents began to drift gradually and broke into pieces, Laurasia
(Angaraland) and Gondwanaland. ( India was a part of Gondwanaland.)
3. In the third stage – During the Mesozoic era, the space between Laurasia and
Gondwanaland got filled with Tethys Sea and it gradually got widened.
4. Fourth stage – around 100 million years ago-Westward drift of North America
and South America led to the opening of Atlantic Ocean.
5. Fifth stage is the Orogenetic Stage-in which mountain building activity took
place. While Himalayas and Alps were formed with the folding of sediments of
Tethys Sea, and westward drift of North and South America led to folded edges
and formation of Rockies and Andes.
70
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Forces responsible for Continental Drift
Due to the cumulative effect of gravitational forces, ‘pole-fleeing force’ and force
of buoyancy, the continental drift was equator wards. The ‘pole-fleeing force’ is
due to increase in centrifugal force from poles towards the equator as the earth
is not perfectly round but there is a bulge at the equator.
Due to tidal currents resulting from rotation of the earth, the continental drift
was westwards. Tidal currents are due to the attraction of moon.
However, later, these two forces were found to be insufficient reasons for drifting
of the continents which is counted as the criticism of Wegener’s theory.
Evidence in support of the Continental Drift Theory
To justify his theory, Alfred Wegener came up with some evidence which are listed
below.
1. Jig-Saw Fit evidence or the Matching of continents– the shorelines of South
America and Africa when are facing each other shows a remarkable fit. Similarly,
Africa, Madagascar and east coast of India fit into each other when matched.
Figure showing jig-saw fitting of continents of Africa with South America
71
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
2. Rocks of Same Age across the Oceans– The radiometric dating methods have
correlated rock formation across different continents. It suggests that the belt of
ancient rocks formed 2,000 million years ago from Brazil coast matches the
mountain belt found in Western Africa. the Caledonian and Appalachian
mountains also show similarity. It also suggests that the early marine deposits
along the coastline of Africa and South America belong to the Jurassic age
indicating that the ocean did not exist prior to that time.
Figure showing matching mountain ranges across different continents
3. Tillite evidence- Tillite refers to the sedimentary rock made out of deposits of
glaciers. In revealing evidence, it has been found that the Gondwana system of
sediments from India has its counterparts in six various landmasses of the
Southern Hemisphere- Africa, Falkland Island, Madagascar, Antarctica and
Australia and India. It demonstrates that these landmasses had remarkably same
antiquities.
72
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing continuity of glaciers across different landmasses
4. Placer deposit- In the Ghana coast (West Africa), rich placer deposits of gold are
found. But there is a complete absence of source rock in the area. It is amazing
to know that the gold-bearing veins are in Brazil. It suggests that the gold deposits
of Ghana are derived from the Brazil plateau when the two continents lay side by
side.
5. Fossil evidence- The observations that Lemurs occur in Africa, Madagascar and
India suggest that a contiguous landmass “Lemuria” existed connecting the
three. Another amazing fact is that skeleton of Mesosaurus (a small reptile,
adapted to shallow brackish water) are found only in South Africa and Iraver
formations of Brazil.
73
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing fossils of similar species in different continents
Criticisms of the Continental Drift Theory
There are flaws in jig-saw fitting, though it seems appropriate if 500-fathom
Isobath is considered as an outline of the continents.
Wegener talks about the role of forces like buoyancy, tidal currents and gravity.
But these were not strong enough to drift continents.
He advocates directional movement either westward or equatorward but
movements have taken place in all directions.
Alfred Wegener failed to explain the Pre-carboniferous history. He did not explain
that why the drift began only in Mesozoic-era and not before.
The theory did not take oceans into consideration.
The theory did not explain the formation of oceanic ridges and Island arcs.
Earth’s crust is believed to be too rigid to permit large-scale motions. Wegener’s
ideas have not offered a suitable mechanism justifying the displacement of larger
masses for long journeys.
74
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Present status of the Continental Drift Theory
The Continental Drift Theory was not accepted by most of the scientific community
and was hotly debated off and on for decades even after his death in 1930. In the
1920s, the concept of conventional currents in the upper mantle was developed.
But Alfred Wegener was not able to incorporate the concept of conventional
currents as the most justifying reason for the movement of continents due to his
untimely death.
Although, the Continental Drift theory have become obsolete the main idea of the
theory of drift of continent was the driving force behind all other modern theories
including the theory of plate tectonics and seafloor spreading.
75
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
9. See Floor Spreading – Paleomagnetism
The theory of Sea Floor Spreading states that new oceanic crust is being formed
continuously at mid-oceanic ridges, while the older rocks move away from the
ridge. That is, it explains why the age, thickness, and density of the oceanic crust
increases with distance from the mid-oceanic ridge.
Sea Floor Spreading
In this article series, we are learning about the theories which explain the present
distribution of oceans and continents, the concepts which explain the distribution
of earthquakes and volcanoes, folds and faults. In the previous article, we have
learned about the Continental Drift Theory given by Alfred Wegener. Although
Wegener is right in stating that the continents drift over time, but his assumption
that the continents were blocks of rock that slid across the ocean floor has been
found to be incorrect. What we know today is that the continents and ocean floor
are part of the same layer of the earth.
In this article, we will learn about Sea Floor Spreading which explains continental
drift by the help of the theory of plate tectonics. But, before coming directly into
the concept of Sea Floor Spreading, we must understand some basic concepts that
explain Sea Floor Spreading.
These concepts are Ocean Floor Mapping, Distribution of earthquakes and
volcanoes, Convectional Current theory and Paleomagnetism.
Ocean Floor Mapping
The ocean floor is found to be having mountain ranges, plains, canyons, submarine
ridges, deep trenches and other relief features. It has also been found that along
the mid-oceanic ridges, volcanic eruptions are most active. Further, the dating of
rocks suggests that the oceanic crust rocks are much younger than the continental
rocks. Also, the rocks which are equidistant from the crest of oceanic ridges on both
sides have been found with utmost similarities in terms of their age, constituents,
chemical composition and magnetic properties. And the sediments on the ocean
floor near the ridge has been found to be thinner.
76
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing typical relief features of the ocean floor
Convectional Current Theory
Convectional current theory was proposed by Arthur Holmes.
First of all, it is to be noted that the heat which is generated from the radioactive
decay of substances deep inside the Earth (the mantle) creates magma which
consists of molten rocks, volatiles, dissolved gases among other material.
The Convectional current theory states that this magma, heat and gases seek a
path to escape which leads to the formation of convection currents in the mantle.
According to the theory of Seafloor spreading, convectional cells are the force
behind drifting of continents.
Also, please note that the ocean plates get subducted under the continental
plates (since ocean plates are denser than continental plates), when these two
types of plates converge.
The collision of plates is followed by earthquakes and volcanoes.
77
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The Distribution of Earthquakes and Volcanoes
Another observation is that, if we map the distribution of earthquakes and
volcanoes, it is surprising to know that volcanic activities, earthquakes and other
associated activities at plate margins are a result of convection currents in the
mantle.
Do observe in the map shown below, that the volcanic activity in the oceans is
almost parallel to the coastlines.
Since magma is less dense than the crust, it rises up to the oceanic crust
with convection currents, leading to the formation of a volcano.
78
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing ridges as the hot-spots of the earthquake and volcanic activity
Palaeomagnetism
Palaeomagnetism, in simple words, refers to the study of the earth’s magnetic
properties. The magnetic studies have revealed that the ocean floor consists of
parallel bands of oceanic crust which have alternating magnetic polarity. These
magnetic bands are symmetric and are mirrored around the mid-oceanic ridge.
Alternating magnetic polarity or the reversal of magnetic polarity refers to change
in the Earth’s magnetic field, that is, the north magnetic pole becomes south
magnetic pole and vice versa.
These alternating magnetic bands had happened because the new rocks which are
formed near the ridge, while the older rocks, which formed millions of years ago
when the magnetic field was reversed, have been pushed farther away.
Hence, this further explains the seafloor spreading.
79
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing parallel bands of oceanic crust with alternating magnetic polarity
around the mid oceanic ridge
The concept of Sea Floor Spreading
The concept of seafloor spreading was put forward by H.Harry Hess, an American
geologist.
He suggested that new sea-floor forms at the oceanic ridges and spread outwards
from the line of origin. Further, he claimed that continents would be pushed aside
by the same forces that cause the ocean to grow. That is, constant eruptions at the
crest of oceanic ridges cause the oceanic crust to rupture and new lava to
wedge out of it, pushing the oceanic crust on either side. The ocean floor thus
spreads.
80
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing sea-floor spreading
The intense heat generated by radioactive substances in the mantle beneath the
lithosphere seeks a path to escape and forms convection currents. The rising
convectional currents continuously bang thick but strong oceanic SIMA crust and
the hammering effect leads to the development of cracks, joints, fractures and
disintegration of oceanic ridges. Pyrospheric material oozes out and pushes the
older oceanic SIMA away from the rift zone. Seafloor spreading occurs at diverging
plate boundaries.
Upwelling of the magmatic material leads to the formation of mid-oceanic ridges
and substitution of older material by newer one. Successive eruption results into
seafloor spreading.
81
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The rate of movement is slow, 2.5 cms per year and resembles two giant conveyor
belts carrying sea floor from the zones of accretion(divergence area- mid-oceanic
ridge) to zones of consumption ( convergence area- trenches).
The theory of seafloor spreading solved many of the unsolved problems
1. It solved the problem of younger age crust found at the mid-oceanic ridges and
older rocks being found as we go away from middle part of the ridges.
2. It also explained why the sediments at the central parts of the oceanic ridges are
relatively thin.
3. The sea-floor spreading also proved the drifting of continents as propounded by
Alfred Wegener and helped in the development of the theory of plate-tectonics.
In the next article, we will learn about a related concept, the theory of plate-
tectonics.
82
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
10. Plate Tectonics
In this geomorphology study notes, we have till now covered concepts like
continental drifting, seafloor spreading and convectional current theory which
gives the explanation for many geomorphological features present on the earth.
But still, there remain some unanswered questions like- how do the fold mountains
form, what are the causes behind the occurrence of earthquakes, what are the
reasons behind volcanic activity on land and so on. For answering these and other
similar questions, we will learn about a very important concept in this article, that
is, the theory of Plate Tectonics.
Formulation of the theory of plate tectonics
With the emergence of the concept of seafloor spreading, and wealth of new
evidence at the beginning of the 1950s and 1960s, the interest in the problem of
distributions of oceans and continents was revived. Also, the following six
developments were instrumental in the formulation of the theory of plate
tectonics:
Development of mid-oceanic ridges and sea floor spreading
Palaeomagnetism
The findings of the age of ocean floors
Discoveries of island arcs and submarine trenches
The precise documentation of volcanoes and earthquakes, identification of
susceptible seismic zones, and spots vulnerable to volcanic activity
Identification of hotspots, their strengths, size and retrospective ejections.
What is a ‘tectonic plate’, ‘plate tectonics’ and ‘tectonic activity’?
The Earth consists of four concentric layers: inner core, outer
core, mantle and crust. The crust and uppermost of solid mantle are known as
lithosphere. Whereas asthenosphere is highly viscous, mechanically weak
and semi-molten region of the upper mantle of the Earth. And, lithosphere floats
over asthenosphere.
83
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Figure showing the structure of the earth
A tectonic plate is a massive, irregularly shaped slab of solid rock, generally
composed of both continental and oceanic lithosphere. Plates move horizontally
over the asthenosphere as rigid units.
On the basis of size, a tectonic plate may be a major plate or a minor plate. For
example, Pacific plate is a major plate whereas Nazca plate is a minor plate.
On the basis of nature, a plate may be referred to as continental plate or oceanic
plate depending on which of the two occupy a large portion of the plate. For
example, Pacific plate is mostly an oceanic plate whereas Eurasian plate may be
called as a continental plate.
While a tectonic plate is a rigid lithospheric slab, plate tectonics is a collective term
for evolution, nature and motion, deformation, the interaction of plate margins and
resultant landforms.
The earth’s crust is continuously experiencing movements in horizontal as well as
vertical direction resulting in breaking and bending of crustal rocks and this process
of deformation is known as the tectonic activity.
84
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Theory of Plate Tectonics
The theory of plate tectonics proposes that the earth’s lithosphere is divided into
seven major and several minor plates. The movement of the plates results in the
building up of stresses within the plates and the continental rocks above, which
leads to folding, faulting and volcanic activity. The major plates are surrounded by
fold mountains, ridges, trenches and faults.
These plates have been moving very slowly across the globe throughout the history
of the earth. Moreover, it may be noted that all the plates without exception, have
moved in the geological past, and shall continue to move in the future as well.
Alfred Wegener in his theory of continental drift had thought that continents move,
but, this is incorrect. He further believed that all continents were initially existent
as a super-continent, Pangea. However, later discoveries have revealed that
continental masses, resting on plates have been moving, and Pangea was a result
of the convergence of different continental masses that were part of one or the
other plates.
Figure showing seven major plates and some minor plates
The seven major plates are:
85
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
1. North American plate (with the western Atlantic floor separated from the South
American plate along the Caribbean islands)
2. South American plate (with western Atlantic floor separated from the North
American plate along the Caribbean islands)
3. Pacific plate
4. Antarctica and the surrounding oceanic plate
5. Eurasia and the adjacent oceanic platee)
6. Africa with the eastern Atlantic floor plate
7. India-Australia-New Zealand plate
86
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
While Pacific plate is the largest of them all, South American plate is the smallest.
Minor plates:
1. Carribbean Plate
2. Cocos Plate
3. Caroline Plate
4. Juan de Fuca Plate
5. Juan Fernandez micro Plate
6. Iranian Plate
7. South sandwich Plate
8. Myanmar Plate
9. Anatolian Plate
10. Nazca Plate
11. Nubian Plate
12. Philippines Plate
13. Okhotsk Plate
14. Scotian Plate
15. Eastern micro Plate
16. Somalian Plate
17. Arabian Plate
18. Solomon Plate
19. Fiji Plate
87
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
20. Bismarck Plate
The force behind the Movement of Plates
Previously, we have studied that intense heat is generated from the radioactive
decay of substances deep inside the Earth (the mantle) which creates magma
consisting of molten rocks, volatiles and dissolved gases. These produce
convectional currents when the magma, heat and gases seek a path to escape in
the mantle. The force behind the movement of the plates are these convectional
currents generated by the upwelling of hot magma which causes the overlying
lithospheric slabs to uplift and stretch.
Figure showing how convection currents play role in movement of plates
Rates of Plate Movement
The rate of plate movement is determined by the bands of normal and reverse
magnetic fields that parallel the mid-oceanic ridge. The rates of plate movement
have a considerable variation. For example, while the Arctic Ridge has the slowest
rate (less than 2.5 cm/yr), the East Pacific Rise in the South Pacific has the fastest
88
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
rate (more than 15 cm/yr). An interesting fact is that the movement of Indian plate
from south to equator was one of the fastest plate movements in history.
Importance of the theory of Plate Tectonics
For geologists, it is a fundamental principle for study. It is the unifying theory of
geology, which further explains large-scale geological phenomena, such as
earthquakes, volcanoes, and the existence of ocean basins and continents.
Plate tectonics theory explains why there are lots of volcanoes in Iceland and
Japan, but far fewer in Russia and Africa. This is because Iceland was created by
a mid-oceanic ridge. Similarly, Japan is located on a fault line. The constant
pressure around the fault line causes many earthquakes and volcanic eruptions.
For geographers, the theory of Plate tectonics aids in the interpretation of
landforms. It ultimately explains why and where deformation of Earth’s surface
occurs.
Further, the concept of plate tectonics explains mineralogy. New minerals pour
up from the core along with the magmatic ejections. The plate boundaries are the
pathways through which rocks from the mantle come out as deposits on
lithosphere. These rocks are the source of many minerals. The famous Pacific
Ring of fire known for its violent volcanic activity is also a ring of mineral deposits.
89
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
11. Divergent Boundary
The Earth’s crust is made up of 6 major plates; they are the Pacific plate, American
plate, Eurasian plate, African plate, Indo-Australian Plate and Antarctic plate along
with some minor plates. These plates float on the molten magma of plastic
asthenosphere. These plates form plate boundaries which are of three types
Convergent plate boundaries: in which the lithospheric plates move towards
each other.
Divergent plate boundaries: in which the lithospheric plates move away from
each other.
Transform plate boundaries: in which the lithospheric plates slide past one
another.
Divergent Plate Boundaries
Divergent plate boundaries can be defined as the locations where the lithospheric
plates move away from each other. Divergent plate boundaries occur above the
rising convection currents in the asthenosphere. These rising currents push up the
bottom of the lithospheric plate, and the magma flows laterally below it. This
lateral flow of the current drags the lithospheric plates in the direction of flow of
current. At the point of the crest of upliftment the lithospheric plate is stretched,
which breaks it and pulls the above plates apart, creating a divergent plate
boundary.
divergent plate boundaries
90
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Facts associated with divergent plate boundaries
According to the continental drift theory, all the different continents were part of
one giant supercontinent which was known as Pangea. Due to the plate tectonics
and rising Convection currents in the asthenosphere, the supercontinent Pangea
broke apart and the different continents emerged and they began their slow
migration towards their present locations. The divergent plate boundaries are the
places where the lithospheric crust is being extended, thinned and broken due to
extension stress. At the divergent plate boundaries, new lithospheric plates are
created while the old lithospheric plates get destroyed somewhere else at the
convergent plate boundaries.
The divergent plate boundaries and the continents initially produce rifts, which
ultimately becomes rift valleys. Most of the active divergent plate boundaries occur
between the oceanic tectonic plates. The divergent plate boundaries are also
associated with volcanic Islands which occur when the lithospheric plates move
apart from each other and produce gaps from which the molten lava comes out. At
the divergent boundaries, new crust is created as the Magma is pushed up from
the mental to the surface. The divergent plate boundaries are also associated with
earthquakes which strike along the rift.
Divergent plate boundaries – Oceanic
When the divergent plate boundaries occur below the oceanic lithosphere, the
rising convection currents from the magma lifts the lithospheric plate and produces
a mid-oceanic ridge. The lithospheric plate is stretched due to the extensional
forces and produce a deep fissure. When this fissure opens, the pressure on the
heated magma is reduced. New magma flows through this fissure, which comes out
and solidifies. This process is repeated in a cyclic manner.
Divergent plate boundaries examples include the mid-Atlantic Ridge. The area
around the ridge is higher compared to the surrounding sea floor, due to the uplift
of the oceanic plate by the convection currents below it. In these areas, the
divergent plate boundaries are associated with submarine mountain ranges such
as the mid-Atlantic ridge, volcanic activities in the form of fissure eruptions,
seafloor spreading and shallow earthquakes.
91
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Divergent plate boundaries location
Seafloor spreading
Divergent plate boundaries are associated with seafloor spreading whose evidence
has been found with younger oceanic crust near the ridges. The oceanic crust
becomes progressively older away from the spreading centre. There is a gradual
formation of new Oceanic crust close to the ridges and gradual spreading of these
oceanic plates over time. The seafloor spreading over the past 200 million years
has enlarged the size of Atlantic Ocean which has grown from a tiny inlet of water
between Europe, Africa, and the Americas into the vast ocean of present times.
Divergent boundaries – Continental
The divergent boundaries below the thick Continental plate are not vigorous
enough to form a clean single break through the continental plate. When the two
Continental plates are pulled apart, faults develop on both sides of the rift. The
central block of the plate slides downwards and earthquakes occur due to this
movement. In the early part of the rift forming process, streams and rivers flow
through this sinking rift valley which can form a long linear lake. If the rift grows
deeper, its level can fall below the sea level and the ocean water can enter inside
92
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
it. This can lead to the formation of a narrow sea inside the rift. If this rifting process
continues, new ocean basin could be formed in that place.
This type of divergent plate boundaries includes the East African Rift Valley, which
is in the early stage of development. The continental plate of that area has not been
completely rifted as the East African Rift Valley is presently above the sea level. The
example of the completely developed rift valley is the Red Sea, where the plates
have been fully separated and the central Rift Valley has dropped below the sea
level.
93
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
12. Earthquakes
An Earthquake is a tectonic movement caused by endogenetic (originating within the earth) thermal conditions inside earth’s interior which is transmitted through the surface layer of the earth.
What is an Earthquake?
It is shaking or trembling of the earth surface causing energy to release suddenly. An earthquake can range from a faint tremor to a wild motion capable of shaking building apart. Minor tremors caused by gentle waves of vibration within the earth’s crust occur every few minutes. Major earthquakes, usually caused by movement along faults, can be very disastrous particularly in a densely populated area.
Earthquakes themselves may cause only restricted damage in the regions of occurrence, but their aftershocks can be very catastrophic.An aftershock is an earthquake of the smaller magnitude that occurs after the main shock. They are also known to cause Tsunami waves.
Nearly 54% of land area in India is prone to earthquakes. Earthquakes are by far the most unpredictable and highly destructive of all the natural disasters. It not only damages and destroys the settlements, infrastructure etc. but also result in loss of lives of men and animals.
Focus and epicentre of an earthquake
Focus – The point within the earth’s crust where an earthquake originates is called the focus. It is also referred as seismic focus or hypocenter. It generally lies within the depth of 60 kilometres in the earth crust.
Epicentre – The point vertically above the focus on the earth’s surface is known as ‘epicenter’. Earthquake travel in the form of the longitudinal wave from the focus to epicentre. The intensity is the highest at the epicentre. That is why the maximum destruction occurs at and around the epicentre. The intensity of vibrations decreases as one moves away from the epicentre.
94
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Focus and Epicentre of an Earthquake
Causes of earthquakes
Plate Movements
Folding, faulting and displacement of rock strata, upwarping and downwarping of crust are some of the main causes of earthquakes. Some examples of this type of earthquakes are the San Francisco earthquakes of California in 1906, the Assam earthquakes of 1951, the Bihar earthquakes of 1935.
95
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Volcanic eruptions
The violent volcanic eruptions put even the solid rocks under great stress. It causes vibrations in the earth’s crust. But, these earthquakes are limited to the areas of volcanic activity such as the circum-pacific ring of fire and the Mid-Atlantic Ridge. An important example of this type of earthquake includes the earthquake preceding the eruption of Mauna Loa volcano of Hawaii Island in 1868.
Forces within the earth
Gaseous expansion and contraction within the earth can cause stress in strata which build up over time and is released suddenly in the form of earthquakes.
Exogenic forces
An earthquake may also be caused due to landslide and collapse of cave or mines etc. which can cause a sudden release in energy.
Man-made causes
These may range from dam building, mining, dredging, road building, drilling etc.
Types of earthquakes
Tectonic earthquakes – The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
Volcanic Earthquakes – A special class of tectonic earthquake is sometimes recognised as a volcanic earthquake. They are generated due to violent volcanic eruptions. However, these are confined to areas of active volcanoes.
Collapse Earthquakes – In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
Explosion Earthquakes – Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
96
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Reservoir-induced Earthquakes – The earthquakes that occur in the areas of large reservoirs are referred to as reservoir-induced earthquakes.
On the basis of focus, earthquakes can be classified as a shallow focus ( focus located at depth of up to 50 km), intermediate focus ( 50 – 250 km depth) and deep focus earthquakes (foci depth of up to 700 km).
Mechanism of tectonic earthquake
Tectonic earthquakes are the most common earthquakes occurring on earth surface.
They occur due to movement of tectonic plates past each other which builds up stress along the fault line.
A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. The friction between the plates locks them together due to which they
cannot glide past each other. However, at some point of time, their tendency to move apart overcomes
the friction and they slide past one another abruptly. This causes a sudden release of energy along the fault, and the energy waves
radiate in all directions.
Earthquake waves
Earthquake waves are energy waves, generated by the sudden release of energy from within the earth surface during an earthquake. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.
Earthquake waves are basically of two types — body waves and surface waves.
Body Waves
Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.
Travelling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake.
97
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
These waves are of a higher frequency than surface waves.
Surface Waves
The body waves interact with the surface rocks and generate a new set of waves called surface waves.
These waves move along the surface. The surface waves are the last to report on a seismograph. Surface waves are considered to be the most destructive waves. They cause displacement of rocks, and hence, the collapse of structures
occurs.
Types of Body waves
There are two types of body waves. They are called P and S-waves.
Earthquake Waves
P waves
These are also called ‘primary waves’. P-waves move faster and are the first to arrive at the surface. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials. P-waves vibrate “parallel” to the direction of the wave. This exerts pressure on the material in the direction of the propagation.
98
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Hence, it creates density differences in the material leading to ‘stretching’ and ‘squeezing’ of the material.
S waves
These are called secondary waves. S-waves arrive at the surface with some time lag. They can travel only through a solid medium. S-waves vibrate perpendicular to the direction of the wave in the vertical
plane. Hence, they create ‘troughs’ and ‘crests’ in the material through which they
pass.
Shadow Zone of earthquakes
When an earthquake occurs, earthquake waves radiate out spherically from the earthquake’s focus. Earthquake waves get recorded in seismographs located at far off locations.
A shadow zone is an area of the Earth’s surface where seismographs do not detect any earthquake waves.
For each earthquake, there exists an altogether different shadow zone. As P waves are refracted by the liquid outer core, the shadow zone of P-
waves appears as a band around the earth between 103° and 142° away from the epicentre.
S waves cannot pass through the liquid outer core and are not detected beyond 103°. Thus, the entire zone beyond 103° is referred as Shadow zone of S-waves
The shadow zone of S-wave is much larger than that of the P-waves. It is also a little over 40 percent of the earth surface.
99
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Measurement of earthquakes
The earthquake events are scaled either according to the magnitude or intensity of the shock.
100
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Richter scale – The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.
Mercalli scale – The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.
Earthquake-prone zones in India
Over 55% of the land area in India is vulnerable to earthquakes. Some of the most vulnerable states are Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, and the Darjiling and subdivision of West Bengal and all the seven states of the northeast.
Apart from these regions, the central-western parts of India, particularly Gujarat and Maharashtra have also experienced some severe earthquakes.
Bureau of Indian Standards, based on the past seismic history, grouped the country into four seismic zones, viz. Zone II, III, IV and V. Of these, Zone V is the most seismically active region, while zone II is the least. The current division of India into earthquake-prone zones does not use Zone I.
The Modified Mercalli (MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones, is as follows:
Seismic Zone Description Intensity on MM scale
Zone II Low intensity zone VI (or less)
Zone III Moderate intensity zone VII
Zone IV Severe intensity zone VIII
Zone V Very severe intensity zone IX (and above)
101
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Areas covered under different Seismic Zones
Zone – V covers 10.79% area of the country. It comprises entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Islands.
Zone – IV covers 17.49% area of the country. It comprises remaining parts of Jammu and Kashmir and Himachal Pradesh, National Capital Territory (NCT) of Delhi, Sikkim, Northern Parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.
Zone – III covers 30.79% area of the country. It comprises Kerala, Goa, Lakshadweep islands, remaining parts of Uttar Pradesh, Gujarat and West Bengal, Parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamil Nadu and Karnataka.
Zone – II covers 40.93% area of the country. It comprises major parts of peninsular region. Karnataka Plateau falls in this zone.
102
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Seismic Zone Map of India
Distribution of earthquakes in the world
The occurrence of an earthquake is a phenomenon of almost every part of the world. But, there are two well-defined belts where they occur more frequently. These belts are the Circum-Pacific belt and the Mid-world mountain belt.
The Circum Pacific Belt comprises the western coast of North and South America; the Aleutian Islands and island groups along the eastern coasts of Asia such as Japan and Philippines. As it encircles the Pacific Ocean from end to end, it is named as such. The earthquakes in this belt are associated with the ring of mountains and volcanoes. It is estimated that about 68 % of earthquakes of the world occur in this belt alone.
The Mid-world Mountain Belt extends from the Alps with their extension into the Mediterranean the Caucasus and the Himalayan region and continues into
103
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Indonesia. About 21 % of total earthquakes of the world originate in this belt. Remaining 11 % occur in the other parts of the world.
Consequences or effects of the earthquakes
Damage to property: when an earthquake occurs, all buildings from cottage to palaces and stronger skyscrapers are greatly damaged or totally destroyed. Earthquakes also cause a great deal of infrastructural damage. Underground pipelines and railway lines are damaged or broken. Dams on river collapse, resultant floods cause havoc.
Loss of lives: Duration of tremors of an earthquake is normally of only a few seconds, but thousands of people may die in this short period. More than 25,000 people died in Gujarat earthquake of 2001. Earthquakes also cause the death of wildlife and result in a destruction of their habitat.
Floods: Flood may result as an indirect consequence of an earthquake due to dam or levee failure.
Changes in river courses: Sometimes river channels are blocked or their courses are changed due to the impact of the earthquake.
Tsunamis: Tsunamis are extremely high sea wave caused by an earthquake. It wreaks havoc on settlement of coastal areas. It sinks large ships. The effect of a tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high. Tsunamis are waves generated by the tremors and not an earthquake in itself.
Soil liquefaction: Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits.
Cracks and fissures: Sometimes cracks and fissures develop in roads railway tracks, and fields, making them useless. The well known San Andreas Fault formed during the earthquake of San Francisco (California).
Landslides and Avalanches: landslides and avalanches may be triggered due to an earthquake.
104
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Fires: Earthquakes can cause fires by damaging electrical power or gas lines. it may also become difficult to stop the spread of a fire once it has started.
Prediction of earthquake
Earthquake can occur at any time of the year, day or night. Its impact is very sudden. There are no warning signs of earthquakes. Extensive and sincere research has been conducted in the forecast or prediction of an earthquake.
For the first time in India, a system to detect earthquakes and disseminate warnings was installed in Uttarakhand, in 2015. It issues warnings 1-40 seconds before earthquakes of magnitude 5 occur. All sensors under this system that warn of earthquakes are based on the detection of P and S waves generated during an earthquake. The P wave, which is harmless and travels faster than the S wave, is detected by the sensors for advance warning.
IIT Roorkee is conducting research to develop first of its kind sensors to be deployed in all seismic prone major cities in North India.
105
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
13. Tsunami
Tsunami and Its Causes
“Tsunami” meaning “harbour wave” in literal translation comes from the Japanese
characters for harbour (“tsu”) and wave (“name”). A tsunami also called seismic sea
waves, is one of the most powerful and destructive natural forces. It is a series of
extremely long waves caused by a large and sudden displacement of the ocean due
to earthquake, volcanic eruptions etc. When they reach the coast, they can cause
dangerous coastal flooding and powerful currents that can last for several hours or
days.
Global distribution of Tsunami
Globally, 70% of the confirmed tsunami sources have been in the Pacific Ocean, 9%
in the Caribbean Sea, 15% in the Mediterranean Sea and the Atlantic Ocean and 6%
Indian Ocean. Most of these Tsunamis were generated by earthquakes.
Tsunamis are frequently observed along the Pacific ring of fire, particularly along
the coast of Alaska, Philippines, Japan and other islands of South Asia and
Southeast Asia including Malaysia, Indonesia, Myanmar, Sri Lanka and India etc.
106
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Characteristics of Tsunami
Tsunamis are among Earth’s most infrequent hazards and most of them are small
and nondestructive
Tsunamis generally consist of a series of waves, with periods ranging from
minutes to hours
Tsunamis radiate in all directions from the point of origin and they can cover
entire ocean basins.
There is no season for tsunamis. We cannot predict where, when or how
destructive the next tsunami will be.
Not all tsunamis act the same. And, an individual tsunami may impact coasts
differently. A small tsunami in one place may be very large a few miles away.
Most tsunamis are caused by large earthquakes. Though, not all earthquakes
cause tsunamis.
107
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Tsunamis are waves generated by the tremors and not by an earthquake itself.
The effect of Tsunami would occur only if the epicentre of the tremor is below
oceanic waters and the magnitude is sufficiently high.
A tsunami can strike any ocean coast at any time. They pose a major threat to
coastal communities.
The speed of the wave in the ocean depends upon the depth of water. It is more
in the shallow water than in the ocean deep. As a result of this, the impact of a
tsunami is more near the coast and less over the ocean
Over deep water, the tsunami has very long wavelengths (often hundreds of
kilometres long) when a tsunami enters shallow water, its wave-length gets
reduced and the period remains unchanged, which increases the wave height.
Tsunamis have a small amplitude (wave height) offshore. This can range from few
centimetres to over 30 m height. However, most tsunamis have less than 3 m
wave height.
How is a tsunami different from a wind-generated wave?
Most ocean waves are generated by wind. Tsunamis are not the same as wind
waves. First of all, they have different sources. Also while wind waves only affect
the ocean surface, tsunamis move through the entire water column, from the
ocean surface to the ocean floor. Waves can also be described based on their
108
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
wavelength (horizontal distance between wave crests), period (time between wave
crests), and speed. These characteristics highlight the differences between wind
waves and tsunamis.
Tsunami Wind Wave
Source
Earthquakes, landslides, volcanic
activity, certain types of weather,
near earth objects
Winds that blow across
the surface of the
ocean
Location of
energy
Entire water column, from the
ocean surface to the ocean floor Ocean surface
Wavelength 100-500 Km 20-30 meters
Wave Period 5 minutes – 2 hours 5-20 seconds
Wave Height 10-30 meters from centimeters to
few meters
Wave Speed 800-900 Kmph (in deep water), 30-
50 Kmph (near shore) 10-100 Kmph
Causes of Tsunami
A tsunami is caused by a large and sudden displacement of the ocean. Large
earthquakes below or near the ocean floor are the most common cause. But
landslides, volcanic activity, near earth objects (e.g., asteroids, comets), certain
meteorological conditions and nuclear tests can also cause tsunamis.
109
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Earthquake – Tsunami can be generated when the sea floor ruptures abruptly due
to tectonic earthquakes, causing vertical displacement of the overlying water. Most
of the earthquakes which generate tsunamis occur on thrust faults. These
earthquakes occur mainly in the areas where tectonic plates move toward each
other in subduction zones.
As per data, ten to fifteen percent of the most damaging tsunamis are generated
by strike-slip earthquakes involving a horizontal movement of the earth.
Example – 2004 Indian Ocean Tsunami was an earthquake-induced Tsunami,
caused by an earthquake (Mw 9.2) in the Indian Ocean.
Landslides – “landslide” is a general term that involves the ground movement of
different types, including rock slide, block slide, debris flows, avalanches, and
glacial calving (referring to the breaking off of large pieces of ice from a glacier).
110
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Tsunamis can be generated when a landslide enters the water and displaces it. Such
generation of Tsunami depends on the amount of rock material that displaces the
water, the speed with which it is moving, and the depth it moves to.
Landslide-generated tsunamis may be larger than seismic tsunamis near their
source, but they usually lose energy quickly and rarely affect distant coastlines. A
landslide big enough to cause a transoceanic tsunami has not occurred in the
recorded history.
Example – 1998 Papua New Guinea Tsunami was generated by a landslide cause by
an earthquake.
Volcanoes – volcanoes generated Tsunamis are very infrequent, both above and
below water. However, different types of volcanic activity can displace enough
water to generate tsunamis e.g. submarine explosions, caldera formations etc.
Like other non-seismic tsunamis, such as those generated by landslides, volcanic
tsunamis usually lose energy quickly and rarely affect distant coastlines.
Example – 1883 Indonesia Tsunami was caused by the explosion of
Krakatau volcano.
Near Earth Objects – It is very rare for a near earth object like an asteroid or comet
to reach the earth and its potential to generate Tsunami is still uncertain, as there
are no records of a Tsunami caused by near earth objects, in recent human history.
However, scientists are of the opinion there are two ways near earth objects could
generate a tsunami.
Large objects (more than 1,000 meters in diameter) that make it through Earth’s
atmosphere without burning up could hit the ocean, displacing water and
generating an “impact” tsunami.
If this happens above the ocean, the explosion could release energy into the ocean
and generate an “airburst” tsunami.
Meteotsunamis – Some meteorological conditions, for example, air pressure
disturbances often associated with fast moving weather systems, can displace
bodies of water enough to generate Tsunamis. These “meteotsunamis” are similar
to tsunamis generated by earthquakes, but usually with lower energies.
111
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The development of these Meteotsunamis depends largely on the direction,
intensity of air pressure and ocean depth. Meteotsunamis are regional, and it is
found in some part of the world frequently due to regional factors such as
topography of earth’s surface both below and above the ocean.
Example – 2013 New Jersey, USA Tsunami caused by a high-speed windstorm
associated with thunderstorms.
Nuclear Weapon or tests – it is an example of man-made disaster. Massive
explosions created by a nuclear weapon of nuclear tests have the potential to cause
Tsunami. There have been dangers of using this as a tectonic weapon.
There has been considerable speculation on the possibility of using nuclear
weapons to cause tsunamis near an enemy coastline. In fact, In World War II,
the New Zealand Military Forces, in a failed attempt, tried to create small tsunamis
with explosives.
Effects of Tsunami
After the tsunamis reach the coast, an enormous energy stored in them is released
which causes colossal loss of lives as well as the infrastructure of the place. As the
port cities are economic hubs and densely populated the damage caused by the
tsunami is devastating.
The Tusanami of 2004 in the Indian Ocean is one of the devastating natural
disasters in the modern time. It took a toll of nearly 230000 people leaving in the
coasts of Indian Ocean.
Unfortunately escaping a tsunami is nearly impossible. Hundreds and thousands
of people are killed by tsunamis, most commonly by drowning, electrocution,
explosions from gas and collapsing of buildings etc.
Flooding and contamination of drinking water can cause disease such as Malaria
to spread in the tsunami-hit areas.
Tsunamis not only destroy human life, but also have a devastating effect on
animal and plant life and other natural resources. A tsunami changes the
landscape. It uproots trees and plants and destroys animal habitats.
112
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Contamination of soil and water is the second key environmental impact of a
tsunami.
There may be nuclear pollution due to radiation resulting from damaged nuclear
plants, as it happened in Fukushima, Japan in March 2011.
Tsunamis are extremely dangerous to coastal life and coastal property. They
produce unusually strong currents, rapidly flooding the land and causing great
damage to coastal property and life.
The flow and force of the water and the debris it carries can destroy boats,
vehicles, and buildings and other structures as the tsunami moves across the
land.
Victims of tsunami events often suffer psychological problems such as PTSD (Post
Traumatic Stress Disorder) which can last for days, years or an entire lifetime.
Massive economic costs hit communities and nations when a tsunami happens.,
severely affecting the economy of the nation.
Poor nations are more prone to large-scale destruction as the infrastructure are
not well developed, and warning systems are not robust or unavailable. Also,
their ability to cope with such massive disaster remains inadequate.
The water can be just as threatening (if not more so) as it returns to the sea,
taking debris and people with it. Flooding and dangerous currents can last for
days.
Tsunami Early Warning System
Tsunami is the most unpredictable natural disaster in the world and to prevent the
Tsunami is next to impossible. Hence, the only way to effectively mitigate the
impact of a tsunami is through an early warning system.
Tsunamis are detected in advance using a tsunami warning system (TWS) and early
warnings are issued to safeguard the life of people. It is made up of two equally
important components: a network of sensors to detect tsunamis and a
communications infrastructure to issue timely alarms to permit evacuation of the
coastal areas.
113
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
There are many regional and international early warning systems installed all across
the globe. National governments warn citizens through a variety of means,
including SMS messages, radio and television broadcasts, and sirens from
dedicated platforms, mosque loudspeakers and police vehicles with loudspeakers.
India had volunteered to join the International Tsunami Warning System after the
December 2004 tsunami disaster. The Indian Tsunami Early Warning Centre
(ITEWC) embedded with specific systems called Deep Ocean Assessment and
Reporting of Tsunamis (DART), established in 2007 at Indian National Centre for
Ocean Information Sciences, (INCOIS – ESSO) Hyderabad, autonomous body under
Ministry of Earth Sciences, is up and running to provide tsunami advisories for the
events occurring in the global oceans.
It has been recognized as one of the best systems in the world. The ITEWC includes
a real-time seismic monitoring network of seventeen broadband seismic stations
to detect tsunamigenic earthquakes and to provide timely warnings to the
vulnerable community. It also receives earthquake data from all other global
networks to detect earthquakes (of M>6.5).
Since its inception in October 2007, so far ITEWC has monitored 339 earthquakes
of M > 6.5. ITEWC also acts as one of the Regional Tsunami advisory Service
Provider (RTSP) along with Australia & Indonesia for the Indian Ocean region.
Way forward and recommendation
2004 Indian Ocean Tsunami was a brutal reminder of disaster preparedness in
India. While the current early warning system in India is state of the art, it is still
inadequate in terms of preparedness for Tsunami. Following suggestions can be
observed for enhancing India’s preparedness for future Tsunami events:-
Adopting integrated multi-hazard approach with emphasis on cyclone and
tsunami risk mitigation in coastal areas
Strict implementation of the coastal zone regulations
Plantation of mangroves and coastal forests along the coastline
114
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Identification of vulnerable structures and appropriate retrofitting for tsunami
resistance of all such buildings as well as appropriate planning, designing,
construction of new facilities
Capacity building programmes and public awareness campaigns should be held
at Tsunami prone areas
Streamlining the relief distribution system and evacuation plans in Tsunami
prone areas
Component of planning for reconstruction and rehabilitation should be added to
disaster management plans at all levels
Emphasis on mental health and to socio-psychological issues during post-disaster
period should be accorded in every plan
Conclusion
Tsunami is one of the most hazardous and unpredictable natural force. Tsunamis
have no seasons and they can occur at anytime and anywhere. We certainly cannot
prevent Tsunami. But what we can do is take necessary steps to minimize the
damage caused by it. Tsunami is a global and transnational event. Hence, it is
important that all countries across the world should join hands to evolve new
scientific ways to predict Tsunamis and to design mitigation strategies to cope with
this disastrous force.
115
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
116
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
14. Volcanism
The volcano is a narrow opening in the earth’s crust through which the molten rock
material, magma (lava), volcanic ashes are emitted outward through an eruption.
Such types of openings (vents) are found in those parts of the crust where the rock
strata are relatively weaker than the surrounding areas.
Volcanism
Volcanism refers to an exogenous activity which includes the formation of magma,
its upward movement, ejection of magma (lava) on the earth’s surface, and its
cooling and solidification.
volcano diagram 1 – Structure of Volcano
Magma refers to the molten rock material present inside the earth in the
asthenosphere. When this molten material comes out to the Earth’s surface
through an opening of a volcano, it is known as the Lava. The process through which
this molten material or magma comes out from asthenosphere to the Earth’s
surface is known as volcanism.
117
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Types of volcanic eruptions
Ejection of Lava to the surface occurs through either Fissure eruption or through
Central eruption.
Fissure eruption
In Fissure volcanic eruptions, the Lava comes out to the surface through the cracks
of the rock strata and hence the fissure eruptions are not much explosive. The
fissure eruptions are smooth and the Lava spreads to a larger area, so they form
landscapes such as plateaus etc.
volcano diagram 2 – Lava flow
Central eruption
In the central volcanic eruptions, the lava comes out to the surface through narrow
pipes and thus causes an explosion, during the ejection of magma onto the surface.
The explosive nature of eruptions leads to the formation of mountains which are
known as volcanic mountains. The different volcanic Islands throughout the world
are actually volcanic mountains formed through Central eruptions.
118
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Causes of volcanic eruptions
In the interior of the earth, the radioactive substances undergo chemical
reactions which generate a large amount of heat. Apart from this, some amount
of residual heat which was captured at the centre of the earth during its
formation is also present. This leads to a creation of large temperature difference
between the inner and Outer layers of earth.
This huge temperature difference leads to the formation of convection currents
in the outer Core and the Mantle. Due to this, the molten magma along with the
gaseous materials comes out to the earth’s surface at the first available
opportunity. This mainly occurs in the weak zones of earth surface such as
divergent plate boundaries, and convergent plate boundaries etc.
Sometimes, the earthquakes may expose the fault zones in the rock strata
through which the Magma can escape to the earth’s surface leading to volcanic
eruptions.
Types of Lava in volcanism
Acidic or Andesitic or composite Lava
The acidic or composite Lava is highly viscous and has a high melting point. It has
a high percentage of silica content, low density and light colour.
The acidic lava flows slowly and they rarely travel far before solidification. This
leads to the formation of the cone-like structure having steep sides.
Due to the rapid solidification of this acidic Lava, the openings obstruct the flow
of new Lava, which results in loud explosions and pyroclasts (volcanic bombs).
119
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
volcano diagram 3 – Cone-like Structure
Basic or Shield or Basaltic Lava
The basaltic or basic lava is highly fluid, and their temperature is about 1,000°C.
Basaltic lava is poor in silica, but are rich in Iron and manganese.
They have a dark colour and high fluidity. Due to their high fluidity, the basaltic
Lava is not very explosive, and they spread over great distances as thin sheets of
Lava.
The volcano formed by Basic Lava is gently sloping and they form a flattened
shield or dome with a wide diameter.
Active, dormant and Extinct volcanoes
The volcanoes erupting fairly frequently are known as active volcanoes. Kilauea
volcano of Hawaii, Grímsvötn volcano of Iceland and Etna volcano in Italy etc are
examples of active volcanoes which have been volcanoes erupting in the recent
past.
120
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Those volcanoes in which the eruption has not taken place regularly in the recent
past are known as dormant volcanoes. The volcanoes erupting after undergoing
long intervals of repose are the dormant volcanoes. The Fujiyama volcano of
Japan, Krakatoa volcano of Indonesia and the Narcondam island volcano of
Andaman and Nicobar islands are the examples of dormant volcanoes.
Extinct volcanoes are those where the volcanic eruption had taken place in
historic times but they are not active today. Mount Chimborazo in Ecuador,
Mount Kenya in Eastern Africa, and Popa in Myanmar etc. are the examples of
extinct volcanoes.
Negative effects of volcanic eruptions
Volcanic eruptions are a highly damaging natural disaster and are highly
destructive in nature. volcanic eruptions have been responsible for the
destruction of whole cities and towns by the advancing lava.
Violent earthquakes are associated with volcanic eruptions which have often
caused damage to life and property. The mudflows of volcanic ashes which get
saturated by rainfall can bury the nearby areas.
The earthquakes are associated with volcanism and in coastal areas, they can
cause tsunamis which have often caused the large destruction of life and
property.
Different gases released from volcanic eruptions such as carbon dioxide,
hydrogen fluoride, Sulphur Dioxide etc are hazardous to human life and
environment. The volcanic gases such as Sulphur Dioxide etc have also been
responsible for causing acid rain.
Large volcanic eruptions inject a large number of Sulphur aerosols in the
stratosphere which can lead to the lowering of surface temperature and increase
in the depletion of Ozone layer. The release of SO2 from volcanic eruptions has
been responsible for lowering of earth’s temperature leading to crop failures and
famines.
121
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Positive impacts of the volcanic eruption
Volcanic eruptions are responsible for the formation of new landforms such as
islands, plateaus, Volcanic Islands, and mountains etc. The volcanic lava, ash and
dust are very fertile for the cultivation of different plants. The weathering of
Volcanic rocks leads to the formation of fertile soil.
volcanic eruptions are also the source of mineral resources. They bring useful and
important minerals resources to the surface of Earth. For example, the diamond
mines of the kimberlite rocks of South Africa, are actually the part of an ancient
volcano.
The areas surrounded by the active volcanoes give rise to the formation of springs
and geysers. These springs and geysers can even be used for the generation of
geothermal electricity. The Yellowstone National Park of USA generates
electricity from the geothermal electricity. The Puga Valley of Ladakh in India is
also a promising spot for geothermal electricity.
The landforms formed by the volcanic eruptions are also great tourist spots and
have a great natural beauty. For example, the Yellowstone National Park of USA
is a great tourist spot.
Apart from these, Volcanic rocks are also used as raw materials for various
building and Engineering purposes etc.
Hot springs and Geysers
When the underground water which percolates down through the porous rocks
is subjected to the heat of the underlying Rock Strata which is in the contact of
hot magma it gives rise to geysers and hot springs.
When the water comes in contact with the intense heat of these rocks, it gets
heated and rises in the form of capillaries and narrow roots through the porous
rocks. When this heated water comes to the surface it undergoes expansion and
gets converted into steam leading to the formation of geysers and Springs.
122
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Diagram- Geyser
Geysers: when the heated water at high pressure comes out of the surface and
bursts into steam, it is known as Geysers. In most of the cases, a carter like
structure is formed at its mouth.
Springs: When the hot water comes out to the surface in a smooth manner it is
known as a spring.
Most of the world’s geysers are found in the areas of Iceland, New Zealand and
the Yellowstone National Park of USA. The hot springs and geysers of Japan and
Hawaii are great tourist attractions.
Geysers are found in very few regions, while the hot water springs are found all
over the world.
123
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
volcano diagram – Volcanic_hot_spring
Distribution of the volcanoes around the world
Till now, around 480 major active volcanoes have been found out of which
around 400 are found in the areas around the Pacific ocean. While the others are
in the Alpine Himalayan belt, Atlantic Ocean, Indian ocean etc. The Himalayas do
not have an active volcano.
The converging plate margins and the mid-oceanic ridges are the areas of high
volcanic activity and earthquakes. The volcanic zones and earthquake zones are
more prominent around the converging plate boundaries.
Pacific Ring of Fire
The circum Pacific region or Pacific Ring of Fire has the largest concentration of
active volcanoes. It has almost two-thirds of active volcanoes.
124
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The Aleutian islands of Kamchatka, Japan, the areas of Philippines, Indonesia,
Islands of Solomon, Tonga and North Island, New Zealand, the Andes to Central
America and up to Alaska are the part of Pacific rim of fire.
volcano diagram – volcanoes in pacific rim
volcanoes along the Atlantic coast
The Atlantic coast has a comparatively fewer number of active volcanoes. But it
has many dormant volcanoes such as Saint Helena, Cape Verde islands etc. The
volcanoes of Iceland and Azores along the Atlantic coast are active volcanoes.
Volcanoes in the Mediterranean region
The Alpine folds, such as Vesuvius, stromboli (also known as the Lighthouse of
Mediterranean) and the Aegean Islands are the areas of the Mediterranean
region where active volcanoes are found.
125
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Volcanoes in the great rift region
Mount Kilimanjaro and Mount Kenya of the East African Rift Valley have some
extinct volcanoes. Mount Cameroon is the only volcano active in West Africa.
Volcanoes in other parts of the world
Other regions such as West Indian Islands have experienced some volcanic activity
in the recent past. Mount Pelee of the Lesser Antilles is a volcanic Island where the
last eruption took place in 1929.
Volcanoes in India
The Barren Island of Andaman and Nicobar Islands which is in the northeast of
Port Blair is a volcanic island. The Barren Island volcano was the last active
recently in 2017 and in 1991 and 1995.
Narcondam which is in the north-east of Barren Island is another volcanic Island
in India. Narcondam volcano has not been active in the recent past. Other parts
of India do not have an active volcano.
Distribution of earthquakes
The distribution of earthquakes in the world coincides closely with that of
volcanoes. The Circum Pacific area, along with the Pacific Ring of Fire, is the
region of greatest systemic activity with most frequent occurrences of
earthquakes. These areas also have the most number of volcanic activity and
Volcanic Islands. Around 70% of world’s total earthquakes take place in the
Circum Pacific belt.
Around 20% of earthquakes take place in the Mediterranean Himalayan belt
which includes the Asia Minor, the Himalayas and the parts of northwestern
China. The earth’s crust is relatively stable in other parts of the world and they
are less prone to the threat of earthquakes. However, no place in the world can
be completely immune to the earthquakes.
126
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Some important volcanic eruptions
Mount Vesuvius in Italy
Mount Vesuvius is a composite volcano, which is around 4000 feet above the bay
of Naples. The volcano erupted in AD 79 and buried the city of Pompeii located
in the Southwest of this volcano. The volcanic ashes and the torrential rainfall
afterwards buried the city and killed its inhabitants.
Since the city was buried with the volcanic ashes, it was good infertility for the
cultivation of crops. This tempted farmers to begin a settled life on the slopes of
Mount Vesuvius. This volcano again erupted in December 1631 and destroyed
around 15 towns and killed their inhabitants.
Mount Krakatoa in the Sunda Strait
In August 1883, Mount Krakatoa located in the centre of the strait, between Java
and Sumatra, exploded violently; this was one of the greatest volcanic explosion
known to the mankind.
The Krakatau island was not inhabited, so nobody was killed due to the lava flows
and ashes. However, it set up the Tsunami waves of over hundred feet high which
submerged the coastal areas of Indonesia and drowned around 36000 people.
127
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mount Tambora, Indonesia in 1815
volcano diagram -Mount_Tambora_Volcano
It is one of the deadliest volcanic eruption in the recent human history which was
responsible for the death of around 120,000 people. The volcano erupted on 10
April 1815 and was the most powerful volcanic eruption in the last 500 years. It
sent volcanic ashes and gases like SO2 in the sky. It also led to the creation a series
of Tsunami waves.
Due to the emission of large amount of SO2, the world experienced large
temperature drop which was responsible for the crop failures in various parts of
the world. Thousands of people died due to starvation in China, while the price
of Grains quadrupled in Switzerland after 2 years of the volcanic eruption.
128
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
15. Volcanic Landforms – Extrusive,
Intrusive
The landforms formed due to the solidification of lava either inside or outside the
earth surface are known as volcanic landforms.
Extrusive and Intrusive Landforms
The landforms formed by the action of volcanoes and volcanic eruptions are
known as volcanic landforms. The geological processes control the characteristics
of various volcanic landforms. On the basis of cooling of magma, the volcanic
landforms are divided into extrusive igneous rocks landforms and intrusive
igneous landforms.
Plutonic rocks are formed when the magma cools within the earth’s crust.
The extrusive igneous rocks are formed when the cooling of Lava occurs above
the Earth’s surface.
Extrusive igneous rocks landforms
When the Lava and other volcanic materials are thrown out to the Earth’s surface
during volcanic eruptions, the extrusive igneous landforms are formed. It includes
volcanic Lava, pyroclastic debris, ash, volcanic bombs, and gases such as Sulphur
dioxide, nitrogen compounds and other gases.
The conical vent and fissure vent
The narrow cylindrical vent through which the lava flows out to the earth’s crust
during a volcanic activity is known as a conical vent. Conical vents are more
common in the composite (or strato volcanic) volcanic features.
The fissure is a narrow linear vent through which the lava comes out to the
earth’s crust during a volcanic eruption. The fissure vents are more commonly
found in the areas of basaltic volcanism.The fissure vents are often few meters
wide, which can be several kilometres long.
129
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Shield volcanoes
Shield volcanoes are characterized by gentle upper slopes and a little steeper
lower slopes. They are composed of relatively fluid lava flows which have been
built over a central vent. Mostly, the low viscosity basaltic lava which is high in
fluidity form Shield volcanoes. It leads to the formation of the extrusive igneous
rocks.
Shield volcanoes are mostly non-explosive, but they can become explosive if
water gets inside the vent.
Shield volcanoes are the largest volcanoes in the world. They extend to greater
heights and distances. Examples of Shield volcanic landforms include Mauna Loa
volcanoes of Hawaii.
Shield volcano
Cinder cone volcanoes
130
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
A Cinder cone has the features of a steep conical hill with loose pyroclastic
fragments which include volcanic clinkers, cinder, volcanic ash (scoria) around
the vent.
Cinder cone volcanoes are made entirely of the loose grainy cinders, and they
lack lava. Cinder cone usually has very steep sides along with a small crater on its
top. They are small volcanoes.
Cinder_cone_diagram
Composite volcanoes
Composite volcanoes (strato-volcanoes) are mainly cone shaped with moderate
steep Sites. The andesitic lava, along with the pyroclastic materials and ashes
which find their way to the ground gets accumulated in the vicinity of vent
openings. This leads to the formation of layers, which makes the volcanic mounts
appear as composite volcanoes.
131
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Composite volcanoes are also known as stratovolcanoes. Most common and
highest volcanoes have the features of composite cones. For example, Stromboli,
the Lighthouse of Mediterranean, mount Fuji etc.
Composite volcanoes are associated with the eruption of a cooler and more
viscous lava than the basaltic lavas. The composite volcanoes often cause
explosive volcanic eruptions.
Composite volcano
Flood basalt provinces (Lava plateaus)
When a very thin and fluid lava comes out to the Earth’s surface, and flow after
intervals for long periods of time, spreading to a large area; it produces a layered,
undulating- wave-like flat surfaces.
These types of extrusive igneous rocks and landforms are known as flood basalt
landforms or Lava plateaus. The Deccan traps of India, Snake basin of USA,
Canadian Shield etc. are the examples of Flood basalt provinces.
132
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mid-oceanic ridge volcanoes
The mid-oceanic ridges occur in the underwater oceans. There is a system of
70000 km long mid-oceanic ridges that stretch along all the major ocean basins.
The central portion of the mid-oceanic ridges is associated with frequent volcanic
eruptions.
The lava which comes out through these eruptions are Basaltic and have less silica
content, so they are less viscous. Due to less viscosity, they flow through longer
distances and cool slowly. This outpour of lava through volcanic eruptions is
responsible for the phenomenon of seafloor spreading.
mid-oceanic ridge
Caldera lake
When the Lava ceases to flow after the volcanic eruption, the creator of
volcanoes turns into a lake, which is known as Caldera lake. The rainwater and
snowmelt often get accumulated in these enclosed depressions leading to the
formation of lakes.
133
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Lonar in Maharashtra, Krakatoa in Indonesia, and Lake Caldera in southern
Oregon etc. are the examples of Caldera lakes.
Caldera lake
Intrusive volcanic landforms
The intrusive igneous rocks or plutonic rocks are formed when the Magma cools
within the earth’s crust and does not erupt to the surface. Various forms of
intrusive igneous rocks are formed due to the intrusive activity of volcanoes.
134
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Intrusive landforms
Batholiths
Batholiths are the intrusive igneous rocks masses formed due to the cooling and
the solidification of Magma inside the earth. These intrusive igneous rocks appear
on the surface after the erosional process erode the materials lying above these
rocks.
The batholiths form the core of large mountains, and they get exposed to the
surface after the erosional activities. Batholiths are granitic intrusive igneous
rocks.
Laccoliths
Laccoliths are the large dome-shaped intrusive igneous rocks which are
connected by a pipe-like conduit with the magma.
These intrusive igneous rocks resemble like a composite volcano structure, but
they are found below the earth’s surface. Example Karnataka plateau.
135
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Lopolith
Lopolith is formed when the Magma moves upwards, and a portion of this magma
moves in a horizontal direction where it finds a weak plane. When it develops into
saucer shape, it is known as Lopolith.
Phacolith
When a wavy mass of intrusive igneous rocks are formed at the base of synclines
or on the top of anticline having a definite conduit with the magma chambers
below, they are called the laccoliths.
Sills
Sills are the intrusive igneous rocks which are formed by the solidified and near
horizontal lava layers inside the earth. The thinner deposits of these rocks are called
sheets, while the thicker horizontal deposits are known as sills.
Dykes
When the Magma moves upwards through the cracks and fissures, and solidifies
almost perpendicularly to the earth’s surface, developing a wall like structure, they
are known as dykes. Dykes are the most common intrusive igneous rocks in
Western Maharashtra and other parts of Deccan traps.
136
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
16. Volcanism Types Based on Out Flow of
Lava
On the basis of outflow of lava, volcanism can be classified into four types-
Exhalative, Effusive, Explosive and subaqueous Volcanism.
Volcanism Types Based on Outflow of Lava
Exhaltive volcanism – vapour or fumes
Exhalative volcanism is characterized by the discharge of materials in the gaseous
form. It includes gases like steam, Sulphur Dioxide, carbon dioxide, carbon
monoxide, hydrogen, nitrogen, hydrogen sulphide, hydrochloric acid etc.
These gases can escape to the earth’s atmosphere from the vents which can be
in the form of hot Spring, geyser, solfataras, and fumaroles etc. The landforms
associated with exhaustive volcanism are sinter mounds, mud volcanoes and the
cones of the of the precipitated minerals etc.
Fumaroles
Effusive Volcanism – Lava outpouring
137
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
When the outflowing lava has low viscosity and high fluidity, various gases
coming out of the volcano can escape easily. The erupting magma forms lava
flows, and these eruptions are known as effusive volcanic eruptions.
Erupting Lava is poor in silica content such as basalt, and they flow through very
large distances. The Deccan traps are formed by the effusive outpouring of Lava.
Many parts of the Deccan trap has developed into the finely grained basalt
plateau.
Lava outpouring
Explosive volcanism – violent ejection of the solid material
If the Magma has a large amount of gases which get trapped inside it, the
pressure increases and builds until the magma erupts explosively out of the vent.
138
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The explosive eruptions can produce pyroclastic flows sweeping down the
valleys, which can destroy everything which comes under their path.
Explosive volcanism leads to the fragmentation and ejection of the solid materials
through the volcanoes. The various materials coming out of the vents of an
explosive volcanism are
Tephra, which includes all the fragmented rejects coming out of the
volcanoes.
Volcanic ashes, which are the finest and sand-sized tephra.
The gravel-sized particles which are in the molten or in the solid state are
known as Lapilli.
The Boulder-sized solid ejects are known as blocks.
When the lumps of Lava are thrown out of the volcano, they are known as
volcanic bombs.
The layers of the volcanic dust and ashes are known as tuff.
The lighter and smaller particles like the lapilli and ash can travel for large
distances and can remain suspended in the atmosphere for longer periods of
time. Volcanic bombs and blocks, which are heavier in weight fall near to the
vents of the volcanoes.
139
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Explosive eruption
Subaqueous volcanism
Subaqueous volcanism takes place in the underwater oceans and seas, and other
water bodies. In the underwater ocean floors, the outflowing lava comes in the
contact of water and consolidates to produce structures, which are like the heap
of pillows. These types of structures belonging to the Precambrian age have been
found in the Karnataka region.
When the outflowing lava is highly viscous and erupts at lesser depths, they
develop a structure of glassy margins on the pillows leading to the formation of
hyaloclastite. Hyaloclastites are most commonly found in Iceland.
140
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
pillow lava
Negative effects of Volcanism
Volcanic eruptions release gases like Sulphur dioxide, carbon dioxide, carbon
monoxide etc in the atmosphere. These gases are hazardous for the people,
agriculture, and environment.
The sulphur dioxide released from volcanic eruptions can cause acid rain which is
harmful to humans as well as biodiversity and environment. The release of
Sulphur aerosols in the stratosphere can lower the surface temperature leading
to the depletion of Ozone layer.
141
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Volcanic_injection
Classification of volcanism based on the mode of eruptions
Hawaiian eruption: in the Hawaiian eruption, the basalt lava comes out in the
form of effusive outpouring from the craters or fissures. In a single flow, the lava
spreads over wide areas, or it flows down the valley is like lava rivers. The great
basalt plateaus of Colombia is its example.
Strombolian eruption: in the Strombolian eruption, more viscous lava has
erupted like a fountain at the regular intervals of about 15 minutes. The
Stromboli volcano is located on the Lipari island near Italy.
Vulcanian Eruption: this volcanic eruption is associated with a short, violent and
a relatively smaller eruption of viscous lava. During volcanic eruptions, volcanic
bombs, blocks and ashes etc are ejected in the surrounding areas. The volcano
142
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
becomes dormant for several decades and even for centuries after each eruption
cycle.
Pelean eruption: it occurs as a result of the flow of very viscous, gas-rich and
acidic lava erupting violently over the crater rim. The lava and gases do not move
upwards toward the sky but spread downslopes as a nuee ardente.
Icelandic type eruption: it is characterized by the outflow of molten basaltic lava
flowing from the long and parallel fissures and often leading to the formation of
Lava plateaus.
143
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Counterclockwise: a Plinian eruption column, Hawaiian pahoehoe flows, and a
lava arc from a Strombolian eruption.
144
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
17. Pseudo Volcanic features
Those topographic features that resemble the volcanic landforms but are non-
volcanic in origin are known as pseudo volcanic features. The pseudo volcanic
features include meteorite crater, mud volcanoes, and salt plugs.
Pseudo Volcanic features
Craters
A crater is a depression usually circular formed by either the extrusion of volcanic
material or by the impact of any meteorite.
A rootless cone is a pseudo creator which resembles a real volcanic crater, but it
is not of volcanic origin. It does not have an actual volcanic vent from which the
lava has erupted. The rootless cone is characterized by the absence of the magma
conduit which connects the crater to the Magma chamber below the Earth’s
surface.
The meteorite craters are formed by the impact of falling meteorites from the
space on earth. They resemble like a crater lake; for example, the Lonar lake in
Buldhana is an example of meteorite crater lake which has been formed as a
result of the impact of a giant meteorite. Other examples of meteorite craters
include Siberian crater, Shiva crater on the Mumbai Offshore basin etc.
Apart from these, the craters developed due to the anthropogenic activities are
also referred as pseudo volcanic features. It includes the craters formed by the
explosion of bombs, mine blasts, etc. which resemble the features of a crater.
145
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Lonar_Meteorite_Crater
Salt plugs or salt dome
The salt plug is a dome-like structure formed by the stratified rocks which contain
the central core of salt, which has been formed by the upward movement of the
salt deposits. Under high pressure, the salt deposits deform plastically and
deform and pierce the overlying sediments like an intrusive landform.
Salt extrusions can take the form of salt hills having many features of plug domes
or lava cones having peaks and sinkholes, which are visually similar to the volcanic
craters formed due to subsidence.
146
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Salt_Dome
Mud volcanoes
A mud volcano is a landform formed by the eruption of mud, water, and gases
etc. Mud volcanoes can be formed due to several geological processes. The mud
volcanoes are not the true volcanoes as they do not produce Lava and are not
necessarily driven by the activities of magma. Thus they can be classified under
the pseudo volcanic features.
The Mud volcanoes have usually been found in the subduction zones around the
world. They mostly release methane gas along with smaller quantities of Nitrogen
and carbon dioxide etc.
Some mud volcanoes are non-volcanic in origin. The volatile hydrocarbons which
have been given off from the petroleum-bearing beds can cause mud eruptions,
which can be called as mud volcanoes. These kinds of mud volcanoes have been
found at Baku along with the Caspian sea, in Burma, and in southern Balochistan
etc.
147
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mud_Volcano
148
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
18. Hotspot Volcanism
A volcano is a rupture in the crust of the Earth from where gases, ashes and molten
rock material – magma – escape to the ground. A volcano is called an active volcano
if the materials mentioned are being released or have been released out in the
recent past. Pacific Ring of Fire has one of the maximum numbers of active
volcanoes in the world. Most of the volcanoes are found underwater.
Hotspot Volcanism
Volcanoes are found where tectonic plates are diverging or converging. The vast
majority of earthquakes and volcanic eruptions occur near plate boundaries or
margins, but there are some exceptions. For example, the Hawaiian Islands, which
are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more
than 3,200 km from the nearest plate boundary.
Hotspot Volcanism refers to this intra-plate volcanism, which describes a volcanic
activity that occurs within tectonic plates. The position of these hotspots on the
Earth’s surface is independent of tectonic plate boundaries.
Hotspot volcanism is unique because of its occurrence. It does not occur at the
boundaries of Earth’s tectonic plates, where all major volcanic activity takes place.
Instead, it occurs within the plates at abnormally hot centres known as mantle
plumes.
149
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Hotspot Volcano
What is a Hotspot?
“Hotspot” refers to an area in the Earth’s mantle from where hot plumes rise
upward, forming volcanoes on the overlying crust.
A hotspot is fed by a region deep within the Earth’s mantle from where these
mantle plumes rise through the process of convection.
The heat from mantle plumes facilitates the melting of rock at the base of
the lithosphere, where the brittle, upper portion of the mantle meets the
Earth’s crust.
High heat and lower pressure at the base of the lithosphere (tectonic plate)
facilitates melting of the rock.
This molten material (rock), called magma, rises through cracks in the crust and
erupts to form volcanoes.
Hot-spots are relatively fixed in comparison to the plates
As the tectonic plate moves over the stationary hot spot, the volcanoes are rafted
away and new ones form in their place, resulting in chains of volcanoes, such as
the Hawaiian Islands.
150
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mantle Plumes
Hotspot volcanism occurs at abnormally hot centres known as the mantle plume
A mantle plume is an upwelling of abnormally hot rock within the Earth’s mantle,
first proposed by Tuzo Wilson in 1963.
In 1971, geophysicist Jason Morgan further developed the hypothesis of mantle
plumes. In this hypothesis, convection in the mantle transports heat from the
core to the Earth’s surface in thermal diapirs.
These mantle plumes are almost like lava lamps, with a rising bulbous head fed
by a long, narrow tail that originates in the mantle.
As the plume head reaches the lithosphere, it spreads into a mushroom shape
that reaches roughly 500 to 1000 kilometres in diameter. These features are
called diapirs.
When the head of a plume encounters the base of the lithosphere, it undergoes
widespread decompression. As a result, melting takes place, and large volumes
of basalt magma are formed which, finds its way to the earth surface when an
explosion takes place.
Layers of Earth
151
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mantle Plume
Distribution of Hotspots
Because of differing definitions of what a hot spot, there is also diverging opinions
about the numbers of hotspots in the world. Forty to fifty hotspots are thought to
exist around the world, although this number varies greatly. Major hot spots in the
world include the Iceland hotspot, under the island of Iceland in the North Atlantic,
the Reunion hotspot, under the island of Reunion in the Indian Ocean etc.
152
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Distribution of Hotspot across the globe
Wilson’s hotspot theory
“Hotspot” theory was given by J. Tuzo Wilson, the Canadian geophysicist in 1963.
Wilson in his study found that in certain locations around the world, such as
Hawaii, volcanism has been active for very long periods of time.
Based on this he gave the idea of “Hotspots” referring to small, long-lasting, and
exceptionally hot regions which existed below the plates and provided a localized
source of high heat energy (mantle plumes) to sustain volcanism.
This led to a new hypothesis by Wilson that the distinctive linear shape of the
Emperor Seamount chain in the Hawaiian Islands resulted from the movement of
the Pacific Plate over a deep, stationary hotspot in the mantle, located below the
current position of the Hawaiian Island.
Heat emerging from this hotspot produced a perennial source of magma by partly
melting the overriding Pacific Plate.
The magma, being lighter than the surrounding solid rock rises through the
interior of the earth to erupt onto the seafloor, forming an active seamount.
153
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Over time, a large number of eruptions cause the seamount to grow until it finally
emerges above sea level forming an island volcano.
He suggested that continuing plate movement eventually carries the island
beyond the hotspot. Hence the source of magma is cutoff and volcanism ceases.
As one island volcano cease to exist, another develops over the hotspot, and the
cycle is repeated.
This process growth and death of volcano, over many millions of years, has left a
long trail of volcanic islands and seamounts across the Pacific Ocean floor.
According to Wilson’s hotspot theory, the farther the volcanoes of the Hawaiian
chain travel beyond the hotspot, the older and more eroded they get.
Cycle of Volcanism
Interestingly, a volcano located above a hot spot does not erupt forever.
154
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Attached to the tectonic plate below, the volcano moves and is eventually cut off
from the hot spot.
Without any source of heat, the volcano becomes extinct and cools. The plate
beneath the volcano (and above the hot spot) also cool.
This cooling causes the rock of the volcano and the tectonic plate to become
denser
The volcano and the plate gradually subside as they move away from the hot
spot.
As the volcano subsides below sea-level, the top is eroded flat by waves.
In time, new and active volcanoes are developed over the hot spot, and this cycle
of volcanism goes on.
Even giant volcanoes, like Mauna Loa on Hawaii, will eventually disappear into
the ocean.
Hotspot Features
Seamounts – Volcanic activity at hot spots can create submarine mountains
known as seamounts. Depending on the amount of volcanic activity, seamounts
can rise hundreds or thousands of meters from the seafloor.
Chain of Islands – Hotspot seamounts that reach the surface of the water can
create entire chains of islands, such as the U.S. state of Hawaii.
Hot spots can also develop beneath continents, for example, The Yellowstone
hotspot, U.S.A
Geysers – Hotspots don’t always create volcanoes that spew rivers of lava.
Sometimes, the water and steam have erupted like a volcano from within the
earth surface due to heating up of the groundwater by the magma. These
eruptions are called geysers. A famous geyser is Old Faithful in Yellowstone
National Park.
155
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Hotspot tracks – While a plume that feeds hot spot volcanoes remains stationary
relative to the mantle, the plate above it usually moves. The result is that a chain
of progressively older volcanoes is created on the overlying plate. Pacific ocean
has some of the best examples of such “hot spot tracks”.
Reunion Hotspot
The Reunion hotspot is a volcanic hotspot
Presently, it’s lying under the Island of Reunion in the Indian Ocean.
The hotspot is believed to have been active for over 66 million years.
About 66-68 million years ago present-day India was above the hot spot
Deccan traps, a vast bed of basalt lava that covers part of central India is thought
to have been formed by a huge eruption of this hotspot 66 million years ago.
The Laccadive Islands, the Maldives, and the Chagos
Archipelago are atolls, resting on tracks created by Reunion Hotspot.
As the plate moved in the northeastern direction more volcanic centres were
formed: the Mauritius Islands from 18-28 million years ago, the Mascarene
Plateau 40 million years ago, the Chagos Ridge 48 million years ago and the
Maldives from 55-60 million years ago.
The youngest volcanoes, Piton de la Fournaise and Piton des Neiges, were formed
in the last 5 million years. Piton de la Fournaise is one of the most active
volcanoes on the Earth surface.
156
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
157
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
19. Fluvial Depositional Landforms
In geography, fluvial processes are associated with rivers and streams and the
deposits and landforms created by them. Landforms are small to medium tracts or
parcels of the earth’s surface.
There are two types of landforms created by the fluvial process – Fluvial Erosional
Landforms and Fluvial Depositional Landforms. Fluvial Depositional landforms are
made by river sediments brought down by extensive erosion in the upper course of
the rivers.
Fluvial Depositional Landforms
Rocks and cliffs are continually weathered and eroded in the youth stage or upper
course of the river.
The river moving downstream on a level plain brings down a heavy load of
sediments from the upper course.
The decrease in stream velocity in the lower course of the river reduces the
transporting power of the streams which leads to deposition of this sediment
load.
Coarser materials are dropped first and finer silt is carried down towards the
mouth of the river
This depositional process leads to the formation of various depositional
landforms through fluvial action such as Delta, Levees and Flood Plain etc.
Alluvial Fans
An alluvial fan is a cone-shaped depositional landform built up by streams, heavy
with sediment load.
Alluvial fans are formed when streams flowing from mountains break into foot
slope plains of low gradient.
158
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Normally very coarse load is carried by streams flowing over mountain slopes.
This load gets dumped as it becomes too heavy to be carried over gentler
gradients by the streams
Furthermore, this load spreads as a broad low to a high cone-shaped deposit
called an alluvial fan that appears as a series of continuous fans.
Alluvial fans in humid areas show normally low cones with a gentle slope from
head to toe and they appear as high cones with a steep slope in arid and semi-
arid climates.
159
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
An alluvial fan at the mouth of Copper Canyon, Death Valley, California
Floodplains
Floodplain is a major landform of river deposition.
Deposition develops a floodplain just as erosion makes valleys.
Rivers in the lower course carry large quantities of sediments
Large sized materials are deposited first when stream channel breaks into a
gentle slope.
Sand, silt and clay and other fine sized sediments are carried over gentler
channels by relatively slow-moving waters
During annual or sporadic floods, these materials are spread over the low lying
adjacent areas. A layer of sediments is thus deposited during each flood,
gradually building up a floodplain
In plains, channels shift laterally and change their courses occasionally leaving
cut-off courses which get filled up gradually by relatively coarse deposits.
The flood deposits of spilt waters carry relatively finer materials like silt and clay.
Active Floodplain – A riverbed made of river deposits is the active floodplain.
Inactive Floodplain – The floodplain above the bank is an inactive floodplain.
Inactive floodplain above the banks basically contains two types of deposits —
flood deposits and channel deposits.
Delta plains – The floodplains in a delta are called delta plains.
Natural Levees
This is an important landform associated with floodplains.
They are found along the banks of large rivers.
160
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
They are low, linear and parallel ridges of coarse deposits along the banks of
rivers on both sides due to deposition action of the stream, appearing as natural
embankments.
At the time of flooding, the water is spilt over the bank. As the speed of flow of
the water comes down, large sized sediments with high specific gravity are
dumped along the bank as ridges.
They are high nearer the banks and slope gently away from the river.
Generally, the levee deposits are coarser
When rivers shift laterally, a series of natural levees can form.
Artificial embankments are formed on the levees to minimize the risk of the
floods.
But sudden bursts in the banks due to the pressure of water can cause disastrous
floods.
An example of such flood can be seen in Hwang Ho river which is also called
“China’s sorrow”.
Floodplain, Natural Levee, Point Bars
161
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Point Bars
Point Bar is also associated with floodplain
Point bars are also known as meander bars.
A point bar is a depositional feature
It is formed by alluvium that accumulates in a linear fashion on the inside bends
of streams and rivers below the slip-off slope.
They are found on the convex side of meanders of large rivers.
They are almost uniform in profile and in width and contain mixed sizes of
sediments.
Long and narrow depressions can be found in between the point bars where
there is more than one ridge
Rivers build a series of them depending upon the water flow and supply of
sediment.
As the point bars are built by the rivers on the convex side, erosion takes place
on the concave side of the bank.
Meanders
In large flood and delta plains, rivers rarely flow in straight courses. Loop-like
channel patterns called meanders develop over flood and delta plains
Normally, in meanders of large rivers, there is active deposition along the convex
bank and undercutting along the concave bank.
If there is no deposition and no erosion or undercutting, the tendency to
meander is reduced.
The concave bank is known as a cut-off bank which shows up as a steep scarp
and the convex bank presents a long, gentle profile and is known as the slip-off
bank
162
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Meander growth and cut-off loops and slip-off and undercut banks
Factors responsible for meandering of the rivers
1. The propensity of water flowing over very gentle gradients to work laterally on
the banks
2. Unconsolidated nature of alluvial deposits making up the banks with many
irregularities which can be used by water exerting pressure laterally
3. Coriolis force acting on the fluid water deflecting it like it deflects the wind
163
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
A
satellite scene depicting meandering Burhi Gandak river, showing a number of
oxbow lakes and cut-offs
Oxbow Lakes
In the lower course of a river, meanders become very much more pronounced
As meanders grow into deep loops, the same may get cut-off due to erosion at
the inflexion points and are left as independent water bodies, known as ox-bow
lakes.
Through subsequent floods that may silt up the lake, oxbow lakes are converted
into swamps in due course of time. It becomes marshy and eventually dries up
164
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Formation of Oxbow Lakes
Braided Channels
A braided channel consists of a network of river channels divided into multiple
threads and separated by small and often temporary islands called ‘eyots’.
Braided channels are commonly found where water velocity is low and the river
is heavy with sediment load
Deposition and lateral erosion of banks are essential for the formation of the
braided pattern.
There is the formation of central bars due to selective deposition of coarser
material which diverts the flow towards the banks causing extensive lateral
erosion
As the valley widens due to continuous lateral erosion, the water column is
reduced and more and more materials get deposited as islands and lateral bars
developing a number of separate channels of water flow.
165
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Braided Channels
Deltas
Deltas are fan-shaped alluvial areas, resembling an alluvial fan
This alluvial tract is, in fact, a seaward extension of the floodplain
The load carried by the rivers is dumped and spread into the mouth of the river
at sea. Further, this load spreads and piles up as a low cone
Unlike in alluvial fans, the deposits making up deltas are very well sorted with
clear stratification. The coarsest sediments are deposited first and the finer
sediments are carried out further, into the sea.
Deltas extend sideways and seaward at an amazing rate
As the delta grows, the river distributaries continue to increase in length and
Delta continues to build up into the sea.
166
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Some deltas are extremely large. For example, the Ganges delta is as big as the
whole west of Malaysia
Satellite image of Ganges Delta
Types of Deltas
There are great variations in size, shape, growth and importance of Deltas. A great
number of factors influence the eventual formation of deltas such as depth of the
river, sedimentation, sea-bed, character of tides, waves and currents etc. owing to
these factors several types of deltas can be found.
Bird’s foot delta – It’s a kind of delta featuring long, stretching distributary
channels, which branch outwards resembling the foot of a bird. Deltas that are less
subjected to wave or tidal action culminate to a bird’s foot delta. Example – the
Mississippi River has a bird’s foot delta extending into the Gulf of Mexico
167
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mississippi River, Birdfoot delta
Arcuate delta – Arcuate is the most common type of delta. This is a fan-shaped
delta. It’s a curved or bowed delta with the convex margin facing the sea. Arcuate
deltas have a smooth coastline due to the action of the waves and the way they are
formed. Examples – The Nile, Ganges and Mekong river deltas
168
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Nile river, Arcuate delta
Cuspate delta – A few rivers have tooth-like projections at their mouth, known as
the cuspate delta. Cuspate deltas are formed where the river flows into a stable
water body (sea or ocean). The sediments brought down by the rivers collide with
the waves. As a result, Sediments are spread evenly on either side of its channel.
Example – Ebro river delta in Spain
169
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Ebro river, Cuspate delta
Estuarine delta – some rivers have their deltas partly submerged in the coastal
waters to form an estuarine delta. This may be due to a drowned valley because of
a rise in sea level. Example – Amazon river delta
Conditions favourable for the formation of delta
Active vertical and lateral erosion in the upper course of the river to provide
extensive sediments to be eventually deposited as deltas
The coast should be sheltered preferable tideless
The sea adjoining the delta should be shallow or else the load will disappear in
the deep waters
There should be no large lakes in the river to ‘filter off’ the sediments
There should be no strong current running at right angles to the river mouth,
washing away the sediments
170
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
20. Fluvial Erosional Landforms
In geography, fluvial processes are associated with movement and energy
associated with rivers and streams, and landforms created by them. Landforms are
small to medium tracts or parcels of the earth’s surface.
There are two types of landforms created by the fluvial process
1. Fluvial Erosional Landforms
2. Fluvial Depositional Landforms
The removal and transport of sediment due to fluvial processes result in erosional
landforms. In this article, we will discuss erosional landforms of Fluvial Process.
Fluvial Erosional Landforms
Unlike other geomorphic agents like wind and ice etc., which are confined to
certain areas, the effect of running water is felt all over the globe wherever water
is present. Thus running water forms the most potent geomorphic agent for
denuding the Earth’s surface through erosion.
Different aspects of Fluvial Erosive Action
In rivers, erosion and transportation go on simultaneously. There are different ways
in which fluvial erosion takes place, such as:-
Corrasion or abrasion – Corrasion is a process of mechanical erosion of the earth’s
surface caused by mechanical grinding of the river’s traction load (coarser material)
against the bed and banks of the river. There are two distinct ways in which
Corrasion can take place:
1. Lateral Corrasion – Lateral Corrasion takes place sideways and widens the V-
shaped valley
2. Vertical Corrasion – it is the downward action. It deepens the river channel
Hydraulic action – It is a mechanical process, in which the moving water current
flows against the banks and bed of a river, thereby removing rock particles. Some
171
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
of the water splashes against the river banks and surges into cracks and crevices.
This helps to disintegrate the rocks. The river water picks up the loose fragments
from its banks and bed and transports them away.
Attrition – this is a form of fluvial erosion in which the bed load is eroded by itself
due to wear and tear of the transported material when they roll and collide into
one another. The coarser boulders are broken down into smaller stones and
pebbles.
Corrosion or solution – this is the chemical or solvent action of water on soluble or
partly-soluble rocks with which the river comes into contact. For example, calcium
carbonate in limestone is easily dissolved and removed in solution.
While the first three processes of fluvial erosion come under mechanical erosion,
the last or the fourth process i.e. corrosion comes under chemical erosion by fluvial
action.
172
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Forms of Erosion
River erosion takes place in three ways:
1. Headward erosion – it is a process by which a river increases its upstream length.
This is achieved by a river cutting back at its source
2. Lateral erosion – it is a process through which river channel is extended in its
width due to sideways erosion at the outside banks of the rivers
3. Vertical erosion – Vertical erosion takes place at the base of the river. The
channel of the river gets deepened through vertical erosion
The fluvial cycle of erosion
Three distinct stages of youth, maturity and old age can be identified during the
lifetime of a stream. At different stages of the erosional cycle, the valley acquires
different profiles. The characteristics related to each stage of landscape
development in running water regimes are summarised as below:
Youth
Streams are few during this stage with poor integration and flow over original
slopes
173
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The valley developed is thus deep, narrow and distinctly V-shaped with no
floodplains or with very narrow floodplains.
Downcutting predominates over lateral corrasion
Streams divides are broad and flat with marshes, swamp and lakes.
Some of the outstanding features which are developed in this stage are gorges,
canyons waterfalls, rapids and river capture etc.
Mature
During this stage, streams are plenty with good integration.
Lateral corrasion tends to replace vertical corrasion
The valleys are still V-shaped but wide and deep due to an active erosion of the
banks;
Trunk streams are broad enough to have wider floodplains within which streams
may flow in meanders confined within the valley.
Swamps and marshes of youth stage, as well as flat and broad inter-stream areas,
disappear. The stream divides turn sharp.
Waterfalls and rapids disappear.
Meander and slip off slopes are the characteristic features of this stage
Old
The river moving downstream across a broad level plain is heavy with sediments.
Vertical corrasion almost ceases in this stage though lateral corrasion still goes
on to erode its banks further
Smaller tributaries during old age are few with gentle gradients.
Streams meander freely over vast floodplains. Divides are broad and flat with
lakes, swamps and marshes.
174
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Depositional features predominate in this stage
Most of the landscape is at or slightly above sea level
Characteristic features of this stage are floodplains, oxbow lakes, natural levees
and Delta etc.
Fluvial Erosional Landforms
Most of the erosional landforms associated with running water are made by
youthful rivers vigorously flowing over steep gradients. With time, stream channels
over steep gradients turn gentler due to continued erosion, and as a consequence,
lose their velocity, facilitating active deposition. There are two components of
running water. One is the sheet that refers to overland flow on the land surface.
Another is streams and rivers that refer to linear flow as in valleys.
River Valleys
The extended depression on the ground through which a stream flows
throughout its course is called a river valley.
At different stages of the erosional cycle, the valley acquires different profiles
Valleys start as small and narrow rills
The rills will gradually develop into long and wide gullies
The gullies will further deepen, widen and lengthen to give rise to valleys.
Depending upon dimensions, shape, types and structure of rocks in which they
are formed, many types of valleys like the V-shaped valley, gorge, canyon, etc.
can be recognised.
175
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
V-shaped river valley
1) V-shaped Valley
The river is very swift as it descends the steep slope, and the predominant action
of the river is vertical corrasion
The valley developed is thus deep, narrow and distinctly V-shaped
176
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Formation of the V-shaped valley
2) Gorge
A gorge is a deep and narrow valley with very steep to straight sides
A gorge is almost equal in width at its top as well as its bottom.
Gorges are formed in hard rocks.
Example – Indus Gorge in Kashmir
177
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
A gorge
3) Canyon
A canyon is a variant of the gorge.
Unlike Gorge, a canyon is wider at its top than at its bottom.
A canyon is characterised by steep step-like side slopes
Canyons commonly form in horizontal bedded sedimentary rocks
Example – Grand Canyon carved by Colorado River, USA
178
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Grand Canyon, Colorado River, USA
Waterfalls and Rapids
When rivers plunge down in a sudden fall of some height, they are called
waterfalls
Their great force usually wears out a plunge pool beneath
Waterfalls are formed because of several factors like the relative resistance of
rocks lying across the river, the relative difference in topographic reliefs e.g. in
Plateau etc.
A rapid is similarly formed due to an abrupt change in gradient of a river due to
variation in resistance of hard and soft rocks traversed by a river
Waterfalls are also transitory like any other landform and will recede gradually
and bring the floor of the valley above waterfalls to the level below.
179
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Potholes and Plunge Pools
Potholes are more or less circular depressions formed over the rocky beds of hill-
streams, because of stream erosion aided by the abrasion of rock fragments.
Once a small and shallow depression forms, pebbles and boulders get collected
in those depressions and get rotated by flowing water and consequently the
depressions grow in dimensions.
Eventually, such depressions are joined leading to deepening of the stream
valley.
At the foot of waterfalls also, large potholes, quite deep and wide, form because
of the sheer impact of water and rotation of boulders. These deep and large holes
at the base of waterfalls are referred to as plunge pools.
These pools also help in the deepening of valleys
180
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Incised or Entrenched Meanders
Incised or entrenched meanders are found cut in hard rocks. They are very deep
and wide.
In streams that flow rapidly over steep gradients, normally erosion is
concentrated on the bottom of the stream channel.
Entrenched meander normally occurs where there is a rapid cutting of the river
bed such that the river does not get to erode the lateral sides.
Meander loops are developed over original gentle surfaces in the initial stages of
development of streams and the same loops get entrenched into the rocks
normally due to erosion or gradual uplift of the land over which they started.
181
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
They are widened and deepened over a long period of time and can be found as
deep gorges and canyons in the areas where hard rocks are found.
They give an indication of the status of original land surfaces over which streams
have developed.
Incised meanders are said to be an impact of river rejuvenation.
River Terraces
River terraces refer to surfaces relating to old valley floor or floodplain levels.
They may be bedrock surfaces without any alluvial cover or alluvial terraces
consisting of stream deposits.
River terraces are basically products of erosion as they result due to vertical
erosion by the stream into its own depositional floodplain.
There can be a number of such terraces. They are found at different heights
indicating former river bed levels.
The river terraces may occur at the same elevation on either side of the rivers in
which case they are called paired terraces
Paired and unpaired river terraces
182
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Peneplain
A peneplain (an almost plain) is a low-relief plain which is formed as a result of
stream erosion
The peneplain is meant to imply the representation of a near-final (or
penultimate) stage of fluvial erosion during times of extended tectonic stability.
A peneplain
Drainage Patterns
The drainage pattern of a stream refers to the typical shape of a river course as
it completes its erosional cycle
They are governed by the topography of the land, resistance and strength of base
rocks and the gradient of the land
There are various types of drainage patterns which are described briefly as
below:-
183
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Various types of drainage patterns
Dendritic drainage pattern
It is the most common form of drainage system.
The drainage pattern resembling the branches of a tree is known as “dendritic”
In a dendritic system, there are many contributing streams, which are then joined
together into the tributaries of the main river
The examples of Dendritic Pattern include the rivers of northern plain such Indus.
Trellis drainage pattern
In the trellis drainage pattern, the primary tributaries of rivers flow parallel to
each other and they are joined by secondary tributaries at the right angle.
The geometry of a trellis drainage system is similar to that of a common
garden trellis used to grow vines.
Trellis drainage is characteristic of folded mountains,
Examples of trellis pattern include the drainage system of the Appalachian
Mountains in North America and Seine and its tributaries in Paris basin (France)
etc.
184
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Parallel drainage pattern
A parallel drainage system is a pattern of rivers caused by steep slopes with
some relief.
The parallel drainage pattern is observed in a uniformly sloping region where the
tributaries seem to be running parallel to each other.
A parallel pattern sometimes indicates the presence of a major fault that cuts
across an area of steeply folded bedrock.
Examples of this system include the rivers of Lesser Himalaya
Rectangular drainage pattern
Rectangular drainage develops on rocks that are of approximately uniform
resistance to erosion, but which have two directions of joining at approximately
right angles.
In the rectangular drainage pattern, the mainstream curve at right angles and the
tributaries join the mainstream at right angles.
Example – Colorado river the USA
Angular drainage pattern
Angular drainage pattern is commonly observed in foothill regions.
Angular drainage patterns form where bedrock joints and faults intersect at more
acute angles than rectangular drainage patterns. Angles are both more and less
than 90 degrees
the mainstream is joined by the tributaries at acute angles.
Radial drainage pattern
When the rivers originate from a hill and flow in all directions, the drainage
pattern is known as ‘radial’.
185
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Volcanoes usually display excellent radial drainage. Other geological features on
which radial drainage commonly develops are domes and laccoliths.
The rivers originating from the Amarkantak range present a good example of it.
Centripetal drainage pattern
When the rivers discharge their waters from all directions in a lake or depression,
the pattern is known as ‘centripetal’.
Examples – streams of Ladakh, Tibet and Loktak Lake in Manipur (India)
Annular drainage pattern
In an annular drainage pattern streams follow a roughly circular or concentric
path along a belt of weak rock, resembling in plan a ring-like pattern.
Example of such system include Black Hill streams of South Dakota, USA
186
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
21. Glacial Landforms
Glacier
A glacier is a large mass of ice that is persistently moving under its own weight
over the land or as linear flows down the slopes of mountains in broad trough-
like valleys
Glaciers are formed in the areas where the accumulation of snow exceeds
its ablation (melting and sublimation) over many years, often centuries.
Glaciers move under the influence of the force of gravity.
The movement of glaciers is slow, unlike water flow. Glaciers flow like very slow
rivers.
Their movement could be a few centimetres to a few metres a day or even less
or more.
Types of Glaciers
Glaciers are categorized by their morphology, thermal characteristics, and
behaviour. Glaciers are mainly of four types – continental glaciers, ice caps,
piedmont glaciers and valley glaciers.
187
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
1. Continental Glaciers – Continental glaciers are continuous masses of ice that are
much larger than alpine glaciers. By definition, they have areas larger than
50,000 km2, some examples of Continental Glaciers are Antartica, Iceland,
Greenland etc.
2. Ice caps – Ice caps are the covers of snow and ice on the mountain ranges from
which the valley or mountain glaciers originate. Though they can also be found
at the lower altitudes. Ice caps have an area of less than 50,000 km2.
3. Piedmont Glaciers – The piedmont glaciers form a continuous ice sheet at the
base of mountains. The Malaspina Glacier in Alaska is one of the most famous
examples of this type of glacier
4. Valley Glaciers – A glacier that fills a valley is called a valley glacier. The valley
glaciers are commonly known as Alpine Glacier and are found in the valleys
created by lofty mountains such as Himalaya in India.
Mechanism of erosion and deposition
Erosion by glaciers is tremendous because of friction caused by sheer weight of
the ice.
The rate of erosion is determined by several factors such as the velocity of flow,
gradient of the slope, the weight of the glacier, the temperature of the ice and
the geological structure of the valley
A glacier erodes its valley through two processes ‘plucking’ and ‘abrasion.’
Plucking – By “Plucking”, the glacier freezes the joints and beds of the underlying
rocks tears out individual blocks and drags them away
Abrasion – By “abrasion”, the glacier scratches, scraps, polishes and scours the
valley floor with the debris frozen into it. These fragments are powerful agents
of denudation
As glaciers move over bedrock, large blocks and fragments of rocks are plucked
from the land by glaciers. This mass of rocks and debris creates huge erosion
potential and erodes the bed and sides of the valley through which glaciers flow.
188
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The movement of glaciers continuously erodes the bedrock and levels of the
plain. Eventually, the slope is so much reduced that no further movement is
possible and so glacier stops and deposits the debris in the vast outwash plain.
Glacial Erosion
Glacial Landforms
Glaciation generally gives rise to erosional features in the highlands and
depositional features on the lowlands, though these processes are not mutually
exclusive because a glacier plays a combined role of erosion, transportation and
deposition throughout its course
189
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Erosional Landforms
Cirque
Cirques are horseshoe shaped, deep, long and wide troughs or basins with very
steep to vertically dropping high walls at its head as well as sides.
Cirques are often found along the head of Glacial Valley
The accumulated ice cuts these cirques while moving down the mountain tops.
After the glacier melts, water fills these cirques, and they are known as cirque
lake.
Horns
Horns form through head-ward erosion of the cirque walls.
If three or more radiating glaciers cut headward until their cirques meet, high,
sharp pointed and steep-sided peaks called horns form.
190
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Horn
Aretes
Arete is a narrow ridge of rock which separates two valleys.
Aretes are typically formed when two glacial cirques erode head-wards towards
one another
The divides between Cirque side walls or head walls get narrow because of
progressive erosion and turn into serrated or saw-toothed ridges referred to as
aretes with very sharp crest and a zig-zag outline.
191
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Arete
Glacial Valleys
Glaciated valleys are trough-like and U-shaped with wide, flat floors and relatively
smooth, and steep sides.
When the glacier disappears, and water fills the deep narrow sections of the
valley, a ribbon lake is formed.
Glacial Valley
192
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Fjords/Fiords
A fjord or fiord is a long, narrow and steep-sided inlet created by a glacier
They are formed where the lower end of a very deep glacial trough is filled with
sea water
Fjords are common in Norway, Chile, and New Zealand etc.
Fjord/Fiord
Hanging Valleys
A hanging valley is a tributary valley that is higher than the main valley. Hanging
valleys are common along glaciated fjords and U-shaped valleys.
The main valley is eroded much more rapidly than the tributary valleys as it
contains a much larger glacier
After the ice has melted tributary valley, therefore, hangs above the main valley
The faces of divides or spurs of such hanging valleys opening into main glacial
valleys are quite often truncated to give them an appearance like triangular
facets.
193
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Often, waterfalls form at or near the outlet of the upper valley
Thus, the hanging valley may form a natural head of water for generating
hydroelectric power
Hanging Valley
Depositional Landforms
Outwash plains
An outwash plain is a plain at the foot of the glacial mountain
They are made up of fluvioglacial sediments, washed out from the terminal
moraines by the streams and channels of the stagnant ice mass.
As it flows, the glacier grinds the underlying rock surface and carries the debris
along.
When the glacier reaches its lowest point and melts, it leaves behind a stratified
deposition material, consisting of rock debris, clay, sand, gravel etc. with larger
194
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
boulders being deposited near the terminal moraine, and smaller particles
travelling further before being deposited.
The stratified surface thus formed is called as an outwash plain and a downward
extension of the deposited finer particles and clay material is called valley train.
Outwash Plain
Moraines
The unassorted coarse and fine debris dropped by the melting glaciers is called
glacial till.
The long ridges of deposits of these glacial till is called as Moraines
Depending on its position, moraines are classified into be ground, lateral, medial
and terminal moraine.
Terminal Moraines – Terminal moraines are long ridges of debris deposited at
the end (toe) of the glaciers.
195
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Lateral Moraines – Lateral moraines form along the sides parallel to the glacial
valleys. These moraines partly or fully owe their origin to glaciofluvial waters
pushing up materials to the sides of glaciers.
There can be many lateral moraines on either side in a glacial valley. The lateral
moraines may join a terminal moraine forming a horse-shoe shaped ridge
Ground Moraines – Many valley glaciers retreating rapidly leave an irregular
sheet of till over their valley floors. Such deposits varying greatly in thickness and
in surface topography are called ground moraines.
Medial Moraines – The moraine in the centre of the glacial valley flanked by
lateral moraines is called medial moraine. They are imperfectly formed as
compared to lateral moraines.
Sometimes medial moraines are indistinguishable from ground moraines.
Types of Moraine
Eskers
An esker is a long, winding sinuous ridge of stratified sand and gravel
196
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Eskers are frequently several kilometres long and, because of their peculiar
uniform shape, are somewhat like railway embankments
When glaciers melt in summer, the water flows on the surface of the ice or seeps
down along the margins or even moves through holes in the ice.
These waters accumulate beneath the glacier and flow like streams in a channel
beneath the ice.
Such streams flow over the ground with ice forming its banks.
The stream underneath carries coarse materials such as boulders, blocks which
gets deposited in the bed and when the glacier melts the deposits forms a sinuous
ridge called eskers.
Eskers
197
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Drumlins
Drumlins are smooth oval shaped ridge-like features composed mainly of glacial
till with some masses of gravel and sand.
The drumlins form due to the dumping of rock debris beneath heavily loaded ice
through fissures in the glacier.
The long axes of drumlins are parallel to the direction of ice movement.
They may measure up to 1000m in length and 30-35 m or so in height.
One end of the drumlins facing the glacier called the stoss end is blunter and
steeper than the other end called the tail.
198
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Drumlins
199
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
22. Karst Landforms
Karst topography is named after the typical topography developed in limestone
rocks of Karst region in the Balkans adjacent to the Adriatic Sea. Karst topography
includes typical landforms in any limestone or dolomitic region, produced by the
action of groundwater through the processes of solution and deposition.
Karst Topography
Limestone is an organically formed sedimentary rock. In its pure state, limestone is
made up of calcite or calcium carbonate but where magnesium is also present it is
termed as dolomite. Limestone is soluble in rainwater.
Conditions for the formation of Karst Topography
A region with a large stretch of water-soluble rocks such as limestone at the
surface or sub-surface level
Limestones should not be porous
These rocks should be dense, thinly bedded and well jointed
A perennial source of water and a low water table to allow the formation of
conspicuous features.
Moderate to abundant rainfall to cause the solvent action of water i.e. solution
of rocks
Mechanism of erosion in Karst region
In Karst regions, rocks are permeable, thinly bedded and highly jointed and
cracked.
Thus there is the general absence of surface drainage as the surface water has
gone underground
200
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
After vertically going down to some depth, the water under the ground flows
horizontally through the bedding planes, joints or through the materials
themselves.
Rocks are eroded due to this downward and horizontal movement of water.
It is through the chemical process of solution and precipitation deposition by
surface water and groundwater, varieties of landforms are developed in rocks
like limestones or dolomites rich in calcium carbonate.
Karst Landforms
Erosional Landforms
Sinkhole
Small to medium-sized round to sub-rounded shallow depressions called swallow
holes form on the surface of limestones through solution where rainwater sinks
into the limestone at a point of weakness
They are also known as sinkholes
Sinkholes are a common feature in limestone/karst areas.
201
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
A sinkhole is an opening more or less circular at the top and funnel-shaped
towards the bottom
There is a great variation in sizes of Sinkholes with areas from a few sq. m to a
hectare and with depth from a less than half a metre to thirty metres or more.
These holes grow in size through continuous solvent action
They are also referred to as solution sinks
Doline
They are also referred to as “Collapse sinks”.
They are less common than sinkholes
They might start as solution forms first, and if the bottom of a sinkhole forms the
roof of a void or cave underground, it might collapse leaving a large hole opening
into a cave or a void below
202
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Uvalas
They are long, narrow to wide trenches, also referred to as “Valley sinks”.
Several sinkholes and dolines may merge together as a result of subsidence to
form a large depression called an Uvala.
Lapies/ Karren
These are grooved, fluted and ridge-like features in an open limestone field.
These ridges or lapies form due to differential solution activity along parallel to
sub-parallel joints.
Eventually, the lapie field may transform into smooth limestone pavements.
Limestone Pavements
A limestone pavement is a natural karst landform consisting of a flat, incised
surface of exposed limestone that resembles an artificial pavement.
These are formed by the solvent action of underground water in the limestones,
causing progressive widening and enlargement of joints and cracks in the
trenches.
203
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The enlarged joints are called grikes and the isolated, rectangular blocks are
termed as clints.
Caves
Cave formation is prominent in areas where there are alternating beds of rocks
(sandstone, shale, quartzite) with limestone or dolomite in between or in areas
where limestones are dense, massive and occurring as thick beds.
Water percolates down either through the materials or through cracks and joints
and moves horizontally along bedding planes.
Gradually, the limestone dissolves along these bedding planes resulting in the
creation of long and narrow gaps called caves.
204
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Polije
A polije is a very large, flat-floored depression in the karst region.
They are often formed by merging of several uvalas or partly due to faulting
They are commonly found in subtropical and tropical latitudes
Some of these may also appear in the temperate region. They may also be found
in boreal regions, though very rarely.
During the rainy season, parts of the floor which are at or near the water table
may become temporary lakes
Drier areas are fertile. Usually covered with thick sediments, they are used
extensively for agricultural purposes
Ponor
A ponor is a natural surface opening in the karst regions
205
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
They are found directly underneath the sinkholes
A ponor is kind of a portal where a surface stream or lake flows either partially or
completely underground into a karst groundwater system.
Landforms due to depositions
Depositional landforms in karst region are developed due to the deposition of
calcium carbonate. The calcium carbonate dissolved during the erosional process
starts to precipitate when the water evaporates or when the solution is super-
saturated.
Stalactites, Stalagmites and Pillars are the most spectacular underground features,
found in the limestone caves.
206
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Stalactites
Stalactites are the sharp, slender, downward-growing icicles of different
diameters that hang from the cave roofs.
Stalactites have a variety of forms
Their bases are normally broad which taper towards the free ends
The water carries calcium in solution and when this lime-charged water
evaporates, it leaves behind the solidified crystalline calcium carbonate.
Stalagmites
Stalagmites form due to dripping water from the surface or through the thin pipe,
of the stalactite, immediately below it
Moisture dripping from the roof trickles down the stalactite and drops to the
floor where stalagmites are formed due to deposition of calcium
207
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Stalagmites may take the shape of a column, a disc, with either a smooth,
rounded bulging end or a miniature crater-like depression.
Pillars
Over a long period, the stalactite is eventually merged with the stalagmite
Thus, the pillars or columns of different diameters are formed.
208
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
23. Marine Landforms
The Coastal Landforms are formed by the constant action of the waves, tides, and
currents. The coastline under the influence of these denudational agents changes
the coastal landforms and gives shapes to various types of marine landform
features.
Agents of Erosion
Waves
Waves accomplish most of the changes along the coasts.
Constant impact of breaking waves drastically affects the coasts.
When waves break, the water is thrown with great force onto the shore, and
simultaneously, there is a great churning of sediments on the sea bottom.
Storm waves and tsunami waves can cause far-reaching changes in a short period
of time than normal breaking waves.
On calm days, when winds are slight, waves do little damage to the shoreline and
may instead help to build up beaches and other depositional features.
Tides and Currents
Tides and Currents, on contact with the shores, make very little direct attack on
the coastline
Tides affect marine erosion mainly by extending a line of erosion into a zone of
erosion. This zone correspond to the area between the low water level and the
high water level
Currents help to move eroded debris and deposit it as silt, sand and gravel along
the coasts
209
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The mechanism of marine erosion
The rate of marine erosion depends on the nature of rocks, the amount of rock
exposed to the sea, the effects of tides and currents, and human interference.
Marine agents of erosion operate in the following ways to transform the coastal
landscape
Corrasion – Corrasion is a process of mechanical erosion. Waves armed with rock
debris break on cliff faces and slowly erode it. On-coming currents and tides
complete the work by sweeping the eroded material into the sea.
Attrition – Attrition occurs when waves cause loose pieces of rock debris such as
boulders, pebbles, shingle and fine sand, to collide with each other. Under attrition,
these materials are broken down into finer, smaller and rounder particles which
are largely responsible for the fine sand that forms the beaches.
Hydraulic action – in their forward surge, waves splashing against the coast may
enter joints and crevices in the rocks. The air trapped inside is immediately
compressed. When the waves retreat, the compressed air expands with explosive
violence. Such repeated action causes enlargements of the cracks and rock
fragments are prised apart.
Solvent action – this refers to chemical erosion of rocks. This process is limited to
limestone coasts. On limestone coasts, the solvent action of seawater on calcium
carbonate sets up chemical changes in the rocks and disintegration takes place.
Types of Coasts
Other than the action of waves, the coastal landforms depend upon the
configuration of land and sea floor and whether the coast is advancing (emerging)
seaward or retreating (submerging) landward.
There are different types of coastlines based on a great variety of coastal features.
However, it is important to discuss, two types of coasts (assuming sea level to be
constant) to explain the concept of evolution of coastal landforms:
1. Submerged coasts (high, rocky coasts)
2. Emerged coasts (low, smooth and gently sloping sedimentary coasts)
210
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Submerged coasts
Submerged coasts are found either because of the sinking of the land or due to
the rise of the sea
The coasts are rocky and river appears to have drowned in the sea creating
estuaries.
Erosional landforms dominate coastal landform and depositional landforms are
absent
Along high rocky coasts, waves break with great force against the land shaping
the hill-sides into cliffs, which further develops a wave-cut platform, caves etc.
As the erosion along the coast takes place a good supply material becomes
available to longshore currents and waves to deposit them as beaches, bars, spits
etc.
Emerged Coasts
Emerged coasts are found due to either uplift of the land or fall in the sea level
They are less common
Along, the low sedimentary coasts the rivers appear to extend their length by
building coastal plains and deltas.
The coastline appears smooth with occasional incursions of water in the form of
lagoons and tidal creeks.
The land slopes gently into the water.
Marshes and swamps may abound along the coasts.
Depositional features dominate.
When waves break over a gently sloping sedimentary coast, the bottom
sediments get churned and move readily building bars, barrier bars, spits and
lagoons.
211
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The maintenance of these depositional features depends upon the steady supply
of materials.
Large rivers which bring lots of sediments build deltas along low sedimentary
coasts.
Marine Landforms
Erosional landforms
Sea cliffs
The most widespread landforms of erosional coasts are sea cliffs.
Generally, any very steep rock face adjoining the coast forms a cliff
Almost all sea cliffs are steep and may range from a few m to 30 m or even more.
Their steep nature is the result of wave-induced erosion near sea level and the
subsequent collapse of rocks at a higher elevation.
212
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
At the base of the cliff, the sea cuts a notch, which gradually undermines the cliff
so that it collapses
The best-known cliffs are the Chalk cliffs of the English channel and the ‘White
Cliffs’ of Dover
Wave-cut platforms
When the sea waves strike against a cliff, the cliff gets eroded gradually and
retreats.
With constant pounding by waves, as the cliffs recede, an eroded base is left
behind, called a wave-cut platform.
The waves level out these platforms to create a flat surface
Such surfaces may measure from a few metres to hundreds of metres wide and
extend to the base of the adjacent cliff.
213
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Sea caves
Prolonged attack of waves against the base of the cliff and the rock debris that
gets smashed against the cliff along with lashing waves create holes in regions of
weakness and
These holes get further widened and deepened to form sea caves.
Example Flamborough head, England
214
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Sea arches
When two caves approach one another from either side of a headland and unite,
they form a bridge like structure, known as arch
These archways may have an arcuate or rectangular shape, with the opening
extending below water level.
The height of an arch can be up to tens of metres above sea level.
It is common for sea arches to form when the waves attack a rock- form from two
opposite sides, the differential erosion
Example – the Neddle Eye near Wick, Scotland
215
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Sea Stack
Continued erosion, under the attack of the wave, can result in the total collapse
of an arch
The seaward portion of headland will remain as an isolated pillar of rock known
as stack
Like all other features, sea stacks is also temporary and eventually, the stack will
also disappear
216
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Stump
The stack is gradually eroded, leaving behind only the stump
Stumps are only just visible above the sea level
Blow holes
The occasional splashing of the waves against the roof of a cave may enlarge the
joints when compressed air is trapped inside
A natural shaft is thus formed which may eventually pierce through the surface
Waves breaking into the cave may force blasts of water from the top
Such shaft is termed as Blow-hole or ‘Gloup’.
Example – Holborn Head, Scotland
Geos
The enlargement of blow-holes and the continued action of waves weaken the
cave roof.
When the roof collapses a long, narrow inlet or creek develops.
Such long and deep clefts are called Geos
Example – the Wife Geo, Scotland
Depositional landforms
Beaches
Beaches are characteristic of shorelines that are dominated by deposition but
may occur as patches along even the rugged shores.
Sands and gravels loosened from the land are moved by waves to be deposited
along the shore as beaches
217
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Most of the sediment making up the beaches comes from land carried by the
streams and rivers or from wave erosion.
Most of the beaches are made up of sand sized materials. Beaches called shingle
beaches contain excessively small pebbles and even cobbles.
Beaches are temporary features.
Dunes
Just behind the beach, the coastal sands lifted and winnowed from over the
beach surfaces will be deposited as sand dunes.
On shore, winds play a major part in the formation of these dunes
Sand dunes forming long ridges parallel to the coastline are very common along
low sedimentary coasts.
Sand dunes are common in the coasts of Belgium, Denmark and the Netherlands
218
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Bars
When a ridge of sand and shingle formed in the sea in the off-shore zone (from
the position of low tide waterline to seaward), it is called a bar
The off-shore bars and barriers commonly form across the mouth of a river or at
the entrance of a bay.
Bars are submerged features and when bars show up above water, they are called
barrier bars.
Generally, bars are approximately parallel to the coast
Tombolo
Tombolo joins two landmasses by a connecting bar
The tombolo is a deposition landform in which an island is attached to the
mainland by a narrow piece of lands such as a spit or bar.
A tombolo is a sandy isthmus.
An example of Tombolo can be found in Chesil beach in England which links the
Isle of Portland with mainland
219
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Barriers
An off-shore bar which is exposed due to further addition of sand is termed a
barrier bar.
The off-shore bars and barriers commonly form across the mouth of a river or at
the entrance of a bay.
They usually occur in chains
They are subject to change during storms and other action, but absorb energy
and protect the coastlines and create areas of protected waters
where wetlands may flourish.
Spits
Barrier bar which gets keyed up to the headland of a bay is called a spit.
Spits are projected depositional landforms with one end attached to the land and
the other end projecting into the sea
Spits may also develop attached to headlands/hills.
The mode of formation of spit is similar to a bar or barrier.
A shorter spit with one end curved towards the land is called a hook.
When barrier bars and spits form at the mouth of a bay and block it, a lagoon
forms.
The lagoons would gradually get filled up by sediments from the land giving rise
to a coastal plain.
220
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
24. Arid Landforms
Deserts are regions with scanty rainfall, i.e. rainfall less than 25cm annually but still,
rainwater plays a crucial role in the geomorphology of the arid landforms. However,
the wind is the dominating agent of gradation in the arid regions of the world.
Arid Landforms
Arid landforms are the characterised by badlands, Mushroom rocks, dunes,
yardang, etc. These features are typical in an arid region. In this article, we will
study various types of arid landforms, their mechanism of formation etc.
Deserts
Deserts are regions with very less precipitation concentrated to a very short
duration. Around 20% of the geographical areas in the world are deserts.
There is a certain definite pattern to the location of the world’ deserts
Almost all the deserts are confined within the 15 to 30-degrees Latitude on both
sides of the equator.
Deserts are generally located in the west coasts of the continent as the
Tradewinds are off-shore
They are bathed by cold currents which produce a desiccating effect so that
moisture is not easily condensed into precipitation
Type of deserts
The works of wind and water in the erosion of elevated uplands, transporting the
eroded material and depositing them elsewhere has given rise to five distinct kinds
of desert landscape
Rocky desert
Rocky deserts are also known as “Hamada”.
They consist of large stretches of bare rocks, swept clear of sand and dust by the
wind
221
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The exposed rocks are thoroughly smoothed and polished
The region is bare and sterile
Example – the best known rocky deserts are those of the Sahara deserts
Stony desert
It is also known as “Reg”
Pebbles and gravels form an extensive sheet of the landscape of these areas.
Stony deserts are more widespread than sandy deserts, contrary to the general
idea of deserts associated with sandy landforms.
Example – Sturt Stony desert, Australia
222
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Sandy deserts
The commonly accepted idea of a desert is the sandy landscape with dunes.
Vast stretches of dunes are deposited by winds, in these types of desert.
The wind direction can be observed from the patterns of the ripples on dunes.
Example – Thar desert in India is a sandy desert
223
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Badlands
Badlands are arid regions where the hills are badly eroded under the action of
water due to occasional rainstorms or flow of river streams
They are represented by gullies and ravines
Example – famous ravines of Chambal in India, ravines in South Dakota, USA etc.
224
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mountain deserts
Mountain deserts are found are found on highlands such as plateaux and
mountain ranges
Erosion has dissected the desert highlands into harsh, serrated outlines of chaotic
peaks and craggy ranges
They have steep-sided slopes and sharp and irregular edges carved by the action
of frost
Example – Tibesti Mountains in the Sahara desert
225
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
The mechanism of Arid Erosion
Arid landforms are the result of many combined factors, one reacting upon the
other. Low precipitation and high rate of evaporation are the major causes of
aridity. The desert rocks devoid of vegetation, exposed to mechanical and chemical
weathering processes due to drastic diurnal temperature changes, decay faster and
wind and the torrential rains help in removing the weathered materials easily.
Weathering
This is the most potent factor of denudation in arid regions.
Weathering is defined as the breakdown of rocks by agents of weathering acting
in situ.
Mechanical weathering and Chemical weathering dominates in the arid
landforms
Without abundant water in the arid environment, the chemical breakdown of
rocks proceeds extremely slowly. However, the mechanical breakdown of rock
proceeds relatively quickly in the arid climate.
Drastic diurnal temperature changes in deserts cause stress in the rocks due to
continuous expansion and contraction. This stress helps in speeding up the
weathering process through exfoliation of the outer rock surface
226
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Wind action
As an erosional agent, the wind is more effective in arid regions than humid
regions as in arid regions, there is little moisture and vegetation to bind the loose
material
The wind action creates a number of interesting erosional and depositional
features in the deserts.
Wind erosion is carried out in desert areas in mainly three ways – Deflation,
Abrasion and Attrition
Deflation – Deflation includes lifting and blowing away of dust and smaller
particles from the surface of rocks.
Abrasion – the sand-blasting of a rock surface by winds when they hurl sand
particles against them is called abrasion. The impact of such blasting results in
rock surface being scratched, polished and worn away.
Attrition – when wind-borne particles roll against one another in a collision they
wear each other away. This process is called attrition
227
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Water action
Many features of deserts owe their formation to mass wasting and running water
as sheet floods.
Though rain is scarce in deserts, it comes down torrentially in a short period of
time.
Stream channels in desert areas are broad, smooth and indefinite and flow for a
brief time after rains.
The desert rocks devoid of vegetation, exposed to mechanical and chemical
weathering processes due to drastic diurnal temperature changes, decay faster
These weathered materials are easily carried away by torrential rainfall
228
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Landforms of wind erosion
Rock pedestal or mushroom rocks
Many rock-outcrops in the deserts easily susceptible to wind deflation and
abrasion are worn out quickly
This leads to wearing away of the softer layer leaving some remnants of resistant
rocks
Grooves and hollows are cut in the rock surface, carving them into fantastic and
grotesque looking pillars called “Pedestals”
Such rock pillar is further eroded near bases
This process of undercutting produces mushroom with a slender stalk and a
broad and rounded pear shaped cap above.
229
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Zeugen
These are tabular masses which have a layer of soft rocks lying beneath a surface
layer of more resistant rocks
The sculpting effects of wind abrasion wear them into a weird looking ridge and
furrow landscape
Their formation is initiated by opening up of joints of surface rocks by mechanical
weathering
Deep furrows are developed by wind abrasion eating into the underlying softer
layers
The hard rocks then stand above the furrows as ridges or Zeugen
Such tabular blocks of Zeugen may stand 10 to 100 feet above the sunken
furrows.
230
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Continuous abrasion by wind gradually lowers the Zeugen and widens the
furrows
Yardangs
The word yardang originated in the interior deserts of central Asia where they
are best developed
Yardangs are a steep-sided irregular ridge of sand lying in the direction of the
prevailing wind
They look quite similar to the ‘ridge and furrow’ landscape of Zeugen
They are formed by the dual action of wind abrasion by dust and sand, and
deflation which is the removal of loose material
231
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Wind abrasion excavates the bands of softer rocks into long, narrow corridors,
separating the steep-sided, over-hanging ridges of hard rocks, called yardangs.
They are commonly found in the Atacama desert, Chile
Mesas (Table) and buttes
Mesa is a Spanish word meaning ‘table’.
It is a flat, table-like land masses with a very resistant horizontal top layer and
very steep sides
The hard stratum on the surface resist denudation by both wind & water, and
thus protects the underlying layer of rocks from being eroded away
Continuous denudation through the ages may reduce Mesas in an area so that
they become isolated flat-topped hills called Buttes.
232
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Inselberg
Inselberg is a German word meaning ‘Island-mountains.’
An inselberg is an isolated residual hill rising abruptly from a gently sloping or
virtually level surrounding plain.
They are characterized by their very steep slopes & rather rounded tops
They are often composed of granite or gneiss
They are probably the relics of an original plateau which has been almost entirely
eroded away
Inselbergs are typical features of many deserts and semi-arid landscapes in old
age
233
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Ventifacts
Ventifacts are pebbles faceted by sand blasting
They are shaped and polished by wind abrasion
Mechanically weathered rock fragments are moved by wind in open settings to
blast against the rock formations carving facets
If wind direction changes another facet is developed
Such rocks have characteristic flat facets with sharp edges
Among the ventifacts, those with the three wind faceted surfaces are known as
Dreikanter.
234
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Landforms of wind deposition
When the velocity of wind decreases, its carrying capacity also decreases. As a
result, the grains of sands starts to settle down, and it leads to the formation of
depositional landforms in a desert.
Depending upon the size of the particles, velocity and direction of the wind,
different depositional landforms can be found in arid and desert areas:
Dunes
Dunes are hills of sand formed by the accumulation of sand & shaped by the
movement of winds
Dry hot deserts are good places for sand dune formation
They may classifies as active and inactive dunes – active or live dunes are
constantly on move and inactive or fixed dunes are rooted with vegetation
235
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Types of dunes
Because of their great contrast in shape, size and alignment, they have been given
classified into several types of dunes viz head dune, tail dune, parabolic dune,
pyramidal dune, transverse dune, longitudinal dune etc. However, there are two
most common types of dunes are barchans and seif which are described as below:-
1) Barchan
They are moon or Crescent shaped live dunes
They may occur individually or in groups
They have their points or wings directed away from wind direction i.e., downwind
They are initiated probably by a chance accumulation of sand at an obstacle, such
as patch of grass or a heap of rocks
236
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
They occur transversely to the wind
Thus, their horns thin out & become lower in the direction of the wind.
The windward side is convex & gently sloping whiles the leeward side is concave
& steep.
The crest of sand dunes moves forward as more sand is accumulated by the
prevailing wind.
The sand is driven up the windward side & on reaching the crest slips down the
leeward side so that the dune advances
The migration of Barchans may be a threat to desert life as they may encroach
on an oasis burying palm trees & houses.
They are most prevalent in the deserts of Turkestan and in the Sahara
237
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
2) Seif
Seif is an Arabic word meaning ‘sword’
They are long, narrow ridges of sand, often over a hundred miles long lying
parallel to the direction of the prevailing winds
Seif is similar to barchan with a small difference; it has only one wing.
Prevailing winds increases the length of the dunes into tapering linear ridges
while occasional crosswinds tend to increase their heights & width
Extensive seif dunes can be found in Sahara desert, West Australian desert, Thar
desert etc.
Loess
The fine dust blown beyond the desert limits is deposited on neighbouring lands
as loess.
It is a yellow, friable (easily crumbled) material is usually very fertile.
238
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Loess is in fact, fine loam, rich in lime, very coherent and extremely porous
Water sinks in readily so that the surface is always dry,
Streams may cut deep valleys through the thick mantle of soft loess to develop
badland topography.
The most extensive deposits are found in north-west China in the loess plateau
of the Hwang-Ho basin.
Landforms due to water action
Desert areas have scanty rainfall. However, there are deserts without rainfall
also.
However, occasional and sudden rainfalls in torrential downpours may produce
devastating effects due to flash floods etc.
Loose materials such as gravel, sand and fine dust are swept down the hillsides.
They cut deep gullies and ravines forming badland topography
239
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
There are so much fine materials in the flash floods that the flow becomes liquid
mud
Bajada
The Bajada is a depositional feature made up alluvial material laid down by the
intermittent streams
Bajada is formed by the coalescence of alluvial fans
These fan-shaped deposits form from the deposition of sediment by a stream
from upland region onto flat land at the base of a mountain
Bajadas are common in arid areas where a large quantity of sediment is deposited
by flash floods
Bajadas frequently contain playa lakes
Playa
In arid areas drainage from upland regions into the lower depression, in times of
sufficient water, create shallow water body or a temporary lake
Such types of shallow lakes are called as playas where water is retained only for
short duration due to evaporation
Quite often the playas contain good deposition of salts.
The playa plain covered up by salts is called alkali flats.
Pediments and Pediplains
A pediment is an erosional plain formed at the base of the surrounding mountain
scarps
They are gently inclined rocky floors close to the mountains at their foot with or
without a thin cover of debris.
They form through the erosion of mountain front through a combination of
lateral erosion by streams and sheet flooding.
240
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Through parallel retreat of slopes, the pediments extend backwards at the
expense of mountain front
Gradually, the mountain gets reduced leaving an inselberg which is a remnant of
the mountain.
That’s how the high relief in desert areas is reduced to low featureless plains
called pediplains.
241
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
25. Lake – Classification of Lakes
Lakes are among the most varied features of the earth’s surface. A lake is a large body of natural water accumulated in a depression. Lake basins are formed due to endogenous geological processes like tectonism and volcanism and exogenous activities like landslides, glaciation, solution, river and wind action.
Lake and Its Classification
They vary tremendously in size, shape, depth and mode of formation. The tiny ones
are no bigger than ponds or pools, but the large ones are so extensive that they
merit the name of seas, e.g. the Caspian Sea. The Caspian Sea is the largest lake
regarding the area. The deepest lake in the Lake Baikal in Siberia.
Lakes occupy about 1.8 % of the earth’s surface. About 280 000 cu.km of water
exists on earth in the form of lakes. This is 0.19% of the total volume of water in
the hydrosphere.
Advantages of lakes
The major role played by lakes and reservoirs is the regulation of stream flow.
242
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Lakes provide water for drinking, factory, irrigation and generating hydel-power.
Lakes are a good refuge for an enormous variety of flora and fauna.
The lives of the people, in a region, are greatly influenced by the presence of a
lake in that area.
In some places, lakes are good sources for water supply for drinking.
Lakes help in the growth of the fishing industry.
The salt lakes yield common salt. For example, Sambar lake
Lakes are helpful in controlling the weather and moderating local climate- Lakes
cool the air in summer and warm it during winter. They also enhance the
humidity.
Lakes have an aesthetic appeal and are helpful in recreation; tourists are
attracted due to lakes which have boating, swimming and a good landscape
around.
Lakes are used for navigation. For example the Great Lakes in North America
Lakes also help in flood control as rivers passing through the lakes in their course
seldom cause disastrous floods. The Wular lake and the Dal lake do not allow the
Jhelum river to be flooded and due to lack of such lakes, the Brahmaputra is
subjected to very great floods every year.
Lakes only a temporary feature?
Lakes are thought to be only a temporary feature of the earth’s crust. Eventually,
they will be eliminated by the dual process of draining and sitting up. In regions of
unreliable rainfall, lakes dry up completely during the dry season. In the hot
deserts, lakes disappear altogether by the combined processes of evaporation,
percolation and outflow. Though the process of lake elimination may not be
completed within our span of life, it takes place relatively quickly regarding
geological time.
243
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Classification of lakes
When there are a large number and variety of lakes, people tend to classify them.
There are several types, kinds and categories of lakes in the world. Classification
helps us to understand and visualize the relationships and helps us to
communicate. The most common classification of lakes is based on the size or
dimension of lakes, whether it is small, big or very large.
Lakes are mainly classified on the basis of:
a) Nature of Inflow-outflow
b) Origin
c) Trophic levels
a) Classification based on inflow-outflow
Temporary and Permanent Lakes
1. Temporary Lakes -These lakes may exist temporarily by filling up small
depressions of undulating grounds after a heavy shower. In such lakes rate of
evaporation is much greater than the rate of recharge through precipitation.
They are usually saline. They are subject to extreme fluctuations in water level.
Example – Badhkal Lake, Faridabad
2. Permanent lakes – Permanent lakes carry more water than could ever be
evaporated. These are very deep. They have some perennial source of inflow of
water such as a glacier. They are usually freshwater lakes. Example – Dal Lake
Freshwater and Salt lakes
1. Freshwater lakes – Most of the lakes in the world are freshwater lakes. They are
usually found in low lying areas and are fed from streams, rivers and runoff from
the surrounding area. e.g. Great Lakes of North America, Lake Baikal in Russia,
Lake Wular and Loktak Lake in India.
2. Salt Lakes – Salt Lake is an inland body of water situated in an arid or semiarid
region, having no outlet to the sea, and containing a high concentration of
244
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
dissolved salt. These lakes exist in regions of low precipitation and intense
evaporation. Because of intense evaporation, the concentration of salts increases
in the water body, turning them saline. Playas or salt lakes are a common feature
of deserts. Example – Great Salt Lake of Utah, USA, Dead Sea etc.
Great Lakes of North America
b) Classification based on origin or mode of formation
The following are the various ways in which lakes can be formed. Each of them is
placed in a different category, though in a few cases the lake could have been
formed by more than one single factor
1) Lakes formed by earth movement
Tectonic Lakes
These lakes are formed by filling up with water in the tectonic depressions
created due to warping, sagging, bending and fracturing of the earth’s crust.
245
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Such depressions give rise to lakes of immense sizes and depths.
Example – Lake Titicaca, Chile, the Caspian Sea etc.
Rift Valley Lakes
These lakes include some of the oldest, deepest and largest lakes around the
globe.
Due to faulting, a rift valley is formed by the sinking of the land between two
parallel faults, deep, narrow and elongated in character.
Water is collected in these troughs
Often their floors are below sea level.
The best example of this is the East African Rift Valley which includes such lakes
as Lake Tanganyika and the Dead Sea etc.
2) Lakes formed by Volcanism
Crater and Caldera Lakes
A natural hollow called a crater is formed by blowing off of the top of the cone
during a volcanic explosion.
Crater may be widened and enlarged by further subsidence into a caldera.
These depressions are normally dry.
In dormant or extinct volcanoes, due to rainfall straight into these depressions
which have no superficial outlet, a crater or caldera lake is formed.
Examples – Lonar crater lake in Maharashtra, India, Crater Lake in Oregon, USA
and Lake Toba in Sumatra etc.
Lava-blocked Lakes
In volcanic regions, it is common to find a stream of lava that flows across a valley
246
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
This stream of lava may occasionally become solidified and block the valley thus
forming a lake basin
This basin may get filled up due damming up of the river due to solidified lava
Example – The Sea of Galilee which is an inland lake was created due to blocking
of the Jordan valley by lava flow
One more type of lake formed due to subsidence of a volcanic land surface is
included under this type. Under this type of lake, the crust of a hollow lava flow
may collapse. The subsidence leaves behind a wide and shallow depression in
which the lake may form. E.g. Myvatn Lake of Iceland
3) Lakes formed by Glaciation
Cirque or tarn lakes
Cirque, a common landform in glaciated mountains, is often found at the heads
of glacial valleys.
A glacier on its way down the valley leaves behind circular hollows.
These circular hollows, in the heads of the valley up in the mountain, are called
cirques.
Cirques are very deep, long and wide troughs or basins.
The head and sides of these cirques have very steep to vertically dropping high
concave walls
Often, a lake of water can be seen within the cirques after the disappearance of
the glacier. Such lakes are referred to as the Cirque or tarn lakes
They are also called as Ribbon lakes
Example – Red tarn in the English Lake District and Chandra Taal (Himachal
Pradesh) in India
247
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Cirque or Tarn Lakes
Kettle Lakes
These are depressions in the outwash plain left by melting of a large mass of
stagnant ice
They are irregular in shape, and also these lakes are not very large or deep
Example – Kettle-lakes of Orkney in Scotland
Rock-Hollow Lakes
These lakes are formed by ice scouring when valley glaciers or ice sheets scoop
out hollows or depressions on the surface
Such lakes are abundant in Finland
Two more types of a glacial lake are formed due to “damming up of valleys by
morainic debris deposited by valley glaciers” and “deposition of glacial drifts in
glaciated lowlands”.
248
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
4) Lakes formed by Erosion
Karst Lakes
Karst lakes are formed in depressions, carved out by solvent action of rainwater
on water-soluble rocks such as limestone, gypsum and dolomite.
The collapse of limestone roofs of underground caves may result in the exposure
of long, narrow lakes that were once underground.
The shallow bed of these lakes is usually an insoluble layer of sediment so that
water is impounded, resulting in the formation of lakes.
Many karst lakes only exist periodically but return regularly after heavy rainfall.
Example – the Lac de Chaillexon in the Jura mountains
Otjikoto Karst Lake in Namibia
249
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Wind deflated lakes
These lakes are formed in arid regions and deserts
The depressions are created in deserts due to deflating action of winds
Groundwater may seep out in these depressions forming lakes
Excessive evaporation causes these to become salt lakes and Playas
Example – Great Basin of Utah, USA
5) Lakes formed by deposition
Ox-bow lakes
In large flood and delta plains, rivers rarely flow in straight courses. Loop-like
channel patterns called meanders develop over flood and delta plains
During a flood, a river may shorten its course by cutting across its meandering
loops, leaving behind a horse-shoe shaped channel as an ox-bow lake
Example – Ox-bow lakes are a common phenomenon in the floodplains of Lower
Mississippi, USA and Rio Grande (Mexico), Kanwar Lake Bird Sanctuary in Bihar,
India is one of Asia’s largest oxbow lakes.
250
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Meandering or river and Formation of Oxbow Lakes
Barrier Lakes
These lakes are formed by landslides, avalanches and such other processes
These processes cause damming up of the river by blocking the valleys
These lakes are short lived as the large piles of loose fragments soon give way
under the pressure of water. The sudden release of water from these lakes like
this can also cause floods
Example – Lake Gormire in Yorkshire, blocked by a landslide
6) Man-made lakes
Artificial lakes
Besides natural lakes, man has now created artificial lakes
Artificial lakes are created by erecting a concrete dam across a river valley
These dams help in creating a reservoir by impounding river water
251
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Guru Gobind Sagar Lake which supports the Bhakra Nangal Hydel Project is an
example of an artificial lake in India
c) Classification based on trophic level
Eutrophic Lake
Eutrophic lakes have very high levels of biological productivity.
The excessive level of nutrients, especially phosphorus and nitrogen gives rise to
an abundance of aquatic plants in these water bodies.
Usually, the water body will be dominated either by aquatic plants or algae.
Eutrophication might occur naturally or due to human impact on the
environment.
Some of Highly Eutrophicated Lake in India include Udaisagar Lake (Rajasthan)
and Dal Lake (Kashmir)
A Eutrophic Lake
252
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Mesotrophic Lake
Lakes with an intermediate level of productivity are called mesotrophic lakes.
The nutrients level of these lakes in medium or moderate.
They usually have clear water with submerged aquatic plants
Oligotrophic Lake
An oligotrophic lake is a lake with low primary productivity, as a result of low
nutrient content.
Algal production in these lakes is relatively low.
Often, they have very clear waters, with high drinking water quality
Paleolakes
A paleolake is a lake that existed in the past when hydrological conditions were
different.
Often, Paleolakes are identified based on relict lacustrine landforms such as
coastal landforms that form recognizable relict shorelines, referred to as paleo-
shorelines.
Paleolakes can also be recognized by characteristic sedimentary deposits that
accumulated in them and any fossils that these sediments might contain.
Evidence of prehistoric hydrological changes during the time of their existence
can be found from the sedimentary deposits of paleo-shorelines and paleo-lakes.
Types of Paleolakes
Former Lake – A former lake is a lake which is no longer in existence. Former lakes
include prehistoric lakes and permanently dried up lakes resulting
from evaporation or human intervention. A good example of a former lake is
Owens Lake in California, USA.
253
NeoStencil – Live Online Classes - IAS/IES/GATE/SSC/PSC | +91 95990 75552 | info@neostencil.com
Shrunken Lake – A shrunken lake is a lake which has drastically decreased in size
over geological time. A good example of a shrunken lake is Agassiz Lake, once
covering much of central North America.
top related