mw gs1 geography - upsc study material for ias prelims and

GEOGRAPHYMAINS WORKBOOK (GS 1)

Geography is a vast and dynamic subject under General Studies 1 of UPSC mains examination. It requires conceptual clarity of various mechanisms, processes and concepts. Questions in GS score geography workbook have been framed in a manner that addresses the conceptual aspects of most relevant questions of Physical, human and economic geography.

How will the workbook help students? It is meant to give students a comprehensive coverage of syllabus along with conceptual understanding of most complex topics under geography. Question and answer format enables students to learn how address the demands of the question specifi cally. For full benefi t students must fi rst practice sets on their own and then compare it with model hints given. This exercise will enable them in enrichment of their answers, build on their answer writing skills and also improve upon time management.The GS SCORE geography workbook consists of questions from the following topics:

Total 20 sets

Physical Geography (8 Sets)

Economic Geography (7 Sets)

Human Geography (5 Sets)

Covering Salient features of world’s physical geography.

Distribution of key natural resources across the world (including South Asia and the Indian sub-continent); factors responsible for the location of primary, secondary, and tertiary sec-tor industries in various parts of the world (including India)

Important Geophysical phenomena such as earthquakes, Tsunami, Volcanic activity, cyclone etc., geographical features and their location- changes in critical geographical fea-tures (including water-bodies and ice-caps) and in fl ora and fauna and the effects of such changes.

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GEOGRAPHYwww.iasscore.in

SET: 1-8QUESTION & ANSWER

PHYSICAL GEOGRAPHYWORKBOOK

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WORKBOOK | GS MAINS PAPER - 1 www.iasscore.in

Why total solar eclipses are rare at any particular location on the earth’s continental Q1. surface? What do you understand by Bailey’s Bead Effect during the Solar Eclipse? Explain the impact of solar eclipse on Earth?

Describe the sources which provide the information about the Earth’s interior. Why Q2. seismology is the only method which gives complete authenticated information about the earth’s interior.

Why Himalayas are more prone to earthquakes than Western Ghats? Locate the Q3. various seismic highways in India by drawing a map of India & discuss the mechanism of propagation of earthquake inside the earth surface.

Evaluate the theory of Continental Drift as given by Wagner. How does the Plate Q4. Tectonic Theory seek to remove the shortcomings of the Continental Drift Theory?

Though oceanic fl oors are considered to be the most fundamental rocks in origin, no Q5. one of them are beyond the age of Mesozoic times. Explain it in the light of sea fl oor spreading theory. How is it different from continental drift theory?

Discuss the main features of Plate Tectonic Theory. Q6.

Discuss briefl y the theory of Isostasy? What is its role in the geo-morphological Q7. context?

What is delta? Illustrate its signifi cance. Explain the factors responsible for sinking Q8. of deltas in India, and strategies to tackle the same

What do you mean by emergent & submergent coast? Identify the emergent and Q9. submergent coast of India and discuss their signifi cance.

The discovery of the mid-oceanic ridges opened doors to not only plate motion but Q10. the age of the ocean fl oor and other geological phenomenon. Elaborate

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PHYSICAL GEOGRAPHY (SET-1)QUESTIONS & ANSWERS

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Why total solar eclipses are rare at any particular location on the earth’s continental 1. surface? What do you understand by Bailey’s Bead Effect during the Solar Eclipse? Explain the impact of solar eclipse on Earth?

(250 words)

APPROACH

Briefl y introduce the total solar eclipses 1. (50 words)

Discuss why total solar eclipses are rare at any particular location 2. (70 words) on the earth’s continental surface

Defi ne Bailey’s Bead Effect during the Solar Eclipse 3. (80 words)

Discuss the impacts of solar eclipse on Earth 4. (50 words)

HINTS

Total solar eclipse is the condition in which the moon moves between the Earth and the Sun and thus covers the entire sun. However, this phenomenon is very rare because:

Most of the time the moon passes a little above or little below the Sun and thus is not able to cover the entire Sun.

Even if the moon comes exactly in line with the sun, the shadow of moon is so small that it covers very little area over the earth

Most of the time this shadow falls over the water bodies and thus humans cannot see solar eclipse over continents.

Condition of perigee must be satisfi ed when sun moon and earth comes in the straight line.

Bailey’s Bead Effect

Moon surface is not exactly plain and contains some mountains and canyons. When moon passes over sun during a solar eclipse, the canyons over the moon allows beads of sunlight to pass through the moon’s periphery. This phenomenon in which bead of light of the sun is seen during solar eclipse is called Bailey’s Bead Effect

Impact of Solar Eclipse on earth

Researchers who studied an eclipse across Europe in 1999 found that the event lowered air temperatures by as much as 5°F across the path of totality.

This brief dip in air temperatures also affected local wind speed and direction

SUPPLEMENTARY NOTES

A total solar eclipse occurs when the disk of the moon appears to completely cover the disk of the sun in the sky. The fact that total solar eclipses occur at all is a quirk of cosmic geometry. The moon orbits an average of 239,000 miles (385,000 kilometres) from Earth — just the right distance to seem the same size in the sky as the much-larger sun. However, these heavenly bodies line up only about once every 18 months.

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Near the beginning and end of total solar eclipse, the thin slice of the Sun visible appears broken up into beads of light. These lights are called ‘Bialy’s Beads’ after the British astronomer Francis Baily who discovered them. They occur because the edge of the Moon is not smooth but jagged with mountain peaks. When just one bead is visible, the effect is often likened to a diamond ring and hence called as Diamond Ring Effect.

Describe the sources which provide the information about the Earth’s interior. Why 2. seismology is the only method which gives complete authenticated information about the earth’s interior.

(250 words)

APPROACH

Briefl y describe the sources which provide the information about the 1. (100 words) Earth’s interior

Defi ne the theories of the interior of the Earth. 2. (100 words)

Conclusion 3. (50 words)

HINTS

Understanding of the earth’s interior is essential to understand the nature of changes that take place over and below the earth’s surface.The sources which provide knowledge about the interior of the earth may be classifi ed into 3 groups,

Artifi cial Sources Theories of the interior of the Earth Natural Sources Artifi cial Sources:

Density:The average density of the crust is around 2.9 to 3.3 g/cm3 While the average density of the Earth is 5.5 Thus, the core of the earth will be, without doubt, more than 5.5. Later, the density of the earth core was found out to be 11

Pressure:Now the question is what is the reason for very high density of the core? Previously it was believed that, as pressure increases, density also increases due to super incumbent force.But there is critical limit beyond which pressure increase does not increase density. Thus, core must be composed of intrinsically heavy metallic materials of high density (i.e. iron and nickel)

Temperature:

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Temperature of the upper part of the magma slab representing upper portion of oceanic crust has been estimated to be 0°C, while temperature at around 70km was found out to be 1200°C.With this rate of increase of temperature, core should be approx. 25000°C. But it would have melted the whole earth.Thus, it is evident that most part of the radioactive materials which releases heat in large amount and thus causes rock melting, are concentrated in the crust.While rate of increase of temperature downwards decreases with increasing depth.

Theories of the interior of the Earth:Edward Suess theory: According to him the earth is made up of three zones of different matter below the outer thin sedimentary cover viz. SIAL, SIMA and NIFE.

Natural Sources:Volcanism: The lava coming from inside the earth shows that the same material has to be there inside the earth. However, all these methods were just speculations or partially proved concepts. The real concrete proof about the interior of the earth was provided by the Seismic studies. The behaviour of seismic waves in different layers of the earth provides the most authentic evidence about the composition & structure of the earth. The different types of waves generated during the occurrence of earthquake, are generally divided into 3 broad categories; (a) Primary waves, (b) Secondary waves, (c) surface waves. Seismic Waves - both P-waves & S-waves travel faster through rigid material than through soft or plastic material. The velocities of these waves travelling through specifi c part of earth give an indication of type of rocks there. Abrupt changes in seismic wave velocities indicate signifi cant changes in earth’s interior. Seismic waves passing through the earth are refracted in ways that show distinct discontinuities within earth’s interior & provide basis for the belief that earth has

A solid inner core, (2) a liquid outer core, (3) a soft asthenosphere, (4) a rigid lithosphere.

Why Himalayas are more prone to earthquakes than Western Ghats? Locate the 3. various seismic highways in India by drawing a map of India & discuss the mechanism of propagation of earthquake inside the earth surface.

(250 words)

APPROACH

Briefl y introduce Earthquake & Why Himalayas are more prone to 1. (70 words) earthquakes than Western Ghats

Locate the various seismic highways in India by drawing a map of India 2. (60 words)

Discuss the mechanism of propagation of earthquake inside the 3. (60 words) earth surface


HINTS

An earthquake can be defi ned as a sudden violent shaking of the ground as a result of movements in the

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earth’s crust or volcanic action. These earthquakes result from a series of earth movements brought about by a sudden release of energy during the tectonic activities in the earth’s crust.

India is prone to almost all natural disasters ranging from earthquakes, landslides, tsunami, drought, fl oods, and cyclones due to its varied physiographic. The Himalayas and the Western Ghats have been formed by the plate movements and are prone to a variety of disasters including earthquakes and landslides. However, the Himalayas have more chances of earthquakes when compared to Western Ghats. The reasons are:The Himalayas are Young – tertiary mountains whereas Western Ghats are Older-block Mountain. Himalayas has not yet reached its isostatic equilibrium which makes it more prone to frequent earth quakes but on the other hand Western Ghats are located on stable part of Indian plate and the chances of colliding of plates in this region are less when compared to Himalayas. Indian plate is moving northwards and subsiding under Eurasian plate, 5-10 cm a year. This leads to rising of Himalayas every year whereas Moving of Indian plate does not affect Western Ghats. This also affects the earthquakes that come in the region.

Unplanned growth of houses in the hill areas increases the damage and the loss that occurs due to the disasters in the region whereas limiting the damage in Western Ghats.

Thus, the main reason for more earthquakes in the Himalayas is due to proneness to more plate movements and due to instability of the Himalayan Mountains.

The map given depicts the different seismic zones of India.

Mechanism of propagation of earthquake inside the earth surfaceEarthquake waves are basically of two types — body waves and surface 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. The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. 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 refl ect or refract when coming across materials with different densities.

The propagation of different waves

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As the waves move or propagate, they cause vibration in the body of the rocks through which they pass. P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material. Other three waves vibrate perpendicular to the direction of propagation. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass. Surface waves are considered to be the most damaging waves.

Evaluate the theory of Continental Drift as given by Wagner. How does the Plate 4. Tectonic Theory seek to remove the shortcomings of the Continental Drift Theory?

(200 words)

APPROACH

Give a detailed description of Continental Drift Theory 1. (50 words)

What provided base for the development of Plate tectonic theory 2. (60 words)

Differentiate the Plate Tectonic Theory with the Continental Drift Theory 3. (60 words)


HINTS

Continental drift is the movement of the Earth’s continents relative to each other, thus appearing to “drift” across the ocean bed. The speculation that continents might have ‘drifted’ was fi rst put forward by Abraham Ortelius in 1596. The concept was independently and more fully developed by Alfred Wegener in 1912, but his theory was rejected by some for lack of a mechanism (though this was supplied later by Arthur Holmes) and others because of prior theoretical commitments. The idea of continental drift has been subsumed by the theory of plate tectonics, which explains how the continents move.

Alfred Wegener a German meteorologist who put forth a comprehensive argument in the form of “the continental drift” in 1912.

According to Wegener, all the continents formed a single continental mass and Mega Ocean surrounded the same. The super continent was named Pangaea, which meant all earth and Mega Ocean surrounded it named Panthalassa, meaning all water. He argued that, around 200 million years ago, the super continent, Pangaea fi rst broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern component respectively. Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today.

When Wegener proposed continental drift, many geologists were contractionists. They thought Earth’s incredible mountains were created because our planet was cooling and shrinking since its formation. But Wegener’s continental drift theory was great step ahead of contractionist theory. It provided base for development of Plate tectonic theory.

However, the theory has many fl aws:-He failed to explain the forces that lead to the drift of huge continental masses. As per the theory force of buoyancy, differential gravitational force along with tidal force of sun and moon are the real cause, which are not suffi cient enough to drift huge land masses.

nitially the theory considers that there is no resistance offered by the denser SIMA layer but later Wegener adopts the force full resistance offered by SIMA to explain the origin of mountains.

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The theory did not talk about the direction of the drift and chronological sequence of this displacement.

At that time people hardly have any understanding about ocean bed topography.

Also Wegener provided those evidences which supports theory and neglected those which did not support.

Wegener did not explain about how Pangaea got formed .He don’t talk about pre Pangaea condition of earth.

Plate tectonics is the widely accepted theory that Earth’s crust is fractured into rigid, moving plates. In the 1950s and 1960s, scientists discovered the plate edges through magnetic surveys of the ocean fl oor and through the seismic listening networks built to monitor nuclear testing. Alternating patterns of magnetic anomalies on the ocean fl oor indicated seafl oor spreading, where new plate material is born. Magnetic minerals aligned in ancient rocks on continents also showed that the continents have shifted relative to one another.

Though Wegner was a pioneer in bringing the concept of continental drift to limelight but the limitations in his theory ignited minds to improve the concept and explain the process. This gives rise to Plate Tectonic Theory.

Plate tectonics revolve around the concept of continental drift and sea fl oor spreading. The theory describes constructive plate margins or divergent plates leading to upwelling of lava from mantle that are responsible for forming new oceanic ridges.

It explains the destructive plate margins when two plates collide along a margin, leading to one plate overriding the other. The overridden plate is sub ducted into the mantle thereby losing a part of the crust.

Strahler & Stahler who are behind this new theory considered the constant thermal convection currents from the inner part of earth as the forces responsible for this movement along with geo magnetic fi eld of the earth.

To overcome the limitations of continental drift and to explain the cause of the drift incorporating seafl oor spreading moulded into plate tectonic theory in 1968. According to which, it was assumed that the Earth was divided into lithosphere and asthenosphere at about 7000km. As asthenosphere is said to be made of viscous fl owing material which can bear non hydrostatic force only for 1000-10000years or longer and convection currents can occur here. While lithosphere comprising of Earth’s crust and upper mantle having variable thickness of 200km, can bear non hydrostatic force for a 100 million years or so. The lithosphere is further divided into a number of large and small plates, which are in constant relative motion due to convection currents in the asthenosphere. It is this movement of tectonic plates which leads to the continental drift. This caused Pangaea to break up which is still continuing, that proves the Wegener’s hypothesis. Due to this tectonic framework, the continents have been coming together and then fragmenting and drifting apart for billions of years.

The plate tectonic was based on the evidence of seafl oor spreading and palaeomagnetism. Both concepts were backed by strong scientifi c evidence. The theory assumed that continents are part of “plates”, mass of lithosphere, which fl oats over asthenosphere, evidence for the same has been found. The theory suggests plate movement based thermal convective current movement in mantle.

Plate tectonic based on strong scientifi c evidence conclusively explained the breaking up Pangaea and drifting of continent as on present day. It also explains the expansion of oceans fl oor and orogeny. Thus the theory explains all events along with strong scientifi c evidence. And hence PT is a large improvement over CDT.

Though oceanic fl oors are considered to be the most fundamental rocks in origin, no 5. one of them are beyond the age of Mesozoic times. Explain it in the light of sea fl oor spreading theory. How is it different from continental drift theory?

(250 words)

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APPROACH

Discuss the theory of sea fl oor spreading. 1. (50 words)

“Though oceanic fl oors are considered to be the most fundamental 2. (100 words) rocks in origin, no one of them are beyond the age of Mesozoic times” Give reasons behind this anomaly

Discuss the difference between sea fl oor spreading and continental 3. (100 words) drift theory are as follows

HINTS

Seafl oor spreading is a geologic process in which tectonic plates—large slabs of Earth’s lithosphere—split apart from each other.The theory of sea fl oor spreading was propounded by Harry Hess. During Glomar expedition of the Atlantic Ocean he observed that oceanic rocks are the fi rst to form over the earth surface but still no one of them are beyond Mesozoic times. There were several reasons behind this anomaly as follows

There is a continuous release of magma along the mid oceanic ridge due to divergence and thus the oceanic fl oor is continuously getting formed.

Also, there if there would have been only divergent boundaries then earth size by now would have increased enormously which is not true.

Thus, he said that there must be some convergent boundary which exists between continental margins which sub ducts the oceanic slab and recycle them.

It may be concluded that due to subduction of the plates along the plate boundaries, the old plates are continuously consumed.

The difference between sea fl oor spreading and continental drift theory are as follows:CDT explains movement of continental plates only while SFS theory explains movement of oceanic plates only.

Forces responsible for the movement of the continents according to CDT are buoyancy force, gravity, tidal currents while according to SFS the oceanic plates move due to convection currents.

CDT theory is too general with vague conclusions while SFS theory is partial in its explanation.

SUPPLEMENTARY NOTES

Seafl oor spreading is a geologic process in which tectonic plates—large slabs of Earth’s lithosphere—split apart from each other.Seafl oor spreading and other tectonic activity processes are the result of mantle convection. Mantle convection is the slow, churning motion of Earth’s mantle. Convection currents carry heat from the lower mantle and core to the lithosphere. Convection currents also “recycle” lithospheric materials back to the mantle.Seafl oor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense. The less-dense material rises, often forming a mountain or elevated area of the seafl oor.Eventually, the crust cracks. Hot magma fueled by mantle convection bubbles up to fi ll these fractures and spills onto the crust. This bubbled-up magma is cooled by frigid seawater to form igneous rock. This rock (basalt) becomes a new part of Earth’s crust.

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Mid-Ocean RidgesSeafl oor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean fl oor. The Mid-Atlantic Ridge, for instance, separates the North American plate from the Eurasian plate, and the South American plate from the African plate. The East Pacifi c Rise is a mid-ocean ridge that runs through the eastern Pacifi c Ocean and separates the Pacifi c plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate. The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate.Seafl oor spreading is not consistent at all mid-ocean ridges. Slowly spreading ridges are the sites of tall, narrow underwater cliffs and mountains. Rapidly spreading ridges have much gentler slopes. The Mid-Atlantic Ridge, for instance, is a slow spreading centre. It spreads 2-5 centimetres (.8-2 inches) every year and forms an ocean trench about the size of the Grand Canyon. The East Pacifi c Rise, on the other hand, is a fast spreading centre. It spreads about 6-16 centimetres (3-6 inches) every year. There is not an ocean trench at the East Pacifi c Rise, because the seafl oor spreading is too rapid for one to develop!The newest, thinnest crust on Earth is located near the centre of mid-ocean ridge—the actual site of seafl oor spreading. The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge.

Geomagnetic ReversalsThe magnetism of mid-ocean ridges helped scientists fi rst identify the process of seafl oor spreading in the early 20th century. Basalt, the once-molten rock that makes up most new oceanic crust, is a fairly magnetic substance, and scientists began using magnetometers to measure the magnetism of the ocean fl oor in the 1950s. What they discovered was that the magnetism of the ocean fl oor around mid-ocean ridges was divided into matching “stripes” on either side of the ridge. The specifi c magnetism of basalt rock is determined by the Earth’s magnetic fi eld when the magma is cooling.Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The continual process of seafl oor spreading separated the stripes in an orderly pattern.

Geographic FeaturesOceanic crust slowly moves away from mid-ocean ridges and sites of seafl oor spreading. As it moves, it becomes cooler, denser, and thicker. Eventually, older oceanic crust encounters a tectonic boundary with continental crust.In some cases, oceanic crust encounters an active plate margin. An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquakes and volcanoes. Oceanic crust created by seafl oor spreading in the East Pacifi c Rise, for instance, may become part of the Ring of Fire, the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacifi c Ocean basin.In other cases, oceanic crust encounters a passive plate margin. Passive margins are not plate boundaries, but areas where a single tectonic plate transitions from oceanic lithosphere to continental lithosphere. Passive margins are not sites of faults or subduction zones. Thick layers of sediment overlay the transitional crust of a passive margin. The oceanic crust of the Mid-Atlantic Ridge, for instance, will either become part of the passive margin on the North American plate (on the east coast of North America) or the Eurasian plate (on the west coast of Europe).New geographic features can be created through seafl oor spreading. The Red Sea, for example, was created as the African plate and the Arabian plate tore away from each other. Today, only the Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict, seafl oor spreading will completely separate the two continents—and join the Red and Mediterranean Seas.

Mid-ocean ridges and seafl oor spreading can also infl uence sea levels. As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes denser. This increases the volume of the ocean basin and decreases the sea level. For instance, a mid-ocean ridge system

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in Panthalassa—an ancient ocean that surrounded the supercontinent Pangaea—contributed to shallower oceans and higher sea levels in the Palaeozoic era. Panthalassa was an early form of the Pacifi c Ocean, which today experiences less seafl oor spreading and has a much less extensive mid-ocean ridge system. This helps explain why sea levels have fallen dramatically over the past 80 million years.Seafl oor spreading disproves an early part of the theory of continental drift. Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans. Seafl oor spreading proves that the ocean itself is a site of tectonic activity.

Though oceanic fl oors are considered to be the most fundamental rocks in origin, no 6. one of them are beyond the age of Mesozoic times. Explain it in the light of sea fl oor spreading theory. How is it different from continental drift theory?

(250 words)

APPROACH

Give a detailed description of the plate tectonic theory 1. (100 words)

Discuss its role in volcanic eruption, earthquakes, mountain building, etc. 2. (100 words)

HINTS

The Plate tectonics theory states that Earth’s outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacifi c plates. Several minor plates also exist, including the Arabian, Nazca, and Philippines plates. The plates are all moving in different directions and at different speeds (from 2 cm to 10 cm per year) in relationship to each other. The place where the two plates meet is called a plate boundary.

There are three types of plate boundaries:Diverging currents produce tension at the contact-zone of crust leading to fracture. Magnetic material penetrates into the fractroges and gets solidifi ed. This continuous process pushes the blocks in opposite direction and creates a new zone, known as “zone of construction”.

At convergent boundary, two adjacent plates come further and further closer to each other and collide. When both sides are of continental nature, a mountain formation is evident. When one of the two is continental and the other maritime again mountain comes into being along the boundary. In this case, continental plate overrides the maritime. When both plates are of maritime, both of them break, sub duct and penetrate below and, hence, trenches are formed. Along this boundary earthquakes and volcanic activities are prominent. In all these three situations, surface area is reduced, therefore, this is also known as “zone of destruction”.

Transform fault is the one when two adjacent plates slide past each other. Direction of movement may be along or against but they move parallel to each other. Therefore, neither there is any construction of fresh area nor it has any destruction. Hence, it is known as “zone of preservation”.

It provides a unifi ed mechanism explaining: Igneous, sedimentary, and metamorphic rocks.

The distribution of earthquakes and volcanoes.

The origin of continents and ocean basins.

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The distribution of fossil plants and animals.

The genesis and destruction of mountain chains.

Continental drift.

Plate tectonics and distribution of natural resources

Besides being responsible for the major features of Earth’s crust and infl uencing the distribution and evolution of the world’s biota, plate movements also affect the formation and distribution of some natural resources. The formation of many natural resources results from the interaction between plates, and economically valuable concentrations of such deposits are found associated with current and ancient plate boundaries. Consequently, geologists are using plate tectonic theory in their search for petroleum and mineral deposits and in explaining the occurrence of these natural resources.

Many metallic mineral deposits such as copper, gold, lead, silver, tin, and zinc are related to igneous and associated hydrothermal (hot water) activity, so it is not surprising that a close relationship exists between plate boundaries and the occurrence of these valuable deposits.

The magma generated by partial melting of a sub ducting plate rises toward the surface, and as it cools, it precipitates and concentrates various metallic ores. Many of the world’s major metallic ore deposits are associated with convergent plate boundaries, including those in the Andes of South America, the Coast Ranges and Rockies of North America, Japan, the Philippines, Russia, and a zone extending from the eastern Mediterranean region to Pakistan.

In addition, the majority of the world’s gold is associated with sulphide deposits located at ancient convergent plate boundaries in such areas as South Africa, Canada, California, Alaska, Venezuela, Brazil, southern India, Russia, and Western Australia. The copper deposits of western North and South America are an excellent example of the relationship between convergent plate boundaries and the distribution, concentration, and exploitation of valuable metallic ores.

The world’s largest copper deposits are found along this belt. The majority of the copper deposits in the Andes and the south-western United States formed less than 60 million years ago when oceanic plates were sub ducted under the North and South American plates. The rising magma and associated hydrothermal fl uids carried minute amounts of copper, which was originally widely disseminated but eventually became concentrated in the cracks and fractures of the surrounding andesitic. These low-grade copper deposits contain from 0.2 to 2% copper and are extracted from large open-pit mines.

Divergent plate boundaries also yield valuable ore deposits. The island of Cyprus in the Mediterranean is rich in copper and has been supplying all or part of the world’s needs for the last 3000 years. The concentration of copper on Cyprus formed as a result of precipitation adjacent to hydrothermal vents along a divergent plate boundary. This deposit was brought to the surface when the copper-rich seafl oor collided with the European plate, warping the seafl oor and forming Cyprus.

Discuss briefl y the theory of Isostasy? What is its role in the geo-morphological 7. context?

(200 words)

APPROACH

Discuss briefl y the theory of Isostacy 1. (100 words)

Discuss is its role in the geo-morphological context 2. (100 words)

HINTS

Isostasy is the state of gravitational equilibrium between Earth’s crust and mantle such that the crust “fl oats” at an elevation that depends on its thickness and density. The term ‘isostasy’ was coined in the year 1889 by the American geologist Clarence Dutton.

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This concept is invoked to explain how different topographic heights can exist at Earth’s surface. When a certain area of Earth’s crust reaches the state of isostasy, it is said to be in isostatic equilibrium. Isostasy does not upset equilibrium but instead restores it (a negative feedback). It is generally accepted that Earth is a dynamic system that responds to loads in many different ways. However, isostasy provides an important ‘view’ of the processes that are happening in areas that are experiencing vertical movement.

For instance, certain areas (such as the Himalayas which are in its young stage) are not in isostatic equilibrium, which has forced researchers to identify other reasons to explain their topographic heights.

Isostasy controls the regional elevations of continents and sea fl oors in accordance with the densities of their underlying rocks. This means that an excess of mass seen as material above sea level, as in a mountain system, is due to a defi cit of mass, or low-density roots, below sea level. Consequently, high mountains have low-density roots that extend deep into the underlying mantle.

The concept of isostasy played an important role in the development of the hypothesis of plate tectonics.

In the simplest example, isostasy is the principle of buoyancy wherein an object immersed in a fl uid is buoyed with a force equal to the weight of the displaced fl uid. On a geological scale, isostasy can be observed where Earth’s strong crust or lithosphere exerts stress on the weaker mantle or asthenosphere, which, over geological time, fl ows laterally such that the load is accommodated by height adjustments.

Isostatic effects of deposition and erosion:

When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink. A likely, when large amounts of material are eroded away from a region, the land may rise to compensate.

An analogy may be made with an iceberg - it always fl oats with a certain proportion of its mass below the surface of the water. If more ice is added to the top of the iceberg, the iceberg will sink lower in the water.

If a layer of ice is somehow sliced off the top of the iceberg, the remaining iceberg will rise. A likely, the Earth’s lithosphere “fl oats” in the asthenosphere.

Isostatic effects of plate tectonics:

When continents collide, the continental crust may thicken at their edges in the collision. If this happens, much of the thickened crust may move downwards rather than up as with the iceberg analogy. The idea of continental collisions building mountains “up” is consequently rather a simplifi cation. Instead, the crust thickens and the upper part of the thickened crust may become a mountain range. However, some continental collisions are far more complex than this, and the region may not be in isostatic equilibrium, so this subject has to be treated with caution.

Post-glacial rebound effect:

Post-glacial rebound (sometimes called continental rebound, glacial isostasy, and glacial isostatic adjustment) is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial phase, through a process known as isostasy. It affects northern Europe (especially Scotland, Estonia, and northern Denmark), Siberia, Canada, the Great Lakes of Canada and the United States, the coastal region of the US state of Maine, parts of Patagonia, and Antarctica.

Post-glacial rebound produces measurable effects on vertical crustal motion, global sea levels, horizontal crustal motion, gravity fi eld, Earth’s rotational motion and a state of stress and earthquakes.

14


SUPPLEMENTARY NOTES

Although originally defi ned in terms of continental crust and mantle, it has subsequently been interpreted in terms of lithosphere and asthenosphere, particularly with respect to oceanic island volcanoes such as the Hawaiian Islands.Imaginary columns of equal cross-sectional area that rise from the asthenosphere to the surface are assumed to have equal weights everywhere on Earth, even though their constituents and the elevations of their upper surfaces are signifi cantly different.Isostasy, ideal theoretical balance of all large portions of Earth’s lithosphere as though they were fl oating on the denser underlying layer, the asthenosphere, and a section of the upper mantle composed of weak, plastic rock that is about 110 km below the surface. Consequently, as a mountain range is eroded down, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata are eroded away and the lower layers rebound upwards again.

Three principal models of isostasy:The Airy-Heiskanen model: where different topographic heights are accommodated by changes in crustal thickness, in which the crust has a constant densityThe Pratt-Hayford model: where different topographic heights are accommodated by lateral changes in rock density.The Vening Meinesz or fl exural isostasy model: where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending.

Recently, the term post-glacial rebound is gradually being replaced by the term glacial isostatic adjustment. This is in recognition that the response of the Earth to glacial loading and unloading is not limited to the upward rebound movement, but also involves downward land movement, horizontal crustal motion, changes in global sea levels, the Earth’s gravity fi eld, induced earthquakes and changes in the rotational motion. An alternate term that is sometimes used is glacial isostasy, because the uplift near the centre of rebound is due to the tendency towards the restoration of isostatic equilibrium (as in the case of isostasyof mountains). Unfortunately, that term gives the wrong impression that isostatic equilibrium is somehow reached, so by appending “adjustment” at the end, the motion of restoration is emphasized.Studies of glacial rebound give us information about the fl ow law of mantle rocks and also past ice sheet history. The former is important to the study of mantle convection, plate tectonics and the thermal evolution of the Earth. The latter is important to glaciology, pale climate and changes in global sea level. Understanding postglacial rebound is also important to our ability to monitor recent global change.

What is delta? Illustrate its signifi cance. Explain the factors responsible for sinking of 8. deltas in India, and strategies to tackle the same.

(200 words)

APPROACH

What is delta and also defi ne its signifi cance? 1. (50 words)

Explain the factors responsible for sinking of deltas in India. 2. (80 words)

Conclusion - Strategies to tackle the problem. 3. (70 words)

15


HINTS

Delta is a depositional feature of almost triangular shape at the mouth of a river debouching either into a lake or a sea.

A river moves more slowly as it nears its mouth or end. The slowing velocity of the river and the build-up of sediment allow the river to break from its single channel as it nears its mouth. Under the right conditions, a river forms a deltaic lobe. A mature deltaic lobe includes a distributaries network—a series of smaller, shallower channels, called distributaries, that branch off from the mainstream of the river.

In a deltaic lobe, heavier, and coarser material settles fi rst. Smaller, fi ner sediment is carried farther downstream. The fi nest material is deposited beyond the river’s mouth. This material is called alluvium or silt. As silt builds up, new land is formed. This is the delta.

Signifi canceDeltas are important to both human activities and fi sh and other wildlife because they are normally home to very fertile soil as well as a large amount of vegetation. Major ancient civilizations grew along deltas.

Today deltas remain important to humans because they are a source of sand and gravel. In many deltas this material is highly valuable and is used in the construction of highways, buildings and other infrastructure.

In other areas delta land is important in agricultural use. For example the Ganga-Brahmaputra delta in India and Bangladesh is one of the most fertile regions of the world. Fish, other seafood, and crops such as rice and tea are the leading agricultural products of the delta.

In addition to these human uses, river deltas are some of the most bio-diverse areas on the planet and as such it is essential that they remain healthy to provide habitat for the many species of plants, animals, insects and fi sh that live in them. Many animals are indigenous to the shallow, shifting waters of a delta. Fish, crustaceans such as oysters, birds, insects, and even apex predators such as tigers and bears can be part of a delta’s ecosystem. There are many different species of rare, threatened, and endangered species living in deltas and wetlands, e.g., Sundarbans delta.

Deltas can provide a buffer for cyclones, hurricanes, and tsunami. The Mississippi River delta, for example, can act as a barrier and reduces the impact of potentially strong hurricanes in the Gulf of Mexico. Similarly, the deltas on the east coast of India with lots of mangroves have helped many Indian states during tsunamis and cyclones.

Deltas provide excellent site for ports and harbours. The Fraser delta helps make Vancouver one of the busiest cosmopolitan ports in the world.

The Delta is also valued for its aesthetic appeal and for its support of recreational activities.

It is an attractive and growing destination for boating, fi shing, hunting, and ecotourism.

Factors responsible for sinking of deltas Land formed by river sediments naturally subsides and sinks over time. Historically, sediment deposition and accretion by plant growth outpaced the natural subsidence, resulting in coastal land gain. Without land-building deposits from the river, subsidence dominates and massive areas of land sink and disappear below sea-level.

Damming and diverting rivers means that much less sediment now reaches many delta ares, while extraction of gas and groundwater also lowers the land.

The sinking of deltas from India to America is exacerbated by the upstream trapping of sediments by reservoirs and dams, man-made channels and levees that whisk sediment into the oceans beyond coastal fl oodplains, and the accelerated compacting of fl oodplain sediment caused by the extraction of groundwater and oil and natural gas exploration and mining.

16


It has been established that the main reason behind delta subsidence is drastically reducing sediments reaching the delta. Studies estimate that during the past century, there has been a 94% reduction in Krishna’s sediment reaching the delta, 95% reduction from historic load in Narmada, 80% reduction in Indus, 80% reduction in Cauvery, 96% reduction in Sabarmati, 74% reduction in Mahanadi, 74% reduction in Godavari, 50% reduction in Brahmani, etc.Almost unanimous agreement between scientists indicates that the reason behind this drastic decline in sediments is sediment retention by dams and reservoirs in the upstream. Large reservoirs trap as much as 80% of the upstream silt. As a result, most rivers are carrying much less sediment, and some rivers (like Krishna, Indus, Nile, and Colorado) transport virtually no sediment.Groundwater mining that causes land compaction is also a cause for sinking of deltas. Groundwater mining has led to a signifi cant compaction and subsidence of land over the last 15 years in Ganga

Strategies to tackle it:A comprehensive solution for the Delta needs to consider the new understanding of the Delta’s ecosystem that has emerged over the last several years. The state’s policy has been to maintain the Delta as a freshwater system. However, to address the problems of the Delta’s native species, a fundamental change in policy is needed. A Delta that is heterogeneous and variable in terms of its salinity levels and water fl ows is more likely to support native species than is a homogeneously fresh or brackish Delta. Accepting the vision of a variable Delta, as opposed to the commonly held vision of a static Delta, will allow for more sustainable management.

What do you mean by emergent & submergent coast? Identify the emergent and 9. submerging coast of India and discuss their signifi cance.

(200 words)

APPROACH

What do you mean by emergent & submergent coast? 1. (50 words)

Identify the emergent and submergent coast of India. 2. (60 words)

Discuss their signifi cance. 3. (60 words)


HINTS

The Emergent coasts are the result of local tectonic uplift or fall in the global elevation of sea level. They have rocky coastlines with cliffs and nearly fl at platforms that extend inland, which were coastal plains earlier. Whereas submergent Coasts are those that have been fl ooded by ocean waters because of a relative rise in the elevation of sea level at that location. A characteristic of the submergent coast is river valleys or glacially-carved valleys that have been fl ooded by ocean water.In case of India, the west coast is an example of submergent coast. The western coastal plains are narrow in the middle and get broader towards north and south. As compared to the western coastal plain, the eastern coastal plain is broader and is an example of an emergent coast.

The signifi cance of the coastal plains is as follows:

The west coast: It is a narrow indented belt and because of submergence provides natural conditions for the development of ports and harbours. Some of the important natural ports located along the west coast are Kandla,

17


Mazagaon, Marmagao, Mangalore, Cochin, etc. The rivers fl owing through this coastal plain do not form any delta. The southern part of the western coastal plain, i.e. the Malabar coast, has a distinguishing feature - ‘Kayals’ (backwaters) - which are used for fi shing, inland navigation and are also special attraction for tourists. The western coastal plain is relatively rocky mainly formed by coarse grained soil and thus is less fertile.

The east coast: Being broader, it has well-developed deltas formed by the rivers fl owing in to the Bay of Bengal. These include the deltas of the Mahanadi, the Godavari, the Krishna and the Kaveri. Because of its emergent nature, it has less number of ports and harbours. The continental shelf extends up to 500 km into the sea, which makes it diffi cult for the development of good ports and harbours. The eastern coastal plain consists of fertile alluvial soil. Hence, well developed intensive agriculture is practiced on these coastal plains. However, the eastern coastal plain is more prone to cyclone than western coastal plain.

The discovery of the mid-oceanic ridges opened doors to not only plate motion but 10. the age of the ocean fl oor and other geological phenomenon. Elaborate

(200 words)

APPROACH

Briefl y introduce what is mid-oceanic ridge 1. (50 words)

Discuss how the discovery of the mid-oceanic ridges opened doors to not 2. (120 words) only plate motion but the age of the ocean fl oor and other geological phenomenon.


HINTS

A mid-ocean ridge is an underwater mountain system.

Mid-ocean ridges are geologically active, with new magma constantly emerging onto the ocean fl oor and into the crust at and near rifts along the ridge axes. The crystallized magma forms new crust of basalt and gabbros.

The discovery of the mid-oceanic ridges opened doors to not only plate motion but the age of the ocean fl oor. The rocks making up the crust below the sea fl oor are youngest at the axis of the ridge and age with increasing distance from that axis.

A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

Hence the discovery of mid oceanic ridges led to the establishment of following phenomenon: Occurrence of earthquakes near the mid-oceanic ridges and absence of sediments on their summits (tops).Presence of active volcanoes on mid-oceanic ridges.

Absence of rocks older than 135 million years on the ocean fl oor.

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mw gs1 geography - upsc study material for ias prelims and

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