RAS Mains General Studies Paper-II Unit-III Part-A World Geography Notes

RAS/RTS Mains General Studies Paper-II

Unit III- Earth Science (Geography & Geology)

Part A- World

  • Broad Physical Feature: Mountains, Plateaus, Plains, Lakes and Glaciers
  • Earthquakes and Volcanoes: Types, distribution and their impact
  • Earth and its Geological time scale
  • Current Geopolitical Problems

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Broad Physical Feature: Mountains, Plateaus, Plains, Lakes and Glaciers:-

Landforms are the natural features and shapes existent on the face of the earth.

  • Landforms possess many different physical characteristics and are spread out throughout the planet. Together, landforms constitute a specific terrain and their physical arrangement in the landscape forms what is termed as topography.
  • The physical features of landforms include slope, elevation, rock exposure, stratification and rock type. Oceans and continents illustrate the largest grouping of landforms. They are they further subcategorized into many different landforms based on their physical features and shapes. Examples of distinctive landforms include mountains, valleys, plateaus, glaciers, hills, loess, deserts, shorelines, and plains. Features such as volcanoes, lakes, rivers, mid-ocean ridges, and the great ocean basins are also part of landform features.

The earth has an infinite variety of landforms.  Some parts of the lithosphere may be rugged and some flat. These landforms are a result of two processes. Within the earth, a continuous movement is taking place. The first or the internal process leads to the upliftment and sinking of the earth’s surface at several places. The second or the external process is the continuous wearing down and rebuilding of the land surface. The wearing away of the earth’s surface is called erosion. The surface is being lowered by the process of erosion and rebuilt by the process of deposition. These two processes are carried out by running water, ice and wind. Broadly, we can group different landforms depending on elevation and slope as mountains, plateaus and plains.

Different Major Landforms on Earth

Major types of landforms on earth include mountains, valleys, plateaus, glaciers, hills, loess, plains and desserts.

MOUNTAINS

A mountain is any natural elevation of the earth surface.

  • The mountains may have a small summit and a broad base. It is considerably higher than the surrounding area. Some mountains are even higher than the clouds.
  • As you go higher, the climate becomes colder. In some mountains, there are permanently frozen rivers of ice. They are called glaciers.
  • There are some mountains we cannot see as they are under the sea. Because of harsh climate, less people live in the mountain areas.
  • Since the slopes are steep, less land is available for farming. Mountains may be arranged in a line known as range. Many mountain systems consist of a series of parallel ranges extending over hundreds of kilometres.
  • The Himalayas, the Alps and the Andes are mountain ranges of Asia, Europe and South America, respectively. Mountains vary in their heights and shape.

There are three types of mountains

  1. Fold Mountains
  2. Block Mountains
  3. Volcanic Mountains

The Himalayan Mountains and the Alps are young fold mountains with rugged relief and high conical peaks. The Aravali range in India is one of the oldest fold mountain systems in the world.

  • The range has considerably worn down due to the processes of erosion. The Appalachians in North America and the Ural mountains in Russia have rounded features and low elevation.
  • They are very old fold mountains. Block Mountains are created when large areas are broken and displaced vertically.
  • The uplifted blocks are termed as horsts and the lowered blocks are called graben.
  • The Rhine valley and the Vosges Mountain in Europe are examples of such mountain systems. Volcanic mountains are formed due to volcanic activity.
  • Kilimanjaro in Africa and Mt.Fujiyama in Japan are examples of such mountains.
  • The mountains are a storehouse of water.
  • Many rivers have their source in the glaciers in the mountains. Reservoirs are made and the water is harnessed for the use of people. Water from the mountains is also used for irrigation and generation of hydro-electricity.
  • The river valleys and terraces are ideal for cultivation of crops. Mountains have a rich variety of flora and fauna. The forests provide fuel, fodder, shelter and other products like gum, raisins, etc.
  • Mountains provide an idyllic site for tourists. They visit the mountains for their scenic beauty. Several sports like paragliding, hang gliding, river rafting and skiing are popular in the mountains.

Hills

Hills are raised areas on the surface of the earth with distinctive summits, but are not as high as mountains. Hills are created as a result of accumulation of rock debris or sand deposited by wind and glaciers.

  • They can also be created by faulting when the faults go slightly upwards. Hills are generally present in low mountain valleys and plains.
  • The Black Hills are the most known. Deep erosions of areas previously raised by the earth’s crust disturbances carry most of the soil away leaving behind a hill.
  • Human activities may also create hill when soils are dug and piled giant masses. Volcanic eruptions as well create hills after the eruption when the molten materials or lava cools and hardens in a pile.

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PLATEAUS

A plateau is an elevated flat land. It is a flat-topped table land standing above the surrounding area. A plateau may have one or more sides with steep slopes.

  • The height of plateaus often varies from few hundred metres to several thousand metres. Plateaus, like mountains may be young or old.
  • The Deccan plateau in India is one of the oldest plateaus. The East African Plateau in Kenya, Tanzania and Uganda and the Western plateau of Australia are other examples.
  • The Tibet plateau is the highest plateau in the world with a height of 4,000 to 6,000 metres above the mean sea level.
  • Plateaus are very useful because they are rich in mineral deposits. As a result, many of the mining areas in the world are located in the plateau areas. The African plateau is famous for gold and diamond mining.
  • In India huge reserves of iron, coal and manganese are found in the Chhotanagpur plateau. In the plateau areas, there may be several waterfalls as the river falls from a great height.
  • In India, the Hundru falls in the Chhotanagpur plateau on the river Subarnarekha and the Jog falls in Karnataka are examples of such waterfalls. The lava plateaus are rich in black soil that is fertile and good for cultivation.

 

PLAINS

Plains are large stretches of flat land.

  • They are, generally, not more than 200 metres above mean sea level.
  • Some plains are extremely level. Others may be slightly rolling and undulating.
  • Most of the plains are formed by rivers and their tributaries.
  • The rivers flow down the slopes of mountains and erode them.
  • They carry forward the eroded material.
  • Then they deposit their load consisting of stones, sand and silt along their courses and in their valleys.
  • It is from these deposits that plains are formed. Generally, plains are very fertile Construction of transport network is easy.
  • Thus, these plains are very thickly-populated regions of the world. Some of the largest plains made by the rivers are found in Asia and North America. For example, in Asia, these plains are formed by the Ganga and the Brahmaputra in India and the Yangtze in China. Plains are the most useful areas for human habitation.
  • There is great concentration of people as more flat land is available for building houses, as well as for cultivation.Because of fertile soils, the land is highly productive for cultivation. In India too, the Indo-Gangetic plains are the most densely populated regions of the country.

 

GLACIERS

Masses of ice moving as sheets over the land (continental glacier or piedmont glacier if a vast sheet of ice is spread over the plains at the foot of mountains) or as linear flows down the slopes of mountains in broad trough-like valleys (mountain and valley glaciers) are called glaciers.

  • The movement of glaciers is slow unlike water flow. The movement could be a few centimetres to a few metres a day or even less or more.
  • Glaciers move basically because of the force of gravity.Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice. The material plucked from the land by glaciers (usually large-sized angular blocks and fragments) get dragged along the floors or sides of the valleys and cause great damage through abrasion and plucking.
  • Glaciers can cause significant damage to even un-weathered rocks and can reduce high mountains into low hills and plains.
  • As glaciers continue to move, debris gets removed, divides get lowered and eventually the slope is reduced to such an extent that glaciers will stop moving leaving only a mass of low hills and vast outwash plains along with other depositional features.

 

EROSIONAL LANDFORMS

Cirque

Cirques are the most common of landforms in glaciated mountains. The cirques quite often are found at the heads of glacial valleys. The accumulated ice cuts these cirques while moving down the mountain tops. They are deep, long and wide troughs or basins with very steep concave to vertically dropping high walls at its head as well as sides.

  • A lake of water can be seen quite often within the cirques after the glacier disappears. Such lakes are called cirque or tarn lakes. There can be two or more cirques one leading into another down below in a stepped sequence.
  • Horns and Serrated Ridges 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.
  • The divides between cirque side walls or head walls get narrow because of progressive erosion and turn into serrated or saw-toothed ridges sometimes referred to as arêtes with very sharp crest and a zig-zag outline.

 

Glacial Valleys/Troughs

Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, and steep sides. The valleys may contain littered debris or debris shaped as moraines with swampy appearance.

  • There may be lakes gouged out of rocky floor or formed by debris within the valleys. There can be hanging valleys at an elevation on one or both sides of the main glacial 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. Very deep glacial troughs filled with sea water and making up shorelines (in high latitudes) are called fjords/fiords.
  • Depositional Landforms the unassorted coarse and fine debris dropped by the melting glaciers is called glacial till. Most of the rock fragments in till are angular to subangular in form. Streams form by melting ice at the bottom, sides or lower ends of glaciers.
  • Some amount of rock debris small enough to be carried by such melt-water streams is washed down and deposited. Such glaciofluvial deposits are called outwash deposits. Unlike till deposits, the outwash deposits are roughly stratified and assorted. The rock fragments in outwash deposits are somewhat rounded at their edges.

 

Moraines

They are long ridges of deposits of glacial till. Terminal moraines are long ridges of debris deposited at the end (toe) of the glaciers. Lateral moraines form along the sides parallel to the glacial valleys.

  • The lateral moraines may join a terminal moraine forming a horse-shoe shaped ridge. There can be many lateral moraines on either side in a glacial valley.
  • These moraines partly or fully owe their origin to glaciofluvial waters pushing up materials to the sides of glaciers.
  • 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.
  • 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.

 

Deserts

Deserts are the hot and dry areas of the world. They are the arid and semi-arid lands with little or no vegetation. Deserts constitute approximately 20% of the earth’s total land cover and are distinguished by little or no rainfall.

  • The deserts are divided into four major categories including the Semi-Arid Deserts, the Hot and Dry Deserts, the Cold Deserts, and the Coastal Deserts.
  • These deserts are located in different areas of the world. Deserts experience very high temperatures, less cloud cover, low humidity, low atmospheric pressure, and very little rain, which makes them have very little vegetation cover.
  • The soil cover is also rocky and shallow and with very little organic matter and as such, it only supports a few plants adapted to the conditions. Plants such as cacti and short shrubs are the ones adapted to the desert conditions because they can conserve water and tolerate the high temperatures.
  • Animals in the deserts include insects, small carnivores, snakes, lizards, and birds adapted to survive with very little water. These animals hide during the day till nightfall to avoid the heat. An example of a desert is the Sahara of North Africa.

 

Lake:-

A lake (from Latin lacus) is a large body of water (larger and deeper than a pond) within a body of land. As a lake is separated from the ocean, it is not a sea. Some lakes are very big, and people in the past sometimes called them seas. Lakes do not flow, like rivers, but many have rivers flowing into and out of them.

Lakes are classified into various types based on their origin or mode of formation. Here is a description of these different types of lakes:

 

Meteorite (extraterrestrial impact/ crater) lake

Meteorite lakes are formed in the depressions made on land by the impact of a meteor or asteroid crashing on to the Earth’s surface.

  • Over the years, precipitation accumulates in the natural depression, creating a lake. The Lonar Crater Lake, a saline soda lake located in the Indian state of Maharashtra, is an example of a meteorite lake.A study of the sediments at the bottom of such lakes often yields valuable information about extraterrestrial objects.

Anthropogenic lakes

Such lakes are created as a direct or indirect result of human activities. The most common origin of anthropogenic lakes is the creation of reservoirs by damming a river or stream.

  • Such reservoir lakes serve several purposes like the generation of hydroelectricity, storage of water for future needs, Pisciculture, etc.
  • Often, sites excavated by people are left abandoned and are filled up with water from underground aquifers or precipitation, resulting in the formation of man-made lakes.

 

Shoreline lakes

Shoreline lakes are formed along the coastline or between islands and mainland mainly due to the deposition of sediments by rivers, wave action or ocean currents that result in the creation of a water body separated from a larger water body by such deposits.

  • For examples, when estuaries are blocked or beach ridges grow by the action of sea currents, shoreline lakes are created.
  • Similarly, the meeting of two spits dividing a larger lake results in a shoreline lake.
  • When two spits or tom bolos connect the island to the mainland, the lake that is formed in between the two spits or tom bolos is also a shoreline lake.

 

Aeolian lakes

Lakes produced as a result of the action of winds are called Aeolian lakes. Such lakes are usually formed in arid environments where layers of wind-blown sand act as a natural dam in a lake basin, giving birth to an Aeolian lake. Such lakes are also formed due to the accumulation of water via precipitation in the cavity between two sand dunes. Such lakes are called interduneal lakes. An example of an Aeolian lake is Moses Lake in Washington, US.

 

Solution lakes

A solution lake is formed when the bedrock is soluble and the dissolution of the bedrock by Precipitation and percolating water results in the formation of hollows or cavities that can give birth to a lake.

  • The accumulation of precipitation in the cavity can fill it up to create a lake. Also, if the soluble bedrock collapses to form sinkholes in a region where ground water is close to the surface then the water can fill up the sinkhole creating a solution lake.
  • Such lakes are common in areas with Karst topography. Solution lakes are found in many parts of Florida and Croatia’s Dalmatian coast.

 

Landslide lakes

Landslide lakes are created when a river is naturally dammed by the deposition of debris resulting from a rock avalanche, landslide, mudflow, or volcanic eruption.

  • Earthquakes and volcanic eruptions often lead to the formation of such lakes. These lakes are also known as debris dams or barrier lakes.
  • Landslides triggered by earthquakes or heavy rainfall are the most common cause of formation of a landslide lake (about 84% of such lakes result from this cause). Volcanic eruptions are responsible for the formation of 7% of lakes of this type.
  • Landslide lakes usually do not last for long as they are of a rather ‘loose nature.’ Often flooding with a high number of casualties is the end consequence.
  • Floods originating from landslide dams result in either back flooding during the time of formation of the lake or downstream flooding at the time of failure.
  • The SoI Dam located in Tajikistan is a landslide dam triggered by an earthquake, the highest known of its kind.

Fluvial lakes

The flow of a river is usually not straight but the river bends and meanders throughout its course due to the uneven and non-uniform topography of the land. As the river flows, a number of lakes are formed by the running water and are called the fluvial lakes. The oxbow lake is a classic example of a fluvial lake. Carter Lake in Iowa, US, is an example of an oxbow lake.

Tectonic lakes

Tectonic lakes often result in the formation of some of the deepest and largest lakes in the world. As the name suggests, such lakes are formed by the tectonic movements of the Earth’s crust like tilting, folding, faulting, etc. Lake Baikal, the Caspian Sea, and the Sea of Aral are some of the examples of tectonic lakes.

Glacial lakes

Glacial lakes are formed from a melted glacier. As glaciers flow down, the erosive action of the glaciers often creates natural depressions in the bedrock below the glaciers.

  • When the glaciers recede like during the end of the last glacial period about 10,000 years ago, patches of ice in the depression on bedrock created by glacial erosion are left behind.
  • Once the ice in these depressions melts, glacial lakes are created. Glacial lakes are quite common and most of North America’s and Europe’s lakes have a glacial origin. The Great Lakes of North America and the lakes of England’s Lake District are all examples of glacial lakes.

Volcanic lakes

Lakes with a volcanic origin are known as volcanic lakes. These lakes are usually formed in volcanic calderas or craters or when lahars or lava flows interrupt the flow of a river or stream. Volcanic lakes are formed in volcanic craters or calderas when the rate of precipitation is higher than the rate of loss of water via evaporation or drainage through an outlet.

  • An example of a lake formed in a caldera is the Crater Lake which is present within Mount Manama’s caldera in Oregon,
  • The Malheur Lake in Oregon is an example of a volcanic lake that was formed by the damming of a river, the Malheur River, by lava flow.

Organic lakes

Organic lakes are formed by the action of flora or fauna.

  • These lakes are relatively small in size and quite rare in occurrence. An example of an organic lake is a reservoir created by the damming of a river by the action of beavers. Coral lakes or dams created by vegetative growth also lead to the formation of organic lakes.

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