Change Of The Earth's Surface PDF

# CHANGE OF THE EARTH'S SURFACE ## KEY POINTS - Exogenic factors of change of the earth's surface - Works of river - Works of wind - Works of glacier - Sea-wave action ## INTRODUCTION: The surface of the earth is composed of continents (landmass) and oceans (hydrosphere). Hydrosphere and landmass...

# CHANGE OF THE EARTH'S SURFACE ## KEY POINTS - Exogenic factors of change of the earth's surface - Works of river - Works of wind - Works of glacier - Sea-wave action ## INTRODUCTION: The surface of the earth is composed of continents (landmass) and oceans (hydrosphere). Hydrosphere and landmass cover 71% and 29% of the Earth's surface respectively. In the continents, plains, plateaus, hills and mountains, river valleys, deserts and coastal plains are found to occur. Similarly, in the oceans also, submerged plateaus, plains, ridges, trenches, coral reefs are there. Moreover, many islands of various sizes, are also there in the oceans. These features of the land surface and ocean bottoms are the results of certain processes operating through the ages. In the course of time, these features, experience changes. The processes involved in the genesis of these features also get changed with the passage of time. It is to be noted that some of the processes or factors operating in the continents and oceans are exogenic, while the others are endogenic. The exogenic forces operate from outside visibly and the endogemic ones work in the Earth's interior invisibly. The exogenic factors, mainly include sunshine, wind, rainfall, river, glacier and sea waves. On the other hand, the endogenic factors include mainly earthquake and volcanic eruption. The endogenic factors operate in the Earth's interior as well as in the ocean bottoms. These factors are generally responsible for making the land surface and the ocean bottoms irregular. As a result of earthquakes, some portions of the continents and ocean bottoms may be uplifted or subsided. At the time of volcanic eruption also, earthquakes may occur and volcanic hills may be formed in the affected areas. Behind the occurrence of the endogenic events, there is a major process called, plate tectonic. The plates that constitute the Earth’s crust are dynamic and their movements cause earthquakes and volcanic eruptions. However, we shall confine our discussion here to the major exogenic factors only. * [Image of the Earth's Plates, titled "Figure 1.01 Plates of the earth."] ## 1.1 EXOGENIC FACTORS OF EARTH'S SURFACE CHANGE The exogenic factors, basically operate externally upon the Earth's surface. The sources of these factors are the sun and the Earth’s atmosphere. Among the exogenic factors insolation, wind, rainfall, river, glacier and sea waves are important. Moreover, the biotic elements like vegetations and animals also contribute more or less towards changing the Earth's surface. The character and intensity of the external factors operating in the Earth’s surface are not similar over space and time. Their activities are found to vary through time and over regions. About two million years back, that is during the Ice Age, most parts of the Earth’s surface remained under ice cover. At that time, the activities of glaciers were more extensive than what it is today. In the future also, possibly there will be great change in the works of glaciers because of global climate change. It is noteworthy that some specific processes are found to be dominant in specific areas in so far as the changes over the Earth’s surface are concerned. Generally, the works of rivers in the rainy areas, winds in the desert areas, glaciers in the polar and high altitude areas and sea waves in the coastal areas are found to be dominant. These factors create certain specific landforms through their modifying activities in the concerned areas. Mention should be made here that all these are exogenic factors which bring about changes over the Earth’s surface. ## 1.2 WORKS OF RIVER As an external agent, the rivers play far-reaching role over the surface of the Earth. The rivers have been operating in the tropical and sub-tropical areas of the world through ages. As a result, the continents and ocean basins have witnessed a variety of changes. * [Image of a map of the Brahmaputra River basin, titled "Map of Brahmaputra River Basin, Figure 1.03 Brahmaputra river basin."] Generally, rivers along with their tributaries form basins of their own. Thus basin is an area wherefrom a river alongwith its tributaries carry water downward. The water sources of rivers may be rainfall, snow deposits over the mountains or springs coming out from the Earth’s interior. As such, changes over the basin area largely depend upon the works of the concerned river. Here question arises as to how a river brings about changes in its basin area? The works of a river are generally classified into three groups: (a) erosion, (b) transportation and (c) deposition. All rivers more or less perform these activities. ### 1.2.1 Erosion When a river flows down from highland, it naturally erodes the surface of the basin. In fact, when the atmospheric moisture comes down to the Earth in the form of rain, erosion begins over it. When rain water flows as river, naturally its erosive power increases. However, the erosive power of a river depends on the structure of the basin surface, hardness of the underlying rocks, amount of water flow and so on. Mention should be made here that although erosion takes place more or less in all parts of the basin, it becomes clearly visible mainly in the river channel. A river channel means the elongated path through which the river flows from source to mouth. The length and width of the channel, however, vary from river to river. It has already been mentioned that the erosion of river depends upon a large number of factors. Among them, the geological structure of the channel and the erosive capacity of water are the major factors. The channel materials exposed to water flow come under the direct action of water. Such water action is known as hydraulic action. As a result of this, the sands and pebbles in the channel get eroded and then transported down by the river. * [Image of river-bank erosion by the Brahmaputra River, titled "Figure 1.05: Bank erosion by Brahmaputra in Rahmaria area of Dibrugarh."] The erosional capacity of a river changes after it enters a plain from the steep mountainous slope. In the mountainous region, the river channel is narrow and deep with high velocity of flow. With the decline of slope towards the plain, the velocity decreases and the sediments transferred from the highland begin to be deposited. As a result, in the plains, channel depth declines. Then the river starts widening its channel through lateral cutting just to maintain its normal flow. By eroding the banks the river widens its channel to carry the water down. Such kind of river erosion is known, as bank or lateral erosion. The bank failure is thus the result of lateral erosion. The river course becomes zigzag when it flows over the gentle slopes of the plains. The lateral erosion makes the river course further zigzag. Such a zigzag course is called meandering course. In certain parts of the plain, the river channel may be highly meandering. Such a course fails to carry huge amount of water during summer. Then the river straightens its course leaving aside the most winding course. The cut-off part then takes the form of, a lake or beel. Shaped like a horse shoe, this type of lake is called horse shoe lake. Almost all the major rivers of the world have a large number of such lakes in their valleys. The rivers Brahmaputra and the Barak of Assam have created many such lakes in their valleys. These beels help in reducing the flood problem by storing sufficient water during summer. * [Image of a horse-shoe lake created by the Colorado River at Arizona, U.S.A., titled " Figure 1.06: A horse-shoe lake created by Colrado river at Arizona, U.S.A."] ### 1.2.2 Transportational and Deposititional Works: The rivers generate sediments through their erosional activities. These are carried downward and deposited in the channels, banks and mouths. A river carries sediments in three different ways- (a) some are carried in dissolved state, (b) some others in suspended state and (c) other in rolling state over the channels. Generally, the salts present in the soil come into contact with river water and these are transported to the sea or ocean in dissolved state. On the other hand, tiny particles of sand, mud and silt, are transported by water current in suspended state and deposited on the river bed and banks . These are also transported to the oceans. Moreover, relatively big particles are transported down by current in rolling state. These materials are also called bed load. When the water gradually becomes weak over the plain, the rolling loads get deposited over the river bed. The materials thus carried down by the rivers are deposited on the banks and the river mouths to form a variety of features. Among these depositional features, sand bar, floodplain, natural levee and delta are important. Normally, erosional activities take place in the upper course of a river, while in the middle and lower courses sediments are deposited on the bed and the banks. As a result of long continued sediment deposition, alluvial plains are formed on both the banks. Such plains are called floodplain. Generally, during floods, sands, mud and silt carried down by the river spread out to a long distance over the banks. The floodplains are fertile and suitable for cultivation. Extensive floodplains are there on the banks of the great rivers like the Brahmaputra, Ganga, Indus, Mississippi, Nile, Amazan, Howang Ho, etc. On the other hand, the sandbars formed of the sediments on the river bed are significant from various points of view. Sandbars are formed both on the sides and the middle of the river bed. Generally sandbars are temporary and of various sizes and shapes. The speedy flow of the river during summer affects the size and shape of the sandbars. Some time bank erosion completely wipes out them. The sandbars become visible when river water subsides during winter. A large number of sand bars are there on the bed of the Brahmaputra. The great river island Majuli is a kind of sandbar of the river Brahmaputra. It should be noted that the relatively heavy materials (boulders, pebbles and large sands) carried by a river cannot be transported to longer distance over the banks. These are normally deposited near the channel. The materials thus deposited may take the shape of a long embankment. As it is formed naturally, therefore, it is called natural levee. In the case of the big rivers natural levees are clearly seen. In some places on the bank of the Brahmaputra also, such levees are there. The islands formed in the river mouths are highly significant from geographical point of view. In the coastal shallow seas where the rivers debouch, islands are formed by the river deposits. These islands take the shape of Assamese letter 'ব' or the Greek letter delta 'A'. These islands are, therefore, called delta. Most of the rivers of the world form deltas in their mouths. Among them, the deltas of the Ganga and Brahmaputra, the Nile and the Mississippi are famous. ## 1.3 WORKS OF WIND Among the exogenic factors that bring about change over the Earth's surface, the wind is very important. The global wind systems act upon the Earth's surface in many different ways. All parts of the world including the oceans are affected by wind action. Certain parts of the world are regularly affected by wind-related hazards like hurricane, typhoon, tornado, etc. However, wind as an agent to bring about change has far-reaching impact particularly in the desert areas of the world. The formation of geomorphic features in the deserts, their shape and size are primarily determined by the wind action. In the desert environment, the wind through three different activities- erosion, transportation and deposition- creates different features and causes change over them. As the vegetal cover is very thin in the deserts wind can easily act upon them. Sands and dust are easily transported from one part to the other. This type of blowing away of sands and dust is called deflation. Deflation often creates dust storm in the deserts. It is notable that the areas from where sands are carried away become relatively depressed. In such depressed areas lakes may be formed temporarily. The sands and dust transported through deflation get deposited in certain places, where surface may be raised gradually to form hills. These hills are called sand dune. The shape and size of the sand dunes vary depending upon the nature, path of flow and strength of the wind. Moreover, sand dunes may differ with the difference in the natural environment of the deserts. * [Image of an Inselberg in the white desert of Egypt, titled "Figure 1.10: Inselberg in white desert of Egypt."] * [Image of a Yardang, titled " Figure 1.11: Yardang."] Some other easily visible features also, give uniqueness to the desert environment. Most of such features are, however, the results of the erosional activities. The speedy desert winds transport rocks and pebbles along with sands. These transported materials erode the high lands located on their ways. This type of frictional erosion is known as abrasion. Generally, features like inselberg and yardang are formed due to abrasion of the highlands. The low erosional hills generally seen in the deserts are inselbergs. The German term ‘inselberg’ means isolated island mountain. The yardangs on the hand, are a kind of gemorphic feature where ridges and groves occur together. These are the results of wind action over, relatively soft sedimentary rocks. The indigenous people of the Lop Nur region of Tarim basin of China used the word yardang to recognise these features. ## 1.4 WORKS OF GLACIER The extremely cold areas of the Earth’s surface are under ice cover. Such areas include the polar and the high mountainous parts. Huge amount of ice in layers are there in these areas. These deposits of ice come down very slowly over the surface slope. Such moving ice down the slope is called glacier. In the mountains, the glaciers flow through the valleys just like rivers. Like the rivers, the glaciers also perform erosional, transportantional and deposional activities to bring about changes over the concerned areas. Glaciers are mainly of two types (a) continental glacier or ice sheet and mountain or valley glacier. The continental glaciers are covering extensive areas over the polar regions. These glaciers flow down very slowly according to surface slope. The velocity of a glacier, however, depends on a variety of factors. Among them, the amount of ice deposits, structure and slope of the surface, atmospheric temperature are important. Most of such glaciers have a velocity of less than 1 meter a day. The mountain glaciers mainly occur in the Alps of Europe, the Rocky in North America and the Himalayas in Asia. These glaciers normally come down through the valleys in between the mountain ridges. Along with ice, these glaciers carry boulders, pebbles, sands and other organic matters. These transported materials perform erosional activities in the glacial channels. The glacial channels are long with U-shaped cross profile. Due to the friction caused by the transported materials, the glacial channels become deep and steep sided. Several glaciers may flow down from some mountain. In such situation, the erosional activities on all sides of the mountain make its peak sharp and conical. Such peaks are called glacial horn. In the Alps and the Himalayas, many such erosional horns are there. It has already been mentioned, that the glaciers carry lot of materials other than ice. Such materials are called moraine. Moraines are of four types- (a) lateral moraine, (b) ground moraine, (c) medial moraine and (d) terminal or end moraine. Generally, moraines are carried through the portion between the steep valley side and the moving ice. The moraines thus carried along the valley sides are called lateral moraine. On the other hand, moraines are also carried over the valley floor. These are called ground moraine. Again in certain places, two different glaciers may meet. Then the lateral morains of the two glaciers may merge and flow along the middle of the large glacier. Such moraines are called medial moraine. It is worth mentioning here that the glaciers coming down from the mountains gradually confront relatively warm climate and at certain level the glacial ice starts melting. The heavy materials get deposited on the spot where the glacial ice melts. The moraines deposited in the terminous of the glacier are called terminal or end moraine. Beyond terminal moraine downward, the melting ice forms river. Most of the rivers coming down the Himalayas are fed with water from melting glaciers. * [Image of a lateral moraine of the Solomon Glacier in British Columbia, Canada, titled "Figure 1.14: Lateral moraine of Solmon Glacier in British Columbia, Canada."] * [Image of a medial moraine, titled "Figure 1.15: Medial moraine."] Thus the moraines created by the depositional process in the glacial valley change the valley’s morphology. Moreover , some other depositional features can also be seen in the mountainous regions under glacial action. Among these, drumlin, esker and kame are worth mentioning. ## 1.5 SEA-WAVE ACTION A large part of the Earth’s surface (71 per cent) is covered by oceans. The land margins of the oceans are called coast. The contact zone of the oceans and continents is like a long zig-zag line. This line is called coastline. The total length of the coastline of the world is about 0.5 million km. * [Image of the coast of India, titled " Figure 1.17: Costal line of India."] Several exogenic processes operate in the coastal areas. Rainfall, wind and sea-waves are important among them. Out of these three again, the sea-waves constantly operate in the coastal areas and bring about remarkable change in the concerned areas. However, wave action is not similar everywhere. Factors like geological structure, depth of the nearby sea, presence of rivers, wind action, coastal vegetal cover, etc. determine the nature and strength of wave action. The intensity of wave action is also determined by the earthquakes that occasionally occur in the ocean floor. Anyway, the geographical scenario of the coastal areas is primarily the product of the wave action. It is mainly the wave actions that have created certain specific features in the sea coasts. Some of the coastal features are erosional in character, while the others are depositional. If the coast is composed of hard rocks like granite and basalt, then the coast becomes steep due to wave action. Such steep coast is called sea-cliff. Such coasts are generally found to be rugged in character. On the other hand, a plain like feature is found to develop in the sea shore where continuous wave action causes systematic sand deposits. This type of feature with gentle seaward slope formed mainly of sands is called beach. The combined effects of swash and backwash over the sea shore result in such features. The sand particles in such beaches may vary from fine sands to small pebbles. The beaches in the coastal areas attract people for recreation. * [Image of Marina Beach on the coast of Chennai, titled "Figure 1.19: Marina Beach in the sea coast of Chennai."] Sometimes, big earthquakes that occur in the ocean floor produce special type of, waves in the oceans. Such waves attain great height and strength in the sea shore and cause extensive damage to the coasts. This type of devastating waves are called Tsunami. ## Conclusion: The above is an outline of some of the exogenic factors that are constantly causing changes over the Earth’s surface. However, all such factors and resultant features are not discussed here. The exogenic factors generally act upon the Earth’s surface very slowly. These factors intend to make the surface a level one by removing all the irregularities. As opposed to this, the endogenic factors (earthquakes and volcanic eruption) suddenly create extensive change over the surface of the Earth. These factors disturb the balance of the surface and make it uneven. The volcanic eruption forms volcanic mountain. Similarly, the earthquake may uplift some parts to form high lands, while it may cause subsidence in other parts to form lowlands. Anyway, our Earth's surface has attained its present pattern due to the exogenic and endogenic factors operating on it. In future too, these factors will continue their activities to further change the surface character of the Earth.