Change of the Earth’s Surface — Question Answer
Welcome to HSLC Guru. This post provides complete ASSEB Class 9 Social Science Geography Chapter 1 solutions for Change of the Earth’s Surface.
সাৰাংশ: ভূ-পৃষ্ঠ কেতিয়াও স্থিৰ হৈ নাথাকে; ই অবিৰতভাৱে সলনি হৈ থাকে। এই পৰিৱৰ্তন দুবিধ কাৰকৰ ফল—অন্তৰ্জাত (ভূ-ত্বকৰ ভিতৰৰ, যেনে ভূমিকম্প আৰু আগ্নেয়গিৰি) আৰু বহিৰ্জাত (ভূ-পৃষ্ঠৰ ওপৰৰ, যেনে নদী, হিমবাহ, বতাহ আৰু সাগৰীয় ঢৌ)। এই কাৰকসমূহে ক্ষয়, বহন আৰু অৱক্ষেপণৰ যোগেদি উপত্যকা, ব-দ্বীপ, বালিয়াৰি, গ্ৰাবৰেখা আৰু সৈকত আদি বিভিন্ন ভূ-ৰূপ সৃষ্টি কৰে। এই অধ্যায়ত ভূ-পৃষ্ঠৰ এই অবিৰাম পৰিৱৰ্তনৰ কাৰণ আৰু ফলাফল আলোচনা কৰা হৈছে।
Summary: The earth’s surface is never static; it changes continuously. These changes are caused by two kinds of forces — endogenic forces (originating below the crust, such as earthquakes and volcanoes) and exogenic forces (acting on the surface, such as rivers, glaciers, wind and sea-waves). Through erosion, transportation and deposition these agents create varied landforms like valleys, deltas, sand dunes, moraines and beaches. This chapter discusses the causes and consequences of this continuous change of the earth’s surface.
Textbook Questions and Answers
Very Short Answer
1. What is meant by exogenic factor? Mention some exogenic factors.
Answer: The external agents that act upon the earth’s surface from outside and bring about changes in it are called exogenic factors. Some exogenic factors are the sun, the atmosphere, wind, rain, rivers, glaciers and sea-waves.
2. Why are earthquakes and volcanic eruptions called endogenic factors?
Answer: Earthquakes and volcanic eruptions originate from below the crust of the earth, that is, from the interior of the earth, and are therefore called endogenic factors. Unlike exogenic processes such as weathering and erosion which occur on the surface, these two processes are generated from within the earth.
3. What is a tributary? Name two major tributaries of the Brahmaputra.
Answer: The smaller streams of water that flow into and join the main river are called tributaries. Two major tributaries of the Brahmaputra are the Subansiri and the Dhansiri.
4. Why is the wind action strong in the deserts?
Answer: Deserts have very little vegetation, so there is nothing to obstruct the movement of air. As a result, the wind can blow rapidly and freely across the land without obstruction, and its action becomes strong.
5. What is meant by deflation?
Answer: Deflation is the process by which the wind lifts and carries sand and dust from one place in a desert to another. This process frequently gives rise to dust storms.
6. What is an inselberg?
Answer: The low, eroded residual hills seen in deserts are called inselbergs. The term comes from a German word meaning ‘isolated island mountain’.
7. Define a glacier and state how it differs from a river.
Answer: A glacier is a mass of ice that accumulates in very cold regions and slowly moves down a slope, whereas a river is a flowing stream of liquid water. The main difference is that a glacier is a frozen mass of ice, while a river is a body of flowing liquid water.
8. What is a coast?
Answer: The margin of land that lies adjacent to an ocean or a sea is called a coast.
Short Answer
9. What do you mean by a river basin?
Answer: A river basin is the whole area from which a river, together with its tributaries, collects water and carries it downward. The water is supplied by rainfall, snow deposited on mountains and springs from the interior of the earth. The entire tract of land drained by the main river and its tributaries makes up the river basin.
10. Give an outline of river erosion.
Answer: River erosion is the process by which the flowing water of a river wears away and carries down its bed and banks. In hilly regions the river erodes mainly downward, that is vertically, while in the plains lateral or sideways erosion becomes greater. This erosion gives rise to various landforms such as valleys and gorges.
11. What do you mean by lateral erosion of a river?
Answer: In hilly areas a river has a narrow, deep channel and a high velocity; but as it comes down to the plains the slope decreases, the velocity falls and the sediment it carries begins to be deposited. To maintain its normal flow the river then cuts both banks and widens its channel. This widening of the channel by cutting the banks is called lateral erosion.
12. What is an ox-bow lake? How is it formed?
Answer: As a river meanders, it erodes the outer bank and deposits sediment on the inner bank. Gradually the narrow neck of land between two meanders is worn away, and the river takes a new, straighter course, cutting off the meander from the main channel. The horseshoe-shaped lake formed in this way is called an ox-bow lake.
13. What is a floodplain? Write how it is formed.
Answer: A floodplain is the flat area of land that stretches from both banks of a river’s channel to the base of the enclosing valley walls. It is formed as the river meanders and erodes its banks and, during floods, deposits sediment over the land, gradually building up this wide, flat area.
14. Write how and where sandbars are formed in a river channel.
Answer: Sandbars are formed by the mud and sand carried down by a river which get deposited in the middle of the river bed and sometimes near the banks. They are generally temporary and change their shape and size with the current. The Brahmaputra shows numerous sandbars, of which Majuli is a notable sandbar island.
15. How are deltas formed?
Answer: As a river enters a larger body of water such as a sea or a lake, its speed decreases and the sand, mud and silt it carries begin to be deposited. Over many years this sediment accumulates layer upon layer and builds up a triangular landform called a delta. It is a very slow process.
16. How are sand dunes formed?
Answer: Sand grains carried by the wind gather against some obstacle and form small mounds; as more and more sand is deposited over them, dunes develop through a process called ‘saltation’. Their shape is determined by the supply of sand, the pattern of the wind and the presence of vegetation.
17. What is a moraine? Mention the different types of moraine.
Answer: The rock and soil material (other than ice) that is carried and deposited by a glacier is called a moraine. There are four types — lateral moraine, ground moraine, medial moraine and terminal moraine.
18. Write how a glacial horn is formed.
Answer: When several glaciers erode a mountain peak from all sides at the same time, a sharp, cone-shaped summit is produced; this pointed peak is called a glacial horn. It is commonly found in alpine mountain regions.
19. What is a beach? State its importance.
Answer: A beach is a narrow, gently sloping zone along the edge of the sea made up of sand, pebbles and shell fragments. It is important because it helps to protect the people living on the coast by buffering the shore against storms and powerful waves.
20. Write how sea-waves bring about change in the coasts.
Answer: The constant action of the waves shapes the coast differently according to the nature of the rocks there. On coasts of hard rock, steep cliffs are formed, whereas on sandy coasts sediment is deposited and beaches are gradually built up.
Long Answer
21. Give an outline of the distribution of the continents and oceans of the world.
Answer: A large portion of the earth’s surface that stands slightly above sea-level as land is called a continent. The continents contain plains, plateaus, hills, mountains, river valleys, deserts and coastal plains. The remaining large portion of the earth’s surface is covered by oceans and seas, which contain submerged plateaus, plains, ridges, trenches and coral. About 71 per cent of the earth’s surface is covered by water and about 29 per cent by land. The distribution of the continents and oceans is uneven, and it has kept changing over time.
22. Explain why there has been change over the earth’s surface.
Answer: The earth’s surface changes continuously, and these changes are the result of processes accumulating over thousands of years. Growth of population has reduced cropland and vegetative cover, which in turn lowers rainfall and creates drought-affected regions. Industrialisation has polluted the air over the whole world. Melting of ice in the polar regions and the Himalayas deposits excess water into rivers and causes floods. Such changes may in time create deserts and dry up water bodies. It is because of the ceaseless action of both exogenic and endogenic factors that the earth’s surface keeps changing.
23. Think and write about the probable changes of the earth’s surface in future.
Answer: In future the movement of the tectonic plates may bring the continents together to form new supercontinents. Rising temperatures caused by climate change may melt the polar ice and raise the level of sea water. This could lead to significant changes in ocean currents, coastlines and biodiversity. Under the combined influence of human activities and natural processes, the earth’s surface will continue to change in the future as well.
Extra Questions and Answers
1. How many continents and oceans are there on the earth?
Answer: There are seven continents and five oceans on the earth.
2. What is meant by weathering?
Answer: Weathering is the process by which rocks lying in one place are broken down into smaller fragments by the action of wind, water and temperature.
3. What is the main difference between endogenic and exogenic factors?
Answer: Endogenic factors originate within the crust of the earth, such as earthquakes and volcanoes, whereas exogenic factors act on the surface of the earth from outside, such as rivers, wind and glaciers.
4. How is a gorge formed?
Answer: A gorge is a narrow, deep, steep-sided valley formed when a river cuts rapidly downward in a hilly region.
5. Why is Majuli famous?
Answer: Majuli, situated in the river Brahmaputra, is famous as one of the largest river islands in the world.
Key Terms
| Term | Meaning |
|---|---|
| Exogenic factor | An external agent that acts on the earth’s surface, such as wind, river or glacier. |
| Endogenic factor | An internal agent acting from below the earth’s crust, such as an earthquake or volcano. |
| River basin | The whole area from which a river and its tributaries collect water. |
| Delta | A triangular landform built by sediment deposited at a river’s mouth. |
| Ox-bow lake | A horseshoe-shaped lake formed when a river meander is cut off from the main channel. |
| Moraine | Rock and soil material carried and deposited by a glacier. |