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SEBA Class 9 Geography: Changes of Earth's Surface

Give very short answer:

1. Give an outline of the distribution of continents and oceans of the world.
Answer: The continents or landmasses cover 29% of the Earth's surface, while the oceans or hydrosphere cover the remaining 71%.

On the continents, features like plains, plateaus, hills, mountains, river valleys, deserts, and coastal plains are found. In the oceans, there are submerged plateaus, plains, ridges, trenches, and coral reefs. Many islands of various sizes are also found in the oceans.

2. Explain why there has been a change over the Earth's surface.
Answer: The Earth's surface changes due to the continuous action of exogenic and endogenic factors. Exogenic forces like sunshine, wind, rain, river, glacier, and sea-waves act from outside, while endogenic forces like earthquakes and volcanic eruptions act from within. These forces create and modify landforms over time. Plate movements also cause sudden changes like earthquakes and volcanic hills, leading to the uplift or sinking of land and ocean floors.

3. What is meant by an exogenous factor? Mention some exogenic factors.
Answer: Exogenous factors are the external forces that operate upon the Earth's surface. Some exogenic factors are sunshine, wind, rainfall, river, glacier, and sea-waves.

4. Why are earthquakes and volcanic eruptions called endogenic factors?
Answer: Earthquakes and volcanic eruptions are called endogenic factors because they operate inside the Earth’s interior and cause sudden changes on the Earth's surface.

5. What do you mean by a river basin? Draw a diagram of a river basin.
Answer: A river basin is the area from which a river and its tributaries collect water and carry it downward. The water sources of rivers may be rainfall, snow deposits over the mountains or springs coming out from the earth's interior.

(Please draw a simple diagram of a river basin showing the main river, tributaries, and surrounding area, as shown in your textbook’s Fig. 1.03)

6. What is a tributary? Name two major tributaries of the Brahmaputra.

Answer: A tributary is a small river or stream that joins a larger river.

Two major tributaries of the Brahmaputra are the Subansiri and Manas.

7. Give an outline of river erosion.
Answer: River erosion begins when rainwater flows down from highland areas and forms rivers. As the river flows, it erodes the land surface of its basin, especially in the river channel. The erosion depends on many factors, like the geological structure of the channel and the erosive capacity of water. The main types of erosion include hydraulic action, where water removes sand and pebbles, and lateral erosion, which occurs when the river flows in plains and cuts its banks to widen the channel and maintain normal flow.

8. What do you mean by lateral erosion of a river?
Answer: Lateral erosion of a river means the erosion that takes place on the sides or banks of the river channel. When a river enters a plain from a mountainous region, its speed decreases, and sediments begin to deposit. To maintain its normal flow, the river starts cutting its banks. This process of widening the channel by eroding the banks is called lateral erosion. Bank failure is a result of this type of erosion.

9. What is an ox-bow lake? Draw a diagram to show how it is formed.

Answer:

An ox-bow lake is formed when a river flows over the gentle slopes of the plains and takes a zigzag course due to lateral erosion. In some parts, the river becomes highly meandering and cannot carry a huge amount of water during summer. Then the river straightens its course by leaving the most winding part. The cut-off portion becomes a lake or a beel. This type of lake, shaped like a horseshoe, is called an ox-bow lake.

10. What is floodplain? Write how it is formed.
Answer: A floodplain is an alluvial plain formed on both sides of a river due to long-continued sediment deposition.

It is formed when the river carries sands, mud, and silt during floods and spreads them over the banks. These materials slowly build up flat land on both sides of the river. Floodplains are fertile and suitable for cultivation. They are found along the banks of big rivers like the Brahmaputra, Ganga, and Nile.

11. Write how and where sandbars are formed in a river channel.
Answer: Sandbars are formed of the sediments on the river bed. They are formed both on the sides and in 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. Sometimes bank erosion completely wipes them out. The sandbars become visible when the river water subsides during winter. The great river island of Majuli is a kind of sandbar of the river Brahmaputra.

12. How are the deltas formed?
Answer: Deltas are formed in the coastal shallow seas where rivers debouch, or flow out, into the sea. The sediments carried by the river get deposited at the river mouth, forming islands. These islands take the shape of the Assamese letter 'ব' or the Greek letter delta 'Δ'. That is why they are called deltas. Most rivers of the world form deltas at their mouths. Famous examples include the Ganga and Brahmaputra, Nile, and Mississippi deltas.

13. Write why the wind action is strong in the deserts.
Answer: Wind action is strong in the deserts because the vegetal cover is very thin, so wind can easily act upon the land surface. Sands and dust are easily transported from one part to the other, causing strong wind activity.

14. What is meant by deflation?

Answer: In the desert, sands and dust are easily transported from one part to the other. This type of blowing away of sand and dust is called deflation.

15. How are the sand dunes formed?
Answer: Sand dunes are formed when sand and dust transported by deflation get deposited in certain places, where the surface is gradually raised to form hills. These hills are called sand dunes.

16. What is inselberg?

Answer:

An inselberg is a low erosional hill generally seen in the deserts. The word inselberg is a German term that means an isolated island mountain.

17. Define a glacier and state how it differs from a river.
Answer: The extremely cold areas of the Earth's surface are under ice cover. Such areas include the polar and the high mountainous parts. A huge amount of ice in layers is there in these areas. These deposits of ice come down very slowly over the surface slope. Such moving ice down the slope is called a glacier.

A glacier flows with ice and moves very slowly over the surface slope in extremely cold areas. It carries boulders, pebbles, sands, and organic matter along with the ice and has a U-shaped channel. A river, on the other hand, flows with water and moves faster. River channels are usually different in shape and flow more freely compared to glaciers.

18. What is moraine? Explain all types of moraine. Draw a diagram to show different types of moraine.

Answer: 

The glaciers carry a 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.

(a) Lateral moraine: The moraines which carried along the valley sides are called lateral moraines.

(b) Ground moraine: The moraines that are carried over the valley floor. These are called ground moraine.

(c) Medial moraine: If two different glaciers meet. Then the lateral moraines of the two glaciers may merge and flow along the middle of the large glacier. Such moraines are called medial moraines.

(d) Terminal or end moraine: The terminal glacier is the end part of a glacier where the glacial ice starts melting due to a relatively warm climate. The heavy materials carried by the glacier get deposited at this spot. These deposits are called terminal or end moraines.

19. Write how a glacial horn is formed.
Answer: Glacial horns are formed when several glaciers descend from a mountain, flowing through valleys between mountain ridges. These glaciers carry with them ice, boulders, pebbles, sand, and organic matter, which cause erosion in the glacial channels. These channels have a U-shaped cross profile, and the friction from the transported materials makes them deep and steep-sided. As glaciers erode the mountain from all sides, they shape the peak into a sharp, conical form. This peak is called a glacial horn. Many such horns are found in the Alps and the Himalayas.

20. What is a coast?
Answer: The land margins of the oceans or the contact zone where the oceans and continents meet are called the coast.

21. What is a beach? State its importance.
Answer: A beach is a plain-like feature with a gentle seaward slope formed mainly of sand along the shoreline due to continuous wave action. The combined effects of swash and backwash create this feature.

Beaches are important because they attract people for recreation in the coastal areas.

22. Write how sea waves bring about change on the coasts.
Answer: Sea waves constantly operate in the coastal areas and bring about remarkable changes. The nature and strength of wave action depend on factors like geological structure, depth of the nearby sea, presence of rivers, wind action, coastal vegetal cover, etc. The intensity of wave action is also determined by the earthquakes that occasionally occur on the ocean floor. Anyway, the geographical scenario of the coastal areas is primarily the product of the wave action. It is mainly the wave action that has created certain specific features in the sea coasts.

Wave action creates both erosional and depositional features. Hard rock coasts become steep and form sea cliffs, while continuous sand deposits create beaches with gentle slopes. So, wave action shapes the geographical features of coastal areas.

Answer:

24. Think and write about the probable changes of the Earth's surface in future.

Answer: In future, the Earth's surface may undergo many changes due to the continuous action of exogenic and endogenic factors. Rivers may create new floodplains, sandbars, or deltas by erosion and deposition. In deserts, wind may form new sand dunes and erode existing landforms like inselbergs. Glaciers might shrink or change due to climate change, altering mountain shapes. Sea waves may change coastal areas by forming new beaches or cliffs. Also, earthquakes and volcanic eruptions may suddenly raise or lower parts of the surface.