Get summaries, questions, answers, solutions, extra MCQs, PDF for CHANGE OF THE EARTH’S SURFACE: SEBA Class 9 Geography, which is part of the syllabus for students studying under Assam Board. These solutions, however, should only be treated as references and can be modified/changed.
Summary
The surface of our Earth is made of land and oceans. The land has plains, mountains, and plateaus. The ocean floor also has features like plains, ridges, and deep trenches. These features are always changing because of certain natural processes. These processes can be divided into two types. Some work from outside the Earth’s surface, and others work from inside.
Forces that work from the outside are called exogenic forces. These include the actions of rivers, wind, glaciers, and sea-waves. They generally work very slowly to wear down high places and fill in low places, making the surface more level. Forces that work from inside the Earth are called endogenic forces. These include earthquakes and volcanic eruptions. They can cause sudden and large changes, creating new mountains or causing land to sink. The Earth’s surface today is a result of both these outside and inside forces working over a very long time.
Rivers change the land by eroding it, carrying away the material, and then depositing it somewhere else. In hilly areas, fast rivers carve out valleys. On flat land, rivers flow slowly and curve in a zigzag pattern called a meander. If a river cuts a new, straighter path, it can leave behind a curved lake. This is called a horse-shoe lake because its shape looks like the U-shaped shoe worn by a horse. Rivers also build new land by depositing sand and soil. This creates fertile floodplains along their banks and triangular pieces of land called deltas at their mouths.
In deserts, the wind is a powerful agent of change. It picks up and carries sand, which can wear away at rocks. This action can form isolated hills called inselbergs, a German word for “island mountains.” The wind also builds large hills of sand called sand dunes.
In very cold regions, huge sheets of moving ice called glaciers shape the land. They slide down mountains, carving out wide, U-shaped valleys. They also sharpen mountain peaks into points called glacial horns. Glaciers carry along rocks and dirt, known as moraine. When the ice melts, this moraine is left behind, forming hills and ridges.
Along the coasts, sea-waves constantly change the shoreline. They can wear away at the land to form steep sea-cliffs. They also move sand and deposit it to form beaches. Sometimes, an earthquake under the sea can create a very large and destructive wave called a tsunami.
Exercise
1. Give an outline of the distribution of continents and oceans of the world.
Answer: The surface of the earth is composed of continents, which are landmasses, and oceans, which form the hydrosphere. The hydrosphere and landmass cover 71% and 29% of the earth’s surface respectively.
In the continents, features like plains, plateaus, hills and mountains, river valleys, deserts, and coastal plains are found. Similarly, in the oceans, there are submerged plateaus, plains, ridges, trenches, and coral reefs. Many islands of various sizes are also present in the oceans.
2. Explain why there has been change over the earth’s surface.
Answer: The features of the land surface and ocean bottoms experience changes over time because they are the results of certain processes operating through the ages. The processes involved in the creation of these features also get changed with the passage of time.
These processes are of two types: exogenic and endogenic. Exogenic forces, such as sunshine, wind, rainfall, and the works of rivers, glaciers, and sea-waves, operate visibly from outside. Endogenic factors, like earthquakes and volcanic eruptions, work invisibly from the earth’s interior. Together, these exogenic and endogenic factors have given the earth’s surface its present character and will continue their activities to further change the surface character of the earth in the future.
3. What is meant by exoganic factor? Mention some exogenic factors.
Answer: An exogenic factor is a force that operates externally and visibly upon the earth’s surface. The sources of these factors are the sun and the earth’s atmosphere.
Some important exogenic factors include insolation, sunshine, wind, rainfall, river, glacier, and sea-waves. Additionally, biotic elements like vegetation and animals also contribute to changing the earth’s surface.
4. Why are earthquakes and volcanic eruption called endogenic factors.
Answer: Earthquakes and volcanic eruptions are called endogenic factors because they are forces that work invisibly within the earth’s interior. These factors operate in the earth’s interior as well as in the ocean bottoms and are generally responsible for making the land surface and ocean bottoms irregular.
5. What do you mean by a river basin? Draw a diagram of a river basin.
Answer: River basin is an area from which a river, along with its tributaries, carrys water downward. Generally, rivers and their tributaries form basins of their own.
6. What is a tributary? Name two major tributaries of the Brahmaputra.
Answer: A tributary is a smaller river or stream that flows into a larger river. Two major tributaries of the Brahmaputra are the Subansiri and the Manas.
7. Give an outline of river erosion.
Answer: When a river flows down from a highland, it naturally erodes the surface of its basin. The erosive power of a river depends on factors like the geological structure of the channel, the hardness of the underlying rocks, and the amount of water flow. The direct action of water on channel materials is known as hydraulic action, which erodes sand and pebbles.
In mountainous regions, the river channel is narrow and deep due to high velocity. As the river enters a plain, the slope declines, velocity decreases, and it begins to deposit sediments. To maintain its flow, the river starts widening its channel through lateral cutting, which is the erosion of its banks. This is known as bank or lateral erosion and causes the river’s course to become zigzag or meandering.
8. What do you mean by latersal erosion of a river?
Answer: By lateral erosion of a river, I mean the process where a river widens its channel by eroding its banks. This type of erosion, also known as bank erosion, occurs when a river flows over the gentle slopes of a plain. As the river’s velocity decreases on the plain, it deposits sediments, which reduces the channel’s depth. To maintain its normal flow, the river starts cutting sideways into its banks. This lateral erosion is what causes a river’s course to become more zigzag.
9. What is ox-bow lake? Draw diagram to show how it is formed.
Answer: When a river flows over the gentle slopes of the plains, its course can become zigzag, which is called a meandering course. In certain parts of the plain, the river channel may be highly meandering. When such a course fails to carry a huge amount of water, the river straightens its course, leaving aside the most winding part. The cut-off part then takes the form of a lake or beel. Shaped like a horse-shoe, this type of lake is called a horse-shoe lake.
10. What is floodplain? Write how it is formed.
Answer: A floodplain is an alluvial plain formed on both banks of a river as a result of long-continued sediment deposition.
Floodplains are formed when floods occur. During floods, sands, mud, and silt carried down by the river spread out to a long distance over the banks and are deposited there.
11. Write how and where sandbars are formed in a river channel.
Answer: Sandbars are formed from sediments on the river bed. Tiny particles of sand, mud, and silt, as well as relatively big particles called bed load, are transported by the river. When the water gradually becomes weak over the plain, these rolling loads get deposited over the river bed, forming sandbars.
Sandbars are formed both on the sides and in the middle of the river bed.
12. How are the deltas formed?
Answer: Deltas are formed in the coastal shallow seas where rivers debouch, or flow out. Islands are formed in these areas by the river deposits. These islands take the shape of the Assamese letter ‘ব’ or the Greek letter delta ‘Δ’ and are therefore called 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, which allows the wind to easily act upon the land. As a result, sands and dust are easily transported from one part to another.
14. What is meant by deflation?
Answer: The blowing away of sands and dust by the wind, particularly in desert environments, is called deflation. Deflation often creates dust storms in the deserts.
15. How are the sand dunes formed?
Answer: Sand dunes are formed when sands and dust transported through deflation get deposited in certain places. This deposition causes the surface to be raised gradually, forming hills which are called sand dunes.
16. What is inselberg?
Answer: Inselbergs are low erosional hills generally seen in the deserts. The German term ‘inselberg’ means isolated island mountain. They are formed due to the abrasion of highlands by speedy desert winds that transport rocks and pebbles along with sands.
17. Define glacier and state how it differs from a river.
Answer: A glacier is a huge amount of ice in layers, found in extremely cold areas like polar regions and high mountains, that comes down very slowly over the surface slope.
A glacier differs from a river in that a glacier is made of moving ice, while a river is a flow of water. Glaciers move very slowly, with most having a velocity of less than 1 meter a day, whereas rivers can have a high velocity of flow, especially in mountainous regions. Glaciers create valleys with a U-shaped cross profile.
18. What is moraine? Draw a diagram to show different types of moraine.
Answer: Moraine is the term for materials other than ice, such as boulders, pebbles, and sands, that are carried by glaciers.There are four types of moraines:
(a) lateral moraine
(b) ground moraine
(c) medial moraine
(d) terminal or end moraine
19. Write how a glacial horn is formed.
Answer: A glacial horn is formed when several glaciers flow down from a mountain. The erosional activities of these glaciers on all sides of the mountain make its peak sharp and conical. Such a peak is called a glacial horn.
20. What is a coast?
Answer: The land margins of the oceans 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 that develops on the seashore. It is formed mainly of sands deposited by continuous wave action.
The importance of a beach is that the beaches in the coastal areas attract people for recreation.
22. Write how sea-waves bring about change in the coasts.
Answer: Sea-waves constantly operate in the coastal areas and bring about remarkable change through erosional and depositional activities. Through erosion, if the coast is composed of hard rocks, it becomes steep due to wave action, forming a feature called a sea-cliff. Through deposition, continuous wave action causes systematic sand deposits on the seashore, forming a plain-like feature called a beach.
23. Connect by arrows the factors and the features on the basis of their cause-effect relation.
Answer:
- Factor: River
- Feature: Floodplain
- Feature: Delta
- Feature: Sandbar
- Feature: Ox-bow lake
- Feature: V-shaped valley
- Factor: Wind
- Feature: Inselberg
- Feature: Sand dune
- Factor: Glacier
- Feature: Lateral moraine
- Feature: Horn
- Feature: U-shaped valley
- Factor: Sea-wave
- Feature: Beach
- Feature: Coast
24. Think and wirte about the probable changes of the earth’s surface in future.
Answer: In the future, the exogenic and endogenic factors will continue their activities to further change the surface character of the earth. For instance, there will possibly be a great change in the works of glaciers because of global climate change. The factors that cause change will themselves also experience change in the course of time.
25. Prepare a note on the surface characteristics of your village or town. Draw a sketch of the area and try to show there the major features (river, hill, wetland, plain etc).
Answer: Do it yourself. Here are the instructions:
- First, look carefully at the physical surroundings of your village or town.
- Identify if there are rivers, ponds, or wetlands nearby and note their location.
- Observe whether the land is mostly plain, hilly, or has plateaus.
- Check for vegetation cover—whether there are forests, grasslands, or agricultural fields.
- If there are man-made features like roads, bridges, dams, or settlements, mark them too, but focus mainly on natural features.
- On a clean page, draw a rough sketch map of your village or town.
- Use simple symbols:
- Blue line for rivers or streams.
- Wavy blue shape for ponds or wetlands.
- Triangles or shaded areas for hills.
- Dotted area for plains.
- Use simple symbols:
- Label each feature clearly in your sketch.
- Write a short note (one page) describing:
- The main landforms (plains, hills, wetlands, rivers, etc.).
- How these features affect daily life (for example, fertile plains help farming, rivers supply water, hills may limit expansion, wetlands attract birds, etc.).
- End your note with a few lines about how these features make your village/town unique.
Additional
Extra Questions and Answers
Coming soon
Extra MCQs
1: What percentage of the Earth’s surface is covered by hydrosphere?
A. 29%
B. 50%
C. 71%
D. 97%
Answer: C. 71%
31: The moraine deposited at the terminus or end-point of a glacier is known as a __________ moraine.
A. lateral
B. ground
C. medial
D. terminal
Answer: D. terminal
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