Ncert Solutions for Class 7 Social Science Chapter 3 – Our Changing Earth
Page No 17:
Question 1:
Answer the following questions.
(i) Why do the plates move?
(ii) What are exogenic and endogenic forces?
(iii) What is erosion?
(iv) How are flood plains formed?
(v) What are sand dunes?
(vi) How are beaches formed?
(vii) What are ox-bow lakes?
Answer:
(i) The Lithospheric plates move around because of the movement of the molten magma inside the Earth.
(ii) Earth’s movements are divided on the basis of the forces which cause them. The ones that work on the Earth’s surface are called exogenic forces while the ones that work in the Earth’s interior are called endogenic forces. The erosional and depositional activities of wind and water are examples of exogenic forces. Earthquakes and volcanoes are examples of endogenic forces.
(iii) Erosion is the weathering or wearing away of the landscape by different agents like wind, water and ice.
(iv) During its course through a plain, a river sometimes overflows its banks. This leads to the flooding of the neighbouring areas. As it floods, the river water deposits layers of fine soil and sediments on its banks. This leads to the formation of a flat, fertile flood plain.
(v) In deserts, when wind blows, it lifts and transports sand from one place to another. When it stops blowing, the sand particles fall and get deposited in low hill-like structures called sand dunes.
(vi) The erosional and depositional activities of the sea waves give rise to different coastal landforms. A beach is one such coastal landform. It is formed when the sea waves deposit sediments along the sea shore.
(vii) An ox-bow lake is a crescent-shaped lake formed by a meandering river. During its journey through a plain, a river twists and turns to form meanders. Erosion and deposition occur constantly along the sides of a meander, thereby causing the ends of its loop to come closer and closer. In due course of time, the meander loop cuts off from the river and forms a cut-off, crescent-shaped ox-bow lake.
Question 2:
Tick the correct answer.
(i) Which is not an erosional feature of sea waves?
| (a) Cliff | (b) Beach | (c) Sea cave |
(ii) The depositional feature of a glacier is
| (a) Flood plain | (b) Beach | (c) Moraine |
(iii) Which is caused by the sudden movements of the Earth?
| (a) Volcano | (b) Folding | (c) Flood plain |
(iv) Mushroom rocks are found in
| (a) Deserts | (b) River valleys | (c) Glaciers |
(v) Ox-bow lakes are found in
| (a) Glaciers | (b) River valleys | (c) Deserts |
Answer:
(i) Which is not an erosional feature of sea waves?
| (a) Cliff | (b) Beach | (c) Sea cave |
(ii) The depositional feature of a glacier is
| (a) Flood plain | (b) Beach | (c) Moraine |
(iii) Which is caused by the sudden movements of the Earth?
| (a) Volcano | (b) Folding | (c) Flood plain |
(iv) Mushroom rocks are found in
| (a) Deserts | (b) River valleys | (c) Glaciers |
(v) Ox-bow lakes are found in
| (a) Glaciers | (b) River valleys | (c) Deserts |
Page No 18:
Question 3:
Match the following.
| (i) Glacier | (a) Sea shore |
| (ii) Meanders | (b) Mushroom rock |
| (iii) Beach | (c) River of ice |
| (iv) Sand dunes | (d) Rivers |
| (v) Waterfall | (e) Vibrations of Earth |
| (vi) Earthquake | (f) Sea cliff |
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| (g) Hard bed rock |
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| (h) Deserts |
Answer:
| (i) Glacier | (c) River of ice |
| (ii) Meanders | (d) Rivers |
| (iii) Beach | (a) Sea shore |
| (iv) Sand dunes | (h) Deserts |
| (v) Waterfall | (g) Hard bed rock |
| (vi) Earthquake | (e) Vibrations of Earth |
Question 4:
Give reasons.
(i) Some rocks have a shape of a mushroom.
(ii) Flood plains are very fertile.
(iii) Sea caves are turned into stacks.
(iv) Buildings collapse due to earthquakes.
Answer:
(i) In deserts, one can see rocks in the shape of a mushroom—with a narrower base and a wider top. These are known as mushroom rocks. Such rocks are formed when the winds erode the lower section of a rock more than the upper part.
(ii) Flood plains are formed as a result of the depositional activity of rivers. Rivers carry along with them eroded material like fine soil and sediments. When it overflows its banks, it deposits the eroded material and causes flood plains to be formed. The deposited material makes the land fertile.
(iii) Stacks are formed as a result of the erosional activity of the sea waves. When sea waves continuously strike rocks, cracks develop in them. As these cracks become larger and wider, hollow caves get formed on the rocks. These are called sea caves. As the waves keep striking the rocks, the cavities become bigger and bigger, with only the roof remaining at the end. Such structures are known as sea arches. Further erosion breaks the roof, and only walls remain. These wall-like features are known as stacks. Thus, sea caves are ultimately converted into stacks.
(iv) Earthquakes are the sudden vibrations caused within the Earth’s surface as a result of the movement of the Lithospheric plates. Such vibrations, when they are of a high intensity, cause damage to the things on the Earth’s surface. Various human-made (e.g., buildings) and natural (e.g., trees) constructions can break down and collapse under the effect of the vibrations because they are situated on the Earth’s surface.
Question 5:
Activity.
Observe the photographs given below. These are various features made by a river. Identify them and also tell whether they are erosional or depositional or landforms formed by both.
Photograph
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Name of the Feature
|
Type
[Erosional or Depositional or Both]
|
 |
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 |
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 |
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Answer:
Photograph
|
Name of the Feature
|
Type
[Erosional or Depositional or Both]
|
|
Waterfall
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Erosional
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Meander
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Erosional and depositional
|
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Flood plain
|
Depositional
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Page No 19:
Question 6:
For fun.
Solve the crossword puzzle with the help of given clues.
Across
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Down
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| 2. Loop like the bend of a river | 1. Rise and fall of water caused by frictionof wind on water surface |
| 4. Solid form of water | 3. Flow of water in a channel |
| 7. Moving mass of ice | 5. Steep perpendicular face of a rock alongsea coast |
| 9. Sudden descent of water in the bed of a river | 6. Debris of boulder and coarse materialcarried by glacier |
| 11. Natural cavity on weak rocks formed by action of waves | 8. Crescent-shaped lake formed by a meandering river |
| 12. Embankment on a river that keeps itin its channel | 10. Fine sand deposited by the action of the wind |
| 13. Large body of sea water | 13. Isolated mass of rising steep rock near acoastline |
| 14. Dry area where sand dunes are found | 14. Alluvial tracts of land formed by river deposits at mouth of the river |
| 15. Small hill of sand caused by the action of the wind |
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| 16. Flat plain formed by river deposits during time of flood |
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Answer:
