NCERT Solutions for Class 7th: Ch 3 Our Changing Earth Geography Social Studies Our Enviroment
Page No: 17Exercises
1. Answer the following questions.
(i) Why do the plates move?
Answer
The plates move because of the movement of the molten magma inside the earth.
(ii) What are exogenic and endogenic forces?
Answer
The forces that work on the surface of the earth are called as exogenic forces.
The forces which act in the interior of the earth are called as endogenic forces.
(iii) What is erosion?
Answer
The wearing away of the landscape by different agents like water, wind and ice is called erosion.
(iv) How are flood plains formed?
Answer
When river overflows its banks, it flood off its neighbouring areas which deposits layers of fine soil and other
material called sediments along its banks. This leads to the formation of a flat fertile floodplain.
(v) What are sand dunes?
Answer
The low hill like structures formed due to deposition of sand at one place by the wind in the desert is called sand dunes.
(vi) How are beaches formed?
Answer
The sea waves deposit sediments along the shores which leads to the formation of beaches.
(vii) What are ox bow lakes?
Answer
Due to continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer which in some time cuts off from the river and forms a cut-off lake, called an ox-bow
lake.
2. Tick the correct answer.
(i) Which is not an erosional feature of sea waves?
(a) Cliff (b) Beach (c) Sea cave
✓ (b) Beach
(ii) The depositional feature of a glacier is:
(a) Flood plain (b) Beach (c) Moraine
✓ (c) Moraine
(iii) Which is caused by the sudden movements of the earth?
(a) Volcano (b) Folding (c) Flood plain
✓ (a) Volcano
(iv) Mushroom rocks are found in:
(a) Deserts (b) River valleys (c) Glaciers
✓ (a) Deserts
(v) Ox bow lakes are found in:
(a) Glaciers (b) River valleys (c) Deserts
✓ (b) River valleys
Page No: 18
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 |
| - | (g) Hard bed rock |
| - | (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 | (f) Sea cliff |
| (vi) Earthquake | (e) Vibrations of earth |
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 desert, wind is an active agent for erosion which erode the lower section of the rock more than the upper part. Therefore, some rocks have a shape of a mushroom having narrower base and wider top.
(ii) Flood plains are very fertile because floods deposit layers of fine soil and other material called sediments which is ideal for cultivation along its banks.
(iii) When cavities in sea caves become bigger and bigger only the roof of the caves remain forming sea arches. Further, erosion breaks the roof and only walls are left which forms the stacks.
(iv) During earthquakes, vibrations travel outwards from the epicentre as waves and propagate through the surface of the earth which produce sudden movements that leads to the collapse of buildings.
Notes of Chapter 3 Our Changing Earth
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