3. Our Changing Earth


NCERT Solutions for Class 7 Social science Chapter  3. Our Changing Earth

Question.1 Answer the following questions-
 
A. Why do the plates move?
Answer:- The Lithospheric plates move around because of the movement of the molten magma inside the Earth.

B. What are exogenic and endogenic forces?
Answer:- Forces – 1.Exogenic Forces, 2.Endogenic Forces
1. The ones that work on the Earth’s surface are called exogenic forces. Example:- erosion
2. The ones that work in the Earth’s interior are called endogenic forces. Example:- Earthquakes

C. What is erosion?
Answer:- Erosion is defined as the wearing away of the landscape by different agents like water, wind and ice.

D. How are flood plains formed?
Answer:-  1. During rains rivers overflow their banks.
2.This leads to the flooding of the nearby areas.
3. After the flood has receded, a layer of fine material and other material is deposited over the plain in the form of sediments.
4.This leads to the formation of the flood plain
 
E. What are sand dunes?
Answer:- Sand dunes are heaps of sand deposited at a place by the wind’s action in the desert areas.

F. How are beaches formed?
Answer:- The sea waves deposit sediments along the seashores. This results in the formation of beaches.

G. What are ox-bow lakes?
Answer:- When the meander loop is cut-off from the main river, it forms a cut-off lake. Its shape is like an ox-bow.Hence, the cut-off lake is called ox-bow Lake.
 
 
Question.2 Tick the correct answer-
 
1. Which is not an erosional feature of sea waves?
(a) Cliff
(b) Beach
(c) Sea cave
 
2.The depositional feature of a glacier is
(a) Flood plain
(b) Beach
(c) Moraine
 
3.  Which is caused by the sudden movements of the Earth?
(a) Volcano
(b) Folding
(c) Flood plain
 
4.  Mushroom rocks are found in
(a) Deserts
(b) River valleys
(c) Glaciers
 
5. Ox-bow lakes are found in
(a) Glaciers
(b) River valleys
(c) Deserts
 
Answer:- 1. (b) Beach    2. (c) Moraine  3. (a) Volcano    
4. (a) Deserts   5. (b) River valleys,
 
Question.3  Match the following-
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-

a) Some rocks have a shape of a mushroom.
Answer:- 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.

b) Flood plains are very fertile.
Answer:- They are made of the silt and other material brought by floods.Due to spread of the new silt, the flood plains are fertile areas.

c) Sea caves are turned into stacks.
Answer:-  When the cavities in the sea caves become bigger and bigger, only the roof of the caves are leftover, which forms sea arches. Further, due to erosion, the roof vanishes and only the walls stay intact, turning the sea caves into stacks.

d) Buildings collapse due to earthquakes.
Answer:-  Most buildings are not built earthquake-proof. They tear apart due to shallow foundation, sub-standard interior material, and lack of adequate steel. They collapse and fall down like a pack of cards.