NCERT Solutions Class 7 Social Science Chapter 3 Our Changing Earth

 

NCERT Solutions for Class 7 for Social Science Geography Chapter 3 Our Changing Earth

1. Answer the following questions: 

(i) Why do plates move? 

Ans. Plates move because of the movement of molten magma inside the earth. The molten magma moves inside the earth in a circular manner. 

 

(ii) What are exogenic and endogenic forces? 

Ans. The forces which act on the surface of the earth are known as exogenic forces, whereas the forces which act in the interior of the earth are known as endogenic forces. 

 

(iii) What is erosion? 

Ans. The wearing away of landscape by different agent of nature like wind, water, and ice is known as erosion.  

 

(iv) How are flood plains formed? 

Ans. Layers of fine soil and other materials known as sediments are deposited along the banks of river, whenever a river floods. This leads to the formation of a fertile floodplain. 

 

(v) What are sand dunes? 

Ans. When the wind blows in a desert, it transports sand from one place to another. When it stops blowing, the sand falls and gets deposited in low hill-like structures, which are known as sand dunes. 

 

(vi) How are beaches formed? 

Ans. Beaches are formed when sediments carried by the sea waves are deposited along the shores. The finer sediments are deposited near the sea, while the coarser ones are deposited further inland. 

 

(vii) What are ox bow lakes? 

Ans. Rivers start meandering when they enter the plains. In due course of time, the meander loop cuts off from the river and forms a cut-off lake, which is also known as 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 

(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  

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 

Ans. 

(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 

 

4. Give reasons. 

(i) Some rocks have a shape of mushroom. 

Ans. Erosion of rocks in a desert by wind progresses at a different rate at its bottom to that at its top. Winds erode lower section of rocks more than the upper section. Thus, rocks take the shape of a mushroom, i.e., narrower at the base and wider at the top. 

(ii) Flood plains are very fertile. 

Ans. Flood plains are formed due to depositional activity of rivers. Sometimes, the rivers flood and layers of fine sediments and other materials are deposited along its banks. This causes floodplains to be formed. The deposited sediments and other materials make the land fertile. 

(iii) Sea caves are turned into stacks. 

Ans. Continuous striking of sea waves against the rocks causes the rocks to develop hollow like caves on them. These hollows are known as sea caves. They become larger and larger due to continuous erosion, leaving the cave with only a roof, thus forming sea arches. Further erosion breaks the roof and only walls are left. These wall like structures are known as stacks. 

(iv) Buildings collapse due to earthquakes. 

Ans. An earthquake is a sudden release of energy from the earth’s crust that travels in waves. When the intensity of earthquake is high, it can cause damage to many man-made and natural things on the surface of the earth. Buildings can collapse when they cannot stand the intensity of earthquake. 

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. 

6. For fun. 

Solve the crossword puzzle with the help of given clues. 

 

Across: 

2. Loop like the bend of a river: Meander 

4. Solid form of water: Ice 

7. Moving mass of ice: Glacier 

9. Sudden descent of water in the bed of a river: Waterfall 

11. Natural cavity on weak rocks formed by the action of waves: Cave 

12. Embankment on a river that keeps it in its channel: Levee 

13. Large body of water: Sea 

14. Dry area where sand dunes are found: Desert 

15. Small hill of sand caused by the action of wind: Sand dune 

16. Flat plain formed by river deposits during time of flood: Floodplain 

Down: 

1. Rise and fall of water caused by friction of wind on water surface: Wave 

3. Flow of water in a channel: River 

5. Steep perpendicular face of a rock along a sea coast: Cliff 

6. Debris of boulder and coarse material carried by glacier: Moraine 

8. Crescent shaped lake formed by a meandering river: Oxbow lake 

10. Fine sand deposited by the action of wind: Loess 

13. Isolated mass of rising steep rock near a coastline: Stack 

14. Alluvial tracts of land formed by the river deposits at the mouth of a river: Delta