NCERT Solutions for Class 7 Geography Chapter 3 - Our Changing Earth

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Chapter 3 - Our Changing Earth Exercise 17

Solution 1

(i) The Lithospheric plates have a tendency to move, albeit very slowly and over a very long period of time, due to the movement of the molten magma inside Earth.

(ii) Different forces cause different types of Earth’s movements. The forces that are operational on the Earth's surface are called exogenic forces. On the other hand, the ones that work in the interior of Earth are called endogenic forces.

 (ii) Erosion is the deterioration, disintegration and wearing away of Earth’s landscape by different factors like water, wind and ice, etc.

(iv) While coursing through a landscape that is plain, rivers sometimes overflow their banks. This results in the flooding of the nearby lands. As this happens, the water deposits layers of fine soil and sediments on the banks of the river. This results in the formation of a flat, fertile flood plain in the concerned areas.

 (v) In desert landscapes, the wind transports sand particles from one place to another. When the wind stops blowing, the sand falls and gets deposited in low hill-like structures. These structures are known as sand dunes.

 (vi) The erosional and depositional activities of the sea waves form different varieties of coastal landforms. Beaches are one such variety of coastal landforms. A beach is formed when sea waves deposit sediments on the sea shore.

(vii) Ox Bow lakes are crescent shaped lakes that are caused due to the trajectory of meandering rivers. The sides of the meanders created by the rivers are further eroded and deposited onto by the water.  This causes the ends of the loop to come closer and closer. Eventually, the meander loop cuts off from the river course and forms a crescent shaped ox-bow lake.

Solution 2

(i) – (b) Beach

(ii) – (c) Moraine

(iii) – (a) Volcano

(iv) – (a) Deserts

(v) – (b) River valleys

Chapter 3 - Our Changing Earth Exercise 18

Solution 3

(i) – (b) River of ice

(ii) – (c) Rivers

(iii) – (a) Sea shore

(iv) – (f) Deserts

(v) – (e) Hard bed rock

(vi) – (d) Vibrations of Earth

Solution 4

(i) In deserts, mushroom rocks are often found. These rocks have a narrow cylindrical base and a wide top. This happens when the wind erodes the lower part of the rock more than the upper part.

(ii) Flood plains are formed when rivers overflow and deposit eroded material like sediments and fine soil onto the banks. Such material is conducive for cultivation. Hence, flood plains are very fertile.

(iii) Sea caves are formed when sea waves erode the cracks and make them hollow. As the waves keep striking the rocks, the cavities keep becoming bigger and bigger and ultimately form sea arches. Further erosion by the waves breaks the roof, leaving only the walls. Such structures are known as stacks. Thus, through continuous erosion, sea caves are turned into stacks.

(iv) Earthquakes are, in effect, vibrations caused within the Earth surface. These vibrations are caused by the movement of the Lithospheric plates. Hence, earthquakes, especially when they are of high intensity, cause major damage to the structures above Earth’s surface. Hence, buildings are often damaged and tend to collapse due to earthquakes.