Our Changing Earth - Chapter 3 Geography NCERT Class 7

Answer the following questions. 

1. Why do plates move?

Ans. The lithosphere is broken into a number of plates known as the Lithospheric plates. Lithosphere is comprised of the crust and upper mantle. Now the mantle is filled with molten rocks which is also known as magma. Because of the circular movement of molten magma the plate moves slowly.

2. What are exogenous and endogenous forces?
Ans. Exogenous forces - They are those forces that acts in the exterior of the earth like - soil erosion, wind activity, weathering, ocean currents etc.
Endogenous forces - They are those forces that acts in the interior of the earth like earthquake, volcanic eruption, plate movements etc.

3. What is erosion?
Ans. When a landscape is washed away or moved away from one place to another with the help of natural agents like - wind, water and ice. The eroded material is transported and gets deposited a different place eventually. This phenomena is known as erosion.

4. How are flood plains formed?
Ans. Flood plains are formed by the deposition of layers of fine soil and other sediment materials along the bank of a river. Flood plains are flat fertile and they are usually seen near the areas where the river approaches the sea.

5. What are sand dunes?
Ans. When the light grains of sand are carried over large areas with the help of wind, this way of transportation is known as sand dunes.

6. How are beaches formed?
Ans. Beaches are formed due to the movement of sediment deposits to the shoreline by the sea waves.

7. What are ox bow lakes?
Ans.  Due to continuous movement of river, erosion and deposition along the sides of the river is a natural process. Over long course of time, meanders are formed. Many such meander loops are formed along the bank of the river. When the meander loop come closer and closer they tend to cut off from the river and forms a cut-off lake, now these cut-off lake is called an ox-bow lake.

Giver reasons.

1. Some rocks have a shape of a mushroom. Why?

Ans. Mushroom rocks are commonly seen in desserts. It is due to the heavy wind activity that erodes the lower section of the rock more than the upper section. This leads to the lower section of the rock becoming much narrow in shape and top being wider. And that's how mushroom rocks are created.

2. Flood plains are very fertile. Why?

Ans. Flood plains are formed by the deposition of layers of fine soil and other sediment materials along the bank of a river. It is due the continuous flow of river that makes a flood plain flat fertile and they are usually seen near the areas where the river approaches the sea.

3. Sea caves are turned into stacks. Explain?

Ans. When sea waves continuously strike the rocks, major cracks are developed in the rock. Over time they become larger and wider. Thus, hollow like caves are formed on the rocks. They are called sea caves. As these cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches. 

4. Building collapse due to earthquake. Why?

Ans. Earthquake is an endogenous forces, meaning it is generated at the interior of the earth. During an earthquake the earth surface vibrates and because of these vibrations the surface becomes unstable making building and other types of concrete construction collapse.

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