NCERT Book Class 7 Geography Our Changing Earth

Our Changing Earth

The lithosphere is broken into a number of plates known as the Lithospheric plates. You will be surprised to know that these plates move around very slowly – just a few millimetres each year. This is because of the movement of the molten magma inside the earth. The molten magma inside the earth moves in a circular manner as shown in the activity.

The movement of these plates causes changes on the surface of the earth. The earth movements are divided on the basis of the forces which cause them. The forces which act in the interior of the earth are called as Endogenic forces and the forces that work on the surface of the earth are called as Exogenic forces (Fig. 3.1) Endogenic forces sometimes produce sudden movements and at the other times produce slow movements. Sudden movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.

A volcano is a vent (opening) in the earth’s crust through which molten material erupts suddenly (Fig. 3.2) Similarly, when the Lithospheric plates move, the surface of the earth vibrates. The vibrations can travel all round the earth. These vibrations are called earthquakes (Fig. 3.3). The place in the crust where the movement starts is called the focus. The place on the surface above the focus is called the epicentre. Vibrations travel outwards from the epicentre as waves. 

Greatest damage is usually closest to the epicentre and the strength of the earthquake decreases away from the centre. Although earthquakes cannot be predicted, the impact can certainly be minimised if we are prepared before-hand. Some common earthquake prediction methods adopted locally by people include studying animal behaviour; fish in the ponds get agitated, snakes come to the surface.

MAJOR LAND FORMS Do you know?

The landscape is being continuously worn away by two processes – weathering and erosion. Weathering is the breaking up of the rocks on the earth’s surface. Erosion is the wearing away of the landscape by different agents like water, wind and ice. The eroded material is carried away or transported by water, wind, etc. and eventually deposited. This process of erosion and deposition create different landforms on the surface of the earth.

Exercises

1. Answer the following questions.

(i) Why do the plates move?

(ii) What are exogenic and endogenic forces?

(iii) What is erosion?

(iv) How are flood plains formed?

(v) What are sand dunes?

(vi) How are beaches formed?

(vii) What are ox bow lakes?

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. 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.

Please refer to attached file for NCERT Class 7 Geography Our Changing Earth