Page 127 - SST Class 07

3 3 3

The Changing Earth

The earth is the third planet from the sun in the by the internal heat of the earth. The surface
solar system. It is also the only planet where area of the plates is very large in comparison to
living things can survive. About 71 percent of the their thickness. Tectonic activities, such as earth-
earth’s surface is covered with water and the rest quakes and volcano, occur along the plate
29 percent is land. However, only 2.5 percent of margins. The plates are continuously moving.
this water is fresh. The earth’s atmosphere is The movement of the lithospheric plates cause
made up of exosphere, thermosphere, meso- various changes on the surface of the earth. The
sphere, stratosphere and troposphere. And the different types of movements of the plates are
earth’s composition is made up of the crust, due to the various forces acting on it. On the
upper mantle, mantle, outer core and inner core. basis of their source of origin, these forces
The earth’s crust consists of plates, known as divided into two broad categories :
lithospheric or tectonic plates. g The forces which originate from within the

There are six major and 20 minor plates, all of earth are called endogenic forces.
which float independently. g The forces which originate on the surface of
The rate of movement of the plates is extremely the earth are called exogenic forces.
slow, about a few millimeters each year. The g The endogenic forces can produce sudden
energy required to move these plates is produced or slow movements or both. These forces

Arctic Circle
EUROPE
NORTH
AMERICA ASIA

Tropic of Cancer

AFRICA
Equator
SOUTH
AMERICA

Tropic of Capricorn AUSTRALIA

Antarctic Circle
ANTARCTICA
World Map Showing Continents

Social Science-7 127