Page 139 - SST Class 06
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The Realms of the Earth
The earth is the third planet from the sun between and soil, is called the lithosphere (‘lithos’ means
Venus and Mars. The earth is made up of several rock in Latin). The solid uppermost layer of the
unique properties, characteristics, composition lithosphere is also called the crust of the earth.
and they all affect the processes of the earth This resembles an eggshell but, unlike an egg-
differently. Every of these properties and elements shell, it is not uniform in thickness. Certain parts
in the earth’s system are largely categorised into of the crust are thick while other parts are thin.
one of the four major subsystems including water, The thickness varies from 8 to 50 km. The
living things, land and the air. These are regarded earth’s crust is thicker under the continents than
as the four interlocking “wonders” that create the under the oceans. The uneven nature of its
earth’s diversity. Cumulatively, they are primarily structure is clear from the fact that the highest
grouped into biological (living things) and physi- mountain on the surface of the earth, Mt Everest
cal (non-living things). in Nepal, is 8,848 meters high while the deepest
part lies in the Mariana Trench near the
Philippines in the Pacific Ocean at a depth of
11,034 meters below the sea level. Under the
continents, the crust has two different layers
termed as sial and sima. However, is absent
under the oceans. Sial stands for silica and
aluminum while sima stands for silica and mag-
nesium.
CRUST
Average depth about 35 km
These four “wonders” of the earth are dependent Upper crust of aluminium and silica
Lower crust of basalt
upon each other and have been used to make Moho
the study of biological and physical components Zone separating crust and mantle
UPPER MANTLE
of the earth easily comprehendible. They are Upper rigid part 100 km deep
scientifically called the biophysical elements mainly of iron and magnesium
namely the hydrosphere (‘hydro’ for water), LOWER MANTLE
Less rigid and hotter, 3,000 km
biosphere (‘bio’ for living things), lithosphere deep–known as Asthenosphere
(‘litho’ for land) and atmosphere (‘atmo’ for air).
OUTER CORE
These spheres are further divided into various Radius of 3,500 km–33% of the
earth's mass – molten or liquid
sub-spheres.
INNER CORE
LITHOSPHERE Radius of 1,225 km – solid, Structure of the
iron-nickel alloy Earth’s Interior
The surface of the earth, made up of land or rock
Social Science-6 139
