Major Divisions of the Earth’s Surface: 3 Spheres

This article throws light upon the three main spheres of the Earth’s surface. The spheres are: 1. Atmosphere 2. Hydrosphere 3. Lithosphere.

Sphere # 1. Atmosphere:

The envelop of air that covers both the lithosphere and hydrosphere is called atmosphere. It contains water molecules and dust, which may act as nuclei for the condensation of water vapour to form clouds or fog.

It is composed of:

Nitrogen: 78.084% (main component)

Oxygen: 20.946% (the gas significant to man)

Argon: 0.934%

Carbon dioxide: 0.033%

In addition, inert gases, such as neon, helium, krypton and xenon are present. The water vapours present in the air vary in amounts at different places and time.

The air becomes less dense with height until it gets too thin to support life.

Sphere # 2. Hydrosphere:

It is a sphere of water surrounding the earth that contains absorbed air and carries particles of rock as sediments. Water covers almost three-fourths of the earth’s surface. Most of it lies within the ocean basins; it also appears on the surface of land in the form of rivers, ponds, lakes and as ground-water.

The quality of different waters varies (Table 2.1). The sea water contains the highest amount of soluble salts (with EC of 60,000 micro-mhos/cm), most of which is sodium chloride; the river water contains only a fraction of such salts (with EC of 591 micro-mhos/cm or even less).

Sphere # 3. Lithosphere:

It is the solid surface (continents, ocean basins, plains, plateaus and mountains, valleys, sand- dunes, lava flows and fault scarps) and interior of the earth which consists of rocks and minerals. It is covered by gaseous and watery envelops. It amounts to 93.06 per cent of the earth’s mass.

Interior of the Earth:

The earth-ball consists of 3 concentric rings: Crust, Mantle and Core (inner and outer) (see Fig. 2.1).

Crust:

5-56 km thick; consists of rocks with density of 2.6-3.0; separated from mantle by Mohorovicic or Moho or M-discontinuity. The soil scientists are interested in it, especially in its skin. It varies from 5 to 11 km in the oceans and 35 to 56 km in the continents.

Mantle:

2,900 km in thickness; comprises mixed metals and silicates and ultra-basic rocks with density of 3.0 to 4.5.

Core:

The innermost portion of the earth; it is 3,500 km in thickness; comprises metals, like nickel and iron with average density of 9-12. The density of the earth as a whole is 5.5 g/cm³, whereas the density of the common rocks varies from 2.6 to 2.7 g/cm³, and the density of the heaviest rock is 4.0 g/cm³. The density of the core is 9.0 g/cm³. From this it is evident that the density increased from crust to the core.