1. Atmosphere:
The air which envelopes the earth and extends up to a considerable height from the surface of the earth is called the atmosphere. It consists of a mixture of various gases and is held to the earth by gravitational attraction. This envelope of air is densest at sea level and thins rapidly upwards. The atmosphere constitutes a very insignificant percentage of the mass of the earth.
(a) Troposphere:
It extends up to a height of 12 km on an average, froe the surface of the earth. At the equator the thickness of the troposphere is the greatest. It is densest of all the layers. It is the locale of all the vital atmospheric processes which create the climatic and weather conditions on the earth’s surface. About half the mass of air comprising the entire atmosphere is concentrated in this zone.
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The temperature of air in the troposphere decreases at the rate of 1°C per 165 meters of height.
Tropopause is an undefined region lying between troposphere and stratosphere, and there prevails a constant temperature in this region.
(b) Stratosphere:
Here air is at rest. It is an isothermal region and is free of clouds, dust and water vapour. It extends up to a height of about 50 kms. Its upper strata is rich in ozone which prevents ultraviolet radiation by absorbing them and a very little is filtered through it, which does not harm living organisms.
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(c) Mesosphere:
It is a very cold region and extends up to a height of 80 km from the surface of the earth. At a height of 60 kms there is an intermediate layer which is known as radio- waves absorbing layer.
(d) Ionosphere:
It is a region of electrically charged or ionized air lying next to mesosphere. It protects us from falling meteorites. It extends up to a height of 150 kms.
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(e) Exosphere:
It is the uppermost region of ionosphere and is the fringe of atmosphere. Its boundaries are not known.
Composition of atmosphere:
The atmospheric composition of the troposphere reveals two major constituents: molecular nitrogen and molecular oxygen. The minor constituents include carbon dioxide, water vapour and inert gases. The water vapour is concentrated in the lower part of the atmosphere. The composition of atmosphere is as follows:
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Nitrogen 78’09 (volume percentage)
Oxygen 2097 (volume percentage)
Carbondioxide 0.03 (volume percentage)
The rest is made up of argon, helium, neon, krypton etc. Water vapour is present in varying proportions over different parts of the earth’s surface.
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2. The hydrosphere:
It is an irregular but nearly continuous layer of salt and fresh wafer, making up the oceans, seas, lakes, rivers, snow and ice on the surface and also the ground water.
The world ocean covers about 71% of the global surface and its average depth is about 3800 metres.
Composition of sea-water. Sea-water is a solution of salts -a brine – whose ingredients have maintained approximately fixed proportions over a considerable span of geologic time. Of the- various elements combined in these salts chlorine alone makes up 55% by weight of all the dissolved matter and sodium makes up 31 %. Magnesium, calcium, sulphur and potassium are the other four major elements in these salts. Sea-water also holds in solution small amounts of all the gases of the atmosphere.
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Common elements present in the hydro-sphere-ocean water, as shown by Brian Mason is as follows :
The rest is made up by other dissolved gases.
3. The lithosphere:
It is the general term for the entire solid earth realm, i.e., the crust. According to the recent concept, the term lithosphere is used for the crust and upper part of mantle which is considered to be elastically very strong. The lithosphere is underlain by asthenosphere.
As per the estimation, made by Clarke and Washington, the lithosphere consists of 95% igneous rocks, 4% shale and 0’75% sandstone and 0’25% limestone (the metamorphic rocks being altered equivalents of one or other of these rocks).
Chemical composition of the lithosphere. It has been observed that while the continents are distinctly sialic the ocean floor, represent the upper sima, are distinctly basic in nature. The following is the composition in terms of elements (by weight), as given by Clarke and Washington :
From the above, it is evident that 99% of the upper crust is made up of only 10 elements, with oxygen accounting for slightly less than 50% Besides, the above mentioned ten oxides constitute more than 98 % of the lithosphere, with silica being the most abundant.
The mineralogical composition of the lithosphere shows that plagioclase feldspar is the most abundant mineral and apatite is the least one. The estimate, as given by Clarke and Washington is as follows:
The above analysis gives only the average mineralogical composition of the lithosphere, but it does not in any way represent the composition of the earth as a whole or even the crust as a whole.