Speaking in more general terms the earth, within the limits accessible for observation, consists of three important parts viz., the atmosphere (air and gases), the hydrosphere (water) and the lithosphere (the earth's crust).
1. ATMOSPHERE
It is the envelope of air which surrounds the earth. This envelope of air extends upto a considerable height from the surface of the earth. Since the atmosphere is not of the same density throughout and that atmospheric pressure decreases with height, it is a bit difficult to mark the outer limit of the atmosphere. There is, however, definite evidence that the atmosphere extends beyond 160 km. Although almost all of the atmosphere (about 97%) lies within 30kmofthe earth’s surface the upper limit of the atmosphere can be drawn approximately, at a height of 10,000 Km. The atmosphere is held to the earth by the gravitational pull of the latter. The atmosphere is densest at sea level and thins rapidly upwards. It constitutes a very insignificant percentage of the mass of the earth.
Composition of Atmosphere
From the earth’s surface upwards to an altitude of about 80 km, the chemical composition of the atmosphere is highly uniform throughout in terms of the proportions of its component gases. In the uppermost reaches the atmosphere is charged with subatomic particles. Thus, except for the water vapour present, the composition of the atmosphere near the surface of the earth is practically uniform throughout the globe. Pure, dry air has very nearly the following constitution:
Nitrogen 78.03 percent by volume
Oxygen 20.99 “ “
Argon. 0.94 “ “
Carbondioxide 0.03 " "
Hydrogen 0.01 " "
All the component gases of the lower atmosphere are perfectly diffused among one another so as to give the pure, dry air a definite set of physical properties, just as if it were a single gas.
Structure of the Atmosphere
The structure of the atmosphere is highly complex but its layering is now well understood. The atmosphere has been divided into several layers according to temperatures and zones of temperature change. Altitudinally arranged the atmosphere falls into five layers or divisions such as:
(a) Troposphere
(b) Stratosphere
(c) Mesosphere
(d) Ionosphere
(e) Exosphere
Troposphere
It is the lower most layer of the atmosphere. On an average, it extends up to a height of 12 km from the surface of the earth. At the equator, the thickness of the troposphere is the greatest i.e. about 18 km and about 8 kms thick over the poles. The troposphere contains about three-fourths of the total mass of the atmosphere, thus it is the densest of all layers. It is the locale of all the vital atmospheric processes which create the climatic and weather conditions on the earth’s surface. The troposphere is characterised by:
(i) varying moisture content,
(ii) mobility of the air masses, both vertical and horizontal, and
(iii) regular temperature decline with height. The temperature of air in the troposphere decreases at the rate of 1°C per 165 meters of height.
Tropopause
It is an undefined region lying between troposphere and stratosphere. Here the temperature remains constant. The height of tropopause varies with latitude.
Stratosphere
It is the atmospheric zone extending from the upper boundary of the troposphere, to a height around 55 kms. In contrast to the troposphere, a Steady rise in temperature with height is observed here. Here air is at rest. It is an isothermal region and is free of clouds, dust and water vapour. The upper strata is rich in ozone. The ozone layer serves as a shield, protecting the troposphere and earth's surface by absorbing most of the ultra violet radiation found in the sun's rays, thus, is of great importance for the existence of life on the earth’s surface, Since the water vapour content of the stratosphere is negligible, weather creating processes are never generated here. In the upper layers of the Stratosphere the temperature rises to 0°C and higher. A stabilisation of temperature occurs at a height of around 55 kms and is called the STRATOPAUSE. Above it a fall in temperature is recorded.Mesosphere
Above the stratopause lies the mesosphere, which is a very cold region. This layer extends upward to about 80 kms from the surface of the earth. Within the mesosphere, at a height of about 60 kms, there occurs a layer called radiowaves absorbing layer. At the end of the mesosphere, there is a transitional layer of minimum temperature of 80°C. An important feature of the mesosphere is its higher temperature in winter compared with summer, which is apparently due to a small ozone content.Ionosphere
The ionosphere extends from an altitude of about 80 km upward. Studies have shown that the ionosphere extends up to a height of 1000-2000 km from the earth’s surface. The part of the ionosphere lying between 80 - 800 km is called thermosphere, which is characterised by a steady rise in temperature,In the ionosphere, almost all the atoms are ionised i.e this zone is made up primarily of ions (charged atoms). The ionosphere is much rarefied and therefore has a very low total mass despite a huge volume. This layer protects us from falling meteorites as it burns most of them. Besides, it reflects the radio waves making wireless communication between places possible.
Exosphere
Above the ionosphere lies the exosphere. It is the outermost zone of the atmosphere. It is also known as the diffusion zone, where the atmospheric gases diffuse into the open space. It is prevented, in part, however, by the earth’s magnetic field, which retains the ionised particles within the magnetosphere. Much about the exosphere is yet to be known.
2. HYDROSPHERE
It is a term used for the total body of water of the earth. In other words, all the natural waters occurring on or below the surface of the earth is known as hydrosphere. Thus the term includes the oceans, seas, lakes, rivers, snow and ice, underground and atmospheric water.An enormous mass of water is concentrated in oceans and seas which occupy approximately 71 per cent (i.e 361,059,000 sq.km out of 510,000.000 sq.km.) of the surface of the entire globe. Oceans and seas communicate with each other and form a single mass of water called the World Ocean. Its average depth is about 3800 metres. The total volume of the world ocean is about 1.4 billion cubic kilometres i.e about 97 percent of the world's free water is constituted by the world ocean.
The distribution of the oceans and seas is highly irregular in different latitudinal belts, as well as in the Northern and Southern Hemispheres. There are four major oceanic bodies:
(i) the Pacific,
(ii) the Atlantic,
(iii) the Indian and
(iv) the Arctic.
The first three oceans together constitute 90 per cent of the total area of water bodies. The Pacific is the largest ocean, both in surface area and volume.
Composition of Sea-Water
The sea waters are more variable in composition than the atmosphere, including large proportions of mineral matter as well as water and gases. Sea-water is a solution of salts. Dissolved salts, or solutes are added to the sea water from the erosion of the rocks of the earth’s surface and from the eruption of volcanic materials espacially along oceanic ridges. These are lost by precipitation to ocean-floor sediments.These salts result in the property of salinity, but the degree of salinity is not the same everywhere. The concentration of solutes in the water is affected by temporal and regional variations in erosion, precipitation and also by surface evaporation and the addition of water from rain and rivers. Thus, in the North Sea, for example, the percentage of salt is less than that of the Atlantic, in the Baltic it is very much less. In the Mediterranean, on the other hand, the proportion of salt is considerably greater than in any part of the open ocean.
The average salinity of the sea water is 35 parts per thousand i.e on the average 1000 grams of sea-water contains 35 grams of dissolved solids. According to Dittmar, the proportion of these solids are as follows:
Sodium chloride 27.213
Magnesium chloride 3.807
Magnesium sulphate 1.658
Calcium sulphate 1.260
Potassium sulphate 0.863
Calcium carbonate 0.123
Magnesium bromide 0.076
_________
Total. 35,000
The ingredients of sea water have maintained approximately fixed proportion over a considerable span of geologic time. Of the various elements combined in these salts, chlorine alone makes up 55 per cent by weight of all the dissolved matter and sodium makes up 31 per cent. 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.
According to Brian Mason (Principles of Geochemistry, 1952) common elements present in the ocean-water are as follows
Elements Weight percentage
Oxygen. 85.79
Hydrogen 10.67
Chlorine. 1.898
Sodium. 1.056
Magnesium 0.127
Sulphur. 0.088
Calcium 0.040
Potassium 0.038
Bromine 0.007
Carbon (inorganic) 0.003
Strontium 0.001
_________
99.718
The rest is made up by other dissolved gases, As we know, water absorbs oxygen more intensely than nitrogen. Different gases are differently absorbed by water, Thus while the oxygen to nitrogen ratio in the air is 1:4, in water these gases are usually found in a 1:2 ratio. Sea water derives oxygen from the air and also through photosynthesis by marine plants. The carbondioxide content of sea water is also high. Its sources are the atmosphere, river waters, the life activity of marine animais and volcanic eruptions.
3. LITHOSPHERE
It is the general term for the entire solid earth realm i.e. crust. According to the recent concepts, the term lithosphere is used for the crust and the upper part of mantle, which is considered to be elastically very strong. This is the outer, cold part of the earth which is about 50-100 kms thick. Compared to the whole earth, the lithosphere is quite thin. Beneath the lithosphere, the rocks are still solid but are capable of creeping 2 few millimeters per year if the load on them is changed. The lithosphere is underlain by the asthenosphere which is considered to be a comparatively weaker zone.According to the estimation made by Clarke and Washington, the lithosphere consists of 95% igneous rocks, 4% shale 0.75% sandstone, and 0.25% limestone (the metamorphic rocks being the altered equivalents of one or other of these rocks).
The average chemical composition of the lithosphere has been computed by a number of geo-scientists in terms of elements by weight percentage as indicated below:
In terms of oxides the chemical composition of the lithosphere is as follows
The above tables indicate that 99 percent of the upper Crust is made up mainly of 10 elements, with oxygen accounting for slightly less than 50%. Besides, the above mentioned ten oxides constitute more than 98 percent of the lithosphere, with silica being the most abundant one.
The mineralogical composition of the lithosphere has been computed as follows by Clarke and Washington in terms of volume percentage
The above analysis represents only the average mineralogical composition of the lithosphere but it does not represent in any way the composition of the earth as a whole or even the crust as a whole.
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