Our Earth is a cosmic body. It is one of the nine members of he Solar system of which Sun is the central star. The nine planets constituting the Solar system has been named as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. In its shape, he Earth is commonly described as a spheroid, it has an equatorial diameter of 12,757.776km and a polar diameter of 12,713.824km and thus has an equatorial bulge. At present the Earth is the only planet believed to be sustaining life other planets have shown no signs of life on them. For systematic scientific investigations, the earth is commonly differentiated into three parts; they are atmosphere, lithosphere and hydrosphere. Each forms an extensive field of study, volumes of information have been collected about each of these parts during last hundred years or so but findings of last three decades have made our understanding about these parts very clear. Only most important characteristics of these parts have been summarized below.
ATMOSPHERE The outer gaseous part of earth starting from the surface and extending as far as 700km and beyond is termed atmosphere. Although extending for such great distances, the atmosphere makes only one-millionth part of the mass of earth; this is because of its gaseous composition. It is now fairly established that the atmosphere possesses a layered structure. Their well-defined layers or zones of the atmosphere are surface upward, troposphere, stratosphere and ionosphere. LITHOSPHERE It is the solid part of the earth and in a broader sense includes all the solid materials composing the earth from surface downwards, although sometimes-specific terms are used for deeper earth zones. Recent detailed seismic studies of the body of the earth have shown that it is composed of three well-defined shells, Crust, Mantle, Core.
The Crust: - Is the topmost shell of the earth, which has a thickness of 30-40 km in the continents and 5-6 km in the oceans. There is a striking variation in the materials or rocks, as they are called, composing the crust over the continents and ocean floors. The oceanic crust is made up of heavier and darker rocks called basalts compared to light-colored and light-density, granitic rocks of the continental crust. When considered as a part of the total structure of the earth, crust makes only an insignificant part represented by a thin layer, similar to the skin of an apple. As regards he chemical composition of the crust, analyses made by Clarke and Gold Schmith, using rocks from different geographic regions of the crust have all shown that when expressed in terms of oxides, the crust has Silica as the most dominant component, its value lying above 50% by volume in the oceanic crust and above 62% in the continental crust. Alumina is the next important oxide, varying between 13-16% followed by Iron Oxides (8%), Lime (6%), Sodium (4%), Magnesium (4%), Potassium (2.5%) & Titanium (2%). The crust itself shows a complicated structure both in make-up and compositional variations.
The Mantle: - At the base of the crust materials of the earth become greatly different in many properties from those overlying them in the crust. These materials appear to form a nearly homogeneous zone till a depth of 2900 km is reached. This zone of materials lying between crust and a depth of 2900 km is known a MANTLE. It is made up of extremely basic materials, called ultra basic rocks, which are believed to be very rich in iron and magnesium but quite poor in silica. Such rock names as Periodotites, Dunite. This One is characterized with a high density, increasing steadily with depth further; the mantle material is believed to be highly plastic in nature. Many of the most important geological process such as earthquakes and formation of mountains are believed to have their origin in this zone. The Core: - It is the third and the innermost structure shell of the earth, which is clearly marked by the seismic evidence. It starts at a depth of 2900 km below the surface and extends right up to the center of the earth at 6370 km. The material making g the core is found to be from seismic studies only strikingly different from that making the other two shells in one major aspect, in elastic properties. The material has no shear resistance, which makes it nearer to liquid than to a solid body. It has a very high density, above 10gms/cubic centimeter, at the mantle –core boundary. Nothing can be said about the composition of the core. According to one, widely favored view, the core is made up of Iron and Nickel alloy material.
