Origin of the Earth
Earth is the third planet from the sun and is the largest of the terrestrial planets. Everything in world around us is built upon the Earth, grows on Earth or depends on the environment of the Earth in some way. Earth surface is the arena for most of the life and all human activities.
A large number of hypothesis were put forth by different philosophers and scientists regarding the Origin of the Earth. One of the earlier and popular arguments was by German philosopher Immanuel Kant. His argument is known as gaseous hypothesis. According to this hypothesis, a system of a few concentric rings separated from a hot and moving nebula, due to centrifugal force. By the process of condensation of these rings, the planets of the solar system including the Earth were formed.In 1796, a mathematician Laplace revised it. This revised hypothesis is known as nebular hypothesis. The Nebular Hypothesis considered that the planets were formed out of cloud of material associated with a youthful Sun, which was slowly rotating.
Later in 1900, Chamberlain and Moulton gave planetesimal hypothesis in under which they considered that a wandering star approached the Sun. As a result, cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar Surface continued to resolve around the Sun and it slowly condensed into planets. Sir James jeans and Harold Jeffrey put toward tidal hypothesis and supported this argument. At a later date, the arguments considered a companion star and the Sun to be coexisting. These arguments are called Binary Theories.
The Evolution of the earth
The earth has a layered structure. Initially Earth was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. This is far from the present day picture of the Earth. From the surface to deeper depths, the Earth’s interior has different zones and each of these contains materials with different characteristics.
The Earth was mostly in a volatile state during its primordial stage. Due to gradual increase in density, the temperature inside has increased. As a result, the material inside started getting separated depending on their densities. This allowed heavier materials to sink towards the center of the Earth and the lighter ones to move towards the surface. With the Passage of time, it cooled further and solidified and condensed into a smaller size, which later led to the development of the outer surface in the form of a crust.
It is through the process of differentiation and partial melting that the Earth forming material got separated into different layers. Starting from the surface to the center, we have layers like the crust, mantle and core. The last phase in the evolution of the earth relates to the origin and evolution of life. The evolution of life from unicellular bacteria to the modern man can be easily defined by the help of Geological Time Scale.
The Earth is the only planet where some special environment conditions are responsible for the existence and continuation of life. These include just the right distance from the sun, so it has the right temperature range, the presence of water, soil, minerals, suitable atmosphere and a blanked of ozone.
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