Stage I : In this stage planetesimals accretion happened and the earth originated. It was a barren, rocky and hot object-with a thin atmosphere of hydrogen and helium.
Stage II : Development of Lithosphere : The primordial earth was mostly in volatile state. Its density gradually increased, the temperature inside increased. As a result, the material inside started getting separated depending upon their densities. The heavier materials like nickel and iron sank towards the centre of the earth and the lighter ones moved towards the surface. Over a period of time it cooled and solidified and condensed into a smaller size which later led to the formation of crust. There was a giant-impact on the earth during the formation of the moon which heated it up further which lead to differentiated layered structure of the earth. Starting from the surface of the earth to the central part, distinct layers like crust, mantle, outer core and inner core were formed.
Stage III : Evolution of Atmosphere and Hydrosphere : There are three stages in the evolution of the present day atmosphere
(a) Loss of primordial atmosphere containing a thin layer of hydrogen and helium as a result of stripping off by solar winds.
(b) The hol interior of the earth : Contributing to the evolution of the earth. The gases were outpoured from the interior of the earth-during the cooling of the earth through the process called degassing. This started the evolution of the early atmosphere which largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen.
(c) Finally the composition of the atmosphere was modified by the living world through the process of photosynthesis.
Formation of Hydrosphere : The continuous volcanic eruptions contributed water vapour and gases to the atmosphere. As the earth cooled, the water vapour released started getting condensed. The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. The rainwater falling onto the surface got collected in the depressions to give rise to oceans. Thus earth's oceans (hydrosphere) were formed within 500 million years from the formation of the earth. The process of photosynthesis was initially confined to oceans only and thus oceans started filling up with the oxygen. Eventually oceans got saturated with oxygen and the oxygen started to flood the atmosphere.
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