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THE ORIGIN AND EVOLUTION OF THE EARTH 15 (ii) In the next stage, the gas cloud starts iron) to sink towards the centre of the earth getting condensed and the matter and the lighter ones to move towards the around the core develops into small- surface. With passage of time it cooled further rounded objects. These small-rounded andsolidifiedandcondensedintoasmallersize. objectsbytheprocessofcohesiondevelop This later led to the development of the outer into what is called planetesimals. surface in the form of a crust. During the Larger bodies start forming by collision, formation of the moon, due to the giant impact, and gravitational attraction causes the the earth was further heated up. It is through materialtosticktogether.Planetesimals the process of differentiation that the earth are a large number of smaller bodies. forming material got separated into different (iii)In the final stage, these large number of layers. Starting from the surface to the central small planetesimals accrete to form a parts, we have layers like the crust, mantle, fewerlargebodiesintheformofplanets. outercoreandinnercore.Fromthecrusttothe core, the density of the material increases. We E VOLUTION OF THE EARTH shall discuss in detail the properties of each of Do you know that the planet earth initially was thislayerinthenextchapter. a barren, rocky and hot object with a thin Evolution of Atmosphere and Hydrosphere atmosphere of hydrogen and helium. This is far from the present day picture of the earth. The present composition of earth’s atmosphere Hence, there must have been some events– processes, which may have caused this change is chiefly contributed by nitrogen and oxygen. from rocky, barren and hot earth to a beautiful You will be dealing with the composition and structureoftheearth’satmosphereinChapter8. planet with ample amount of water and There are three stages in the evolution of conducive atmosphere favouring the existence of life. In the following section, you will find the present atmosphere. The first stage is out how the period, between the 4,600 million marked by the loss of primordial atmosphere. years and the present, led to the evolution of Inthesecondstage,thehotinterioroftheearth contributed to the evolution of the atmosphere. life on the surface of the planet. Finally, the composition of the atmosphere was The earth has a layered structure. From the outermost end of the atmosphere to the modified by the living world through the centre of the earth, the material that exists is process of photosynthesis. not uniform. The atmospheric matter has the The early atmosphere, with hydrogen and helium, is supposed to have been stripped off least density. From the surface to deeper depths, the earth’s interior has different zones as a result of the solar winds. This happened and each of these contains materials with not only in case of the earth, but also in all the different characteristics. terrestrial planets, which were supposed to have lost their primordial atmosphere through the impact of solar winds. How was the layered structure of the earth developed? During the cooling of the earth, gases and water vapour were released from the interior solid earth. This started the evolution of the Evolution of Lithosphere present atmosphere. The early atmosphere The earth was mostly in a volatile state during largely contained water vapour, nitrogen, its primordial stage. Due to gradual increase carbon dioxide, methane, ammonia and very in density the temperature inside has little of free oxygen. The process through which increased. As a result the material inside the gases were outpoured from the interior is started getting separated depending on their called degassing. Continuous volcanic densities. This allowed heavier materials (like eruptions contributed water vapour and gases Reprint 2026-27