-=Welcome to the apwiki Earth page=- Earth is the third planet of the Solar system. Its diameter is 12,739.2 km (about 7,962 miles). It has its own planet which is called moon. Average distance of Earth to the Sun is 148.8 million kms (93 million miles). It orbits the sun in 365 days. It is the only planet in solar system to support life. Since the earth is spherical, it must have a centre that is called the ''Core'' in geology. Just above the core there is a layer called the ''Mantle'', and above this mantle there is another layer that is called the ''Crust''. Thus the Crust is the uppermost layer of the Earth. The thickness of this layer varies between 5 kilometres under the oceans to 31 kilometres under the land. The crust is made of two layers - the Sial and the Sima. Sial is the upper layer and thinner than the Sima, the thicker under layer. The temperature beneath the crust is, geologists believe, as high as 870 degree celcius. Beneath the crust, ie the Continental sub-layer and the Oceanic layer, lies the mantle. It's thickness is about 2,800 kms and the temperature increases towards the core up to over 2200 degree celcius. It is believed that the mantle forms about three quarters of the whold Earth. Let us keep in mind that we never reached this mantle through any means whatsoever. We have made boreholes in search of fossil oil or petroleum, but only upto 8 kms. This way, we know little even about the crust. Whatever knowledge we have is based on the volcanic erruptions or the earthquake waves. Then begins the core of the Earth towards its centre. It has been calculated that the distance from the surface of the Earth to the centre is about 6371 kms. Thus the thickness of the core of the Earth is more than 3400 kms. This is just like a great ball, and which is believed to have made of molten iron under a great pressure of around 20,000 tons per square inch. This core creates the magnetic field of the Earth. __History of the Earth__ It is estimated by geologists that the Earth is about 4,600 million years old. However, we cannot say it with certainty. It is only becasue, whatever we know, is through the study of the rocks. __Layers of the Earth__ The layers of the Earth are called ''strata''. The crust has been forming and reforming continuously. The strata, is thus made of various layers of rocks from different ages. The uppermost layers are the newest and the innermost layers of rocks are the oldest. It is the study of these rocks that gave clues of emergence of life on Earth. Scientists believe that 70 million years of the Carboniferous period that began about 300 million years ago, coal layers were formed. It is only the top soil that contains life in various forms right from insects to plant roots. However, ths sub-soil does not contain any sign of life. We came to this conclusion because we are yet to find traces of any organic matter whatsoever. Then there are layers of sandstone, which is made of compressed sand. These are also called sedimentary rocks. Millions of years ago the sands were deposited as sediments by either wind or water, it is believed. One of the layer is coal. It is nothing but plants turned to carbon after million of years under the soil. One of the layer is shale, which is made of fine mud particles. It is believed to have formed under millions of years of compression. These are deposits of ancient rivers emptying into some nearby sea. Then there is a layer called limestone. This layer is made of mainly calcium carbonate. These are perhaps naturally produced from dead sea-creatures. All these layers are found on Earth alternately - one layer after another. Its ((Geography))
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