The surface of the earth, as we know now, is covered with three fourths of water. This comprises of the oceans, seas, rivers and lakes. We really do not know how these water bodies were formed. The scientists have propounded the theory that as the earth cooled it was enveloped by thick clouds. These clouds rained down water on the earth.
However the surface of the earth was still very hot and the water would evaporate and go back to the atmosphere. This again would come down as rain. This cycle continued for millions of years till finally the earths surface cooled. The heavy rains then filled the hollows and low-lying areas in the crust. As the continents drifted apart due to the movements of the mantle below, more depressions were caused. The rain gradually filled them to form the present day oceans and seas. It has been estimated that the oceans were formed about 500 to 1000 million years ago. This calculation has been based on the age of the minerals found in them.
The oceans are very interesting in that they reveal the past history of the earth to us. If we study the bottom of the oceans we can see a variety of land formations. Much of the landscape consists of flat plains. However in certain areas, the ocean floors have hills, mountains, valleys, valleys, volcanoes, trenches and ridges. At the edge of the continents there is a hollow shelf which slopes gradually and then dips steeply of the plates. These mountains ranges formed by the movement of the plates.
These mountains can rise thousands of meters above the rest of the sea bed. The Mid Atlantic ridge is the longest mountains range measuring about 15,000 km, even longer than the Andes. It runs along the center of the Atlantic Ocean form the arctic to the Antarctic. Iceland straddles this ridge and was formed by the volcanic outpourings. Ocean trenches are found all along the edges of the Pacific Ocean parts of the ocean floor. The deepest recorded trench is the Mariana Trench, which is 11,034 m deep.
The largest ocean is the Pacific Ocean, which has an area of about 166 million sq.km. This is followed by the Atlantic Ocean, which, measures about 82 million sq.km. The third largest is the Indian Ocean at about 74 million sq.km. The smallest is the Arctic Ocean which is about 12 million sq.km. The term, Antarctic Ocean, is generally used to explain the far reaches and Atlantic Ocean, which extend south of the Antarctic Circle.
Much of the ocean floor is covered with sand and mud brought by the rivers. In certain places there are hot springs which bring up sulphur and other minerals. The water is salty because of the minerals, the main mineral being salt. The mud of the ocean floors is soft and made up of the lime shells and skeletons of the tiny sea animals. With the pressure of the water and the passage of time, these sediments gradually form hard rocks.
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