All living organisms on the earth depend on water. About 71% of the earth's surface is covered by water. The earth is the only known planet in the solar system with abundant water.
The oceans are the single largest continuous body of water encircling land. The oceans contain 97.2% of the world's water. There are four oceans - the Pacific oceans, the Atlantic oceans, the Indian ocean and the Arctic ocean.
The ocean floor which once was considered to be flat has variety of features such as continental shelf, continental slope, abyssal plains and the deeps.
Pacific ocean is the largest ocean. It comprises of thousands of islands. The greater part of the ocean comprises of the deep seas. Mariana trench in the Pacific Ocean is the deepest known part of the ocean with a depth of 11022 metres.
The Atlantic Ocean is almost half the size of the Pacific Ocean. The world's widest shelves like Dogger Bank and Grand Bank are found here. The largest continuous Mid Atlantic Ridge is the important feature of the Atlantic ocean.
Indian ocean is smaller than the other two ocean.
The temperature of the surface water of the oceans varies from one part of the ocean to the other. It is generally high near the equator and low near the poles. There is variation in the vertical distribution of temperature too. Temperature decreases with the increase in depth.
Salinity is defined as the weight in grammes of solid material left after the evaporation of 1000 grammes of sea water. The salinity of the ocean water is caused by a large number of dissolved chemical compounds. The salinity is not uniform everywhere in the ocean. Equatorial and Polar water are less saline than the tropical seas.
Ocean waters are in constant motion. There are three type of movements in the oceans waters - waves, tides and currents. Waves are caused by winds. They move roughly in circular path. Tides are the periodic change in the elevation of the oceans surface at a particular place. They are produced as a result of gravitational attraction of the moon and the centrifugal force produced due to rotation of earth.
Currents are distinct and generally horizontal flow of a mass of water in a fairly defined direction. Currents are formed due to the density of water rotation of the earth and planetary winds. These currents form a clockwise pattern in the northern hemisphere and move in anti clockwise pattern in the southern hemisphere. The currents of the Indian ocean are influenced by Monsoon winds.