Some Physical Features of the Oceans:
The oceans are so vast that they stagger one's imagination. They cover more than 70 per cent of the earth's surface. The depth of oceans varies, from shallow near the coasts to deep in the middle. There are trenches, valleys, and what may becalled hills under theocean waters. Tlie deepest part happens to be in middle
Pacific Ocean and its depth is greater than the height of Mount Everest-the highest peak in the world. Ocean waters are never still. Wind exerts frictional force on the surface of water and generates waves that keep the surface water in motion. Tides are another type of movement of the oceanic water, noticeable in coastal regions. Water level of sea rises and falls twice a day. The gravitational pull of the sun and the moon, is the principal cause of the formation of tides.
Coastal waters rise to a high point called the high tide, when the sun and the moon are on the same side of the earth, and the tide is low when the sun and the moon happen to be on the opposite sides. Ocean currents are yet another form of water movement. Currents are generated in several ways : by changes in the density of sea water, due to temperature differences, by the effect of earth's rotation and also prevailing winds.
These currents transport sea water over long distances, more or less like rivers on the surface of the earth. Ocean water is salty. It has about 35 parts of salt (by weight) per 1000 parts of water. Ordinary salt or sodium chloride is the major salt component of the oceanic water.
Salts of magnesium, calcium and potassium are also present. This substantial amount of salt in sea water, is the result of accumulation of small bits of salt that are carried by rivers from the lands they drain. Oceanic waters have a minimum temperature well below zero Celsius, near the poles, and a maximum of about 28 Celsius in the tropics. Another aspect of marine environment is pressure. The atmosphere exerts a pressure of about 1 kilogram per square cm (= 1 Atmosphere Pressure) at the surface of the sea or land. This pressure increases due to the weight of water by 1 Atmosphere, for every 10 metres of the depth of water. Thus, if you are at a depth of 3000 metres in the ocean, the pressure there would be 300 times that at the earth's surface. It would be quite impossible for human beings to survive at this depth without very special equipment.