Three-fourth of the earth surface is covered by water.
The distribution of water in percentage –
Oceans – 97.3%
Ice-caps – 02.0%
Ground water – 0.68%
Fresh water lakes – 0.009%
Inland seas and salt lakes – 0.009%
Atmosphere – 0.0019%
Rivers – 0.0001%
The movements that occur in oceans can be broadly categorized as: waves, tides and currents.
When the water on the surface of the ocean rises and falls alternately, they are called waves. During a storm, the winds blowing at very high speed form huge waves. These may cause tremendous destruction. An earthquake, a volcanic eruption or underwater landslides can shift large amounts of ocean water. As a result a huge tidal wave called tsunami, that may be as high as 15m, is formed. The largest tsunami ever measured was 150m high. These waves travel at a speed of more than 700 km/h. The tsunami of 2004 caused wide spread damage in the coastal areas of India. The Indira point in the Andaman and Nicobar islands got submerged after the tsunami.
The rhythmic rise and fall of ocean water twice in a day is called a tide. It is high tide when water covers much of the shore by rising to its highest level. It is low tide when water falls to its lowest level and recedes from the shore.
The strong gravitational pull exerted by the sun and the moon on the earth’s surface causes the tides. During the full moon and new mood days, the sun, the moon and the earth are in the same line and the tides are highest. These tides are called spring tides. And when the moon is in its first and last quarter, the ocean waters get drawn in diagonally opposite directions by the gravitational pull of sun and moon resulting in low tides. These tides are called neap tides.
High tides helps in navigation. They raise the water level close to the shores which helps the ships to arrive at the harbor more easily. The rise and fall of water due to tides is being used to generate electricity in some places.
Ocean currents are streams of water flowing constantly on the ocean surface in definite directions. The ocean currents may be warm or cold. Generally, the warm ocean currents originate near the equator and move towards the poles while the cold currents carry water from polar or higher latitudes to tropical or lower latitudes. The Labrador Ocean current is cold current while the Gulf Stream is a warm current. Seas around Japan and the eastern coast of North America are such examples. The areas where a warm and cold current meet also experience foggy weather.