Water is very important for life .It continuously changes its form through water cycle. The process by which water continually changes its form and circulates between oceans, atmosphere and land is known as the water cycle. The major sources of fresh water are the rivers, ponds, springs and glaciers. The ocean bodies and the seas contain salty water. The water of the oceans is salty or saline as it contains large amount of dissolved salts. Most of the salt is sodium chloride or the common table salt that we eat. The earth is covered by three-fourth of water. 97.3% water is saline or salty in ocean and seas, 3% is fresh water that consisting ice-cape, ground water, fresh water lakes, inland and salt lakes.
The movements that occur in oceans can be broadly categorised as: waves, tides and currents which is discussed below:
• Waves: 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. A huge tidal wave called tsunami which 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. per hour. 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.
• Tides: 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. Spring tides occur during the full moon and new moon days when the sun, the moon and the earth are in the same line. Neap tides occur 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 earth resulting in low tides. High tides help in navigation. They raise the water level close to the shores which helps the ships to arrive at the harbour more easily.
Ocean currents are streams of water flowing constantly on the ocean surface in definite directions which may be warm or cold. The Warm Ocean currents originate near the equator and move towards the poles. The cold currents carry water from polar or higher latitudes to tropical or lower latitudes. The ocean current influence the temperature conditions of the area. Warm currents bring about warm temperature over land surface. The areas where the warm and cold currents meet provide the best fishing grounds of the world. Seas around Japan and the eastern coast of North America are such examples.