There are many islands, big or small, in the ocean and according to the origin, they can be grouped into four classes:
1. Volcanic island
2. Continental island
3. Oceanic island or island formed due to orogenic movement and
4. Coral island.
1. It has been proved that there are many local volcanoes at the ocean floor. At the time of eruption huge quantities of lava come out of the volcanoes and start accumulating. In course of time, the upper portion of accumulated lava comes out of water surface and forms island. The Hawaian Island of the Pacific Ocean is one of the classic examples. This type of island is called Volcanic Island.
2. Due to the action of certain natural forces some parts in the coastal region of the continents may assume positive or negative movements. As a result, in course of time, some parts of the continents may detach from the mainland and the detached land portions are called Continental Island. Malagassi and Newfoundland are examples of Continental Islands.
3. Some folded mountains are found to be originated in the ocean floor due to the tectonic movement at the interior of the earth. When the peak of the folded mountain comes out of the water, it is called oceanic island. There are many such oceanic islands on the north eastern side of Asia in the Pacific Ocean. Commender Island and Aleutian Island are of this type.
4. Coral islands are formed by the dead skeletons of Protoplasm called coral. Corals are born in warm water areas of ocean. They are not found beyond 50 metres of deep water and they need clear water. That is why, corals are found in the tropical regions between tropic of Cancer and tropic of Capricorn. They are generally found on the eastern coastal area but they are not seen at all in the Atlantic ocean.
Corals live in groups and after death their skeletons get jointed by the abundant lime available in their bodies. In course of time, the jointed skeletons transform into rocks and gradually their heights increase up to the water surface. This type of coral rock is generally known as reef. In course of time the reefs are filled with sand, mud and other oceanic sediments and transform into island.
Coral Islands are grouped into three classes on the basis of location and shape:
1. Fringing reef
2. Barrier reef
1. Fringing Reefs:
A coral mass built out from the shore may appear as a platform which at low tide is seen to be in continuity with the mainland or nearly so. The width may be 900 metre or more. In outer edge, it falls steeply to deep water.
2. Barrier Reefs:
Barrier reefs are found within 300 km offshore. They develop on lower coral platforms on submerged ridges and are separated from the shore by a deep lagoon. In between the fringing reef and the outer barrier reef there may lay inner barrier reef as found in the north-east coast of Australia. The 200 km complex of reefs known as the Great Barrier Reef is said to be the largest coral structure in the world.
Atolls are low-lying ring-shaped islands, generally divided into separate islands enclosing a deep and level floored lagoon. They resemble barrier reefs but the central island is absent. There are numerous atolls with lagoons in the Indian and Pacific oceans.