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Coral. Coral is a kind of calcareous rock chiefly made of the skeletons of a tiny sea organism called polyp. These tiny organisms extract calcium salt from seawater to build hard skeletons to protect their soft bodies. The tube-like skeletons extend upward and outward as the old polyps die and new ones are born. There are about 2,500 species of coral polyps. They live mainly in colonies, which remain fastened to the rocky sea-floor. When one generation of polyps dies, fresh structures are built upon the old skeletons by a new generation. In due course of time, a great mass of coral reef is formed. The masses of cemented skeletons form hard calcareous rocks, which vary in colour and form, depending upon the species forming them. The progressive growth of corals from submarine topographic features appears in various forms in course of time above the surface of the sea. Corals are generally found in Tropical Seas between 30 degrees N and 30 degrees S. Coral polyp need for their growth a temperature of about 20 degrees C. They live at depths of almost 45 to 55 meters where sunlight is in abundance. Polyps thrive well in clear salt water. Both fresh water and highly saline water are harmful to the growth of the polyp. They avoid the delta regions. An adequate supply of oxygen and microscopic marine food known as plankton is essential for their existence and growth. As the food supply is more plentiful on the seaward side of a growing reef, coral grow on the seaward side more rapidly.

Three types of coral features have been recognized on the basis of their characteristics and mode of occurrence.

Fringing Reef. The fringing reef is a coral platform attached to the coast of a continent or an island. Sometimes, there is a lagoon or a shallow channel between the edge of the reef and the land. The fringing reef is a narrow belt and its width varies between 0.5 and 2.5 km. The fringing reef grows from the deep sea bottom. Its seaward side usually drops steeply into the sea. The surface of the reef is rough and is located above the level of low water. The waves deposit coral fragments and form a boulder zone called a reef flat. The coral polyp does not extend outward because of the sudden and large increase of depth. The fringing reefs occur in New Hebrides Society Islands and off the southern coast of Florida. It is also found in the Gulf of Mannar near Rameshwaram in South India.

Barrier Reef. The barrier reef is the largest of the three types. It may be several km wide and several hundred km long. The essential characteristic of this kind of reef is its distant location from the coast or the island. It is separated from the land by a comparatively broader and deeper lagoon. Lying almost parallel to the coast develops on a coastal platform. The barrier reef is generally very thick which extends below a depth of about 180 meters with very steep seaward slopes. Small channels usually cut across the barrier reefs connecting the lagoon with the open sea. The barrier reefs are formed by the accumulation of corals of various shapes, sizes through the ages. Their surface is covered with boulders, coral debris and sand. Generally, barrier reefs encircle islands in an irregular and broken ring.

The Great Barrier Reef off the north-east coast of Australia is the largest in the world. It is more than 1,900 km long and about 160 km wide. The Great Barrier is 16 km from the coast at its nearest point and 240 km at the farthest. A vast complex of hundreds of separated reefs and island, the Great Barrier Reef is considered as a marine paradise. It attracts thousands of tourists every year from all over the world.

Atoll. The atoll is a ring-like reef, which partly or sometimes completely encloses a shallow lagoon. A cross section of an atoll shows that the lagoon has a level floor but the outer edges of the atoll slope steeply. The lagoon has a depth of 80 to 150 meters having an island or a submerged plateau in it. Generally, a large number of channels cutting across the atoll reef join the lagoon with the open sea. Atolls are located at great distances from the deep sea platforms. Favorable conditions are created at such places by the presence of submarine features, the surface of which may rise to a level fit for coral growth. Such submarine features may include a submerged island, a volcanic cone, or a drowned island, owing to the positive movement of the sea level.

According to their nature, atolls can be divided into three types (i) the true atoll with a circular reef enclosing a shallow lagoon with no island in it; (ii) an atoll which surrounds a lagoon with an island in it; and (iii) a coral island or atoll island which is in fact an atoll reef, built by the process of erosion and deposition of waves with island crowns formed on them.

Atolls are far more common in the Pacific than any other ocean. The Fiji atoll and the Funafuti Atoll in the Ellice Island are well known examples of atolls. A large number of atolls also occur in the Lakshadweep Islands.

The main problem with coral coasts is to account for their origin; in particular, many coral reefs are built in water of depths well beyond the theoretical limit of growth. Two main theories have been put forward. The glacial control theory, suggested by Daly, states that coral shorelines originated during glacial low sea-levels, and have built up with the sea-level rise, in postglacial time. However, borings of many atolls have shown that coral structures exist to depths up to 760 m, well beyond the lowest glacial sea-levels. The alternative subsidence theory was first proposed by Darwin in 1842, and still receives a wide measure of support. He supposed that small islands have slowly subsided with the general downwarping of oceanic parts of the Earth's crust. Thus, what were originally fringing reefs became bar-tier reefs and eventually atolls as the land core submerged. With different rates of subsidence, we now see coral shorelines at all these stages of development. Recent geophysical evidence shows coral bases at a wide variety of depths, which suggests that the subsidence theory is the more feasible in explaining the deepest coral formations

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