These are the best examples of rock formation by the direct action of living organisms. These are masses sf limestone and dolomite, which are essentially the remains of calcareous organisms and algae. In their form, the corals resemble branching trees and shurbs.
The coral reefs are accumulated by coral polyps which live in colonies. Apart from the polyps a large number of organisms take part in building up the coral reef. Calcareous algae are as important as the corals themselves and much of the deposit consists of the shells of Foraminifera, Molluscs, Echinoderms and other creatures.
Corals may not even make up the largest part of the reef. More often the term ‘bioherm’ is used for such structures.
Each coral polyp has a calcareous skeleton, in the form of a small tubular chamber, secreted from the animal’s body. Living in colonies of millions of individuals, they combine to build a structure of great size.
In reef-building corals, the successive generation of polyps settles in and fastens itself to the dead ones of the preceding generation and the reef gradually grows upward and sidewise. Thus, in a reef while the upper part is inhabited by the living polyps, the lower part is hard calcareous skeleton.
Coral polyps need, for their growth, a temperature of about 20°C. They live within depths of about 80 metres where sunlight is abundant. Fresh, muddy and highly saline waters are unfavourable to their growth.
Because of temperature, light and oxygen requirements of the organisms that inhabit them, reefs are built only at or close to sea level. Coral reefs occur in warm, tropical and equatorial waters between the latitudes 30°N and 25°S.
As corals cannot survive long exposure to the air, living reefs cannot grow much above low-tide level and are remarkably flat on top. In recent years a few coral reefs have been found in polar seas also. Coral reefs are particularly numerous in the Pacific and Indian oceans.
On the basis of their forms and relation to the land three main types of reefs are recognized: (i) fringing reefs, (ii) barrier reefs, (iii) atolls.
(i) Fringing Reefs
These are narrow belts of reef which grow around island or along the coast. The reef is closely attached to the coast and the island. At low tide they are seen to be in continuity with the shore. Its seaward edge is higher than the landward portions. It is separated from the coast by a narrow, shallow strip of water.
(ii) Barrier Reefs
These are located at a considerable distance from the coast or the island and are separated from the mainland by a quite broad and relatively deep stretch of sea. This lagoon may be tens of kilometres wide.
Small channels cut across the barrier connect the lagoon with the open sea. The great Barrier Reef of Australia is the best example of this type. It extends for more than 2000 kms with an average width of 150 kms.
These are circular coral Reefs with a shallow central lagoon but no central island. A large number of channels cutting across the atoll reef connect the lagoon with the open sea. Atolls are more common in the Pacific than in any other ocean.
Origin of Coral-Reefs
The standstill theory, the glacial control theory and the subsidence theory-have been propounded to explain the origin of coral reefs. Of these three theories, the subsidence theory by Charles Darwin is still considered valid by most geologists.
According to this theory, the island around which the fringing reef develops subsides slowly, while the corals continue to build upward and outward.
Thus over a period of time the original fringing reef becomes barrier reef and the island is separated from it by a lagoon. With further subsidence the island disappears below the water level and the surrounding reef is termed as atoll.