“A coral reef is a ridge or mound of lime stone the upper surface of which is near the surface of the sea and which is formed of calcium carbonate by the action of organisms chiefly corals” (Vaughan. 1917). Though the reefs are built by stony corals but other organisms such as Foraminifera, Millipora, Tubipores, Heliopores, the Molluscs, Echinoderms, Algae and Sponges take part together in the formation of compact structure, the coral reefs. The coral reefs are formed by in-crushing their skeletal parts on the deposited lime. Coral reefs composed of multiple of organisms vary in shape and colour. The zooxanthellae is responsible for the rich colouration of corals, which may be brown, yellow, or green. The reef building corals require warm, shallow waters, and consequently are limited to continental island shores in tropical and sub-tropical zones. They cannot tolerate temperatures below 18°C and they flourish nicely only above 22°C. Consequently their distribution is limited to the zone existing between about 28°C on either side of the equator. They rarely remain alive at a depth greater than 90 meters. Therefore reef-building corals are confined to certain geographical areas of Atlantic, Indian and Pacific Oceans extending about 30° on each side of the equator.
They inhabit two general regions (i) The Caribbean and related Western Atlantic waters
such as Florida, Bermuda, the Bahamas and the West Indies and (ii) The Indo-Pacific region from the east coast of Africa.
Kinds of coral reefs.
Coral reefs are usually of three kinds (i) Fringing reefs (ii) Barrier reefs and (iii) Atolls
(i) Fringing reef:
The fringing reefs are sea-level flats starting from the sea-shore and extending for a short distance. They are 1/4 or 1/2 mile in width, built upon the salient parts of continental or insular shores. The fronts of these reefs fall off seaward to moderate depths. They are composed largely of dead reef rock, and are occupied by living reef builders chiefly on their outer edge and slope. Sand and other debris are also found on reefs. Reefs of this kind are quite common in East Indies.
(ii) Barrier reef:
Barrier reef is like the fringing reef with a reef front and a flat 20 to 1000 feet wide, but it is separated from the shore by a Lagoon 60 to 300 feet deep and 1/2 to 10 miles or more in width. Lagoon is suitable for navigation of the largest ships. The Great Barrier Reef of northeast coast of Australia which is over 1200 miles long and in some places 90 miles from the shore, presents a most notable example of the barrier reef. Tahiti island is entirely encircled by the barrier reef.
An atoll is a more or less circular or horse shoe-shaped reef partly or wholly
encircling a lagoon, which varies from less than a mile to 40 or 50 miles across. Atoll does not enclose any island. Thus reef may be complete or broken, by a number of channels of which only a few are navigable. The outer side of the atoll reef slopes off steeply into the depth of ocean. The atolls are common in Indo-Pacific regions. They are hundreds of miles away from the nearest land. The largest atoll of the world is Suvadiva of the Maldives.
Formation of Coral Reefs:
The coral reefs grow very slowly. Most of the reefs expand at the rate of 10 to 200mm per year. These reefs are generally built by the tiny polyps of the stony corals (Madreporaria) which secrete around them limestone cups that coalesce to form large masses. Later they take up the form of huge rocks.
Many theories have been put forth to explain the great vertical thickness attained by
the coral reefs. The most favoured theories are:
1. Darwin‘s Subsidence Theory
Charles Darwin’s theory explains the formation of reef. According to him the reefs begin as fringe around slowly sinking shores, which continue to grow upward and outward as the land sinks. Thus the fringing reef turns into a barrier reef when coast subsides. The lagoon separating it from the reef becomes wider and wider. Islands surrounded by barrier reefs finally sink beneath the lagoon. Atolls are formed as accumulation of loose fragments of rocks.
2. Submerged Bank Theory
This theory has been supported by many recent studies. According to this theory the coral formation grow on flat, preexisting surface during or after the submergence of surfaces.
3. Daly Glacial-Control Theory
The main points of this theory are as follows: (i) During the last glacial period much water of the ocean turned into ice forming glaciers due to a very low temperature and thus the level of the ocean was lowered by 60 to 70 metres below the present level, (ii) Various ten-aces were then cut or islands levelled by wave action, (iii) Later, with the rising temperatures, corals began to grow upon these platforms and kept pace with the rising sea-level as the ice melted.
This theory explains nicely about the uniform depth of coral lagoons, whose bottoms, below the debris since deposited, would consist of the platforms cut when the ocean was at its low level.
Theories 3 and 4 supplement each other and at present are most favoured by the students of the problems, although Darwin’s idea (Darwin-Dana subsidence theory) continues to find much support. The submergence theory agrees with Darwin’s subsidence theory in that both consider the reef foundations to be now at great depths than they were when the coral growth started. But the submergence theory does not admit any relationship between that various kinds of reefs and postulates that the barrier reef and atoll have grown upon pre-existing flat platforms. The atolls are considered to have been shaped by winds, waves and currents.
Boring have been made to find out the truth of these theories.
(i) Boring of Funafuti atoll in the South Pacific north of Fiji was made in 1904 by an expedition of the Royal Society of London. The boring was 3 inches to 5 inches in diameter and went up to 1114 feet without reaching the reef base. Twenty eight genera of reef building corals were discovered, 2 of which are now living on the reef in that locality above 60 metres. The material obtained from the boring did not contain any of the deep water corals lives in that locality at the depth to which the boring went. This finding supports subsistence theory.
(ii) Cory (1931) made 3 boring at different distances from the shore into a reef in Samoa (Pacific island) and concluded that the reef rested on a level platform cut by the action of waves. This supports the glacial control theory.
(iii) The Great Barrier Reef Committee made two boring, one in 1928 and the other in 1938 on the Great Barrier Reef. Both boring gave the same result that the coral material extended out only to 400 to 450 feet and below this there was nothing but sand containing shells of various animals. There was no evidence of any underlying platform. Therefore this finding also supports Darwin’s subsidence theory. Thus Darwin’s subsidence theory applies of many reefs but some reefs may have been laid down on pre-existing platforms.
SOME IMPORTANT FACTS ABOUT REEFS
Tropical storms can modify the reefs very much. Quarrying can also damage reefs as in India, where in 1971 Pillai has estimated that nearly 250 cubic meters of reefs material are removed per day for use in the production of cement, calcium carbide, calcium carbonate etc. Recently much concern has been expressed for the safety of some oceanic islands and their reef food chains because some reefs are being destroyed by ever increasing population of the Crown-of-thorns starfish, Acanthaster planei. These starfishes feed upon the living coral and have caused much reef destruction in some parts.
ECONOMIC IMPORTANCE OF CORAL REEFS
The coral reefs are of much importance to oil industry. They form highly favourable sites for the accumulation of petroleum deposits. The coral reefs are of importance for curio trade. Many plant and animals like sponges, molluscs, fishes, echinoderms etc. grow on these reefs. Even some humans inhabit them.
Some corals form highly priced decorative pieces. Corellium rubrem is considered a precious stone. The red coral and organ pipe coral are used in medicine in South India. Skeleton of a few corals are used as building material.
Coral skeletons are also used in the formation of lime, mortar and cement. The skeletons are also used in making ridges which act as natural barriers against sea erosion and cyclonic storms.
The coral reefs serve as good nursery grounds for commercially important fishes. They form more colourful and beautiful fishes.