There are certain special conditions which are necessary prerequisites for the growth and development of corals. That is why the reef building corals and their associates are not uniformly deposited throughout the tropical warm ocean waters.
For the growth of corals in such large numbers, as is necessary to give rise to coral deposits, there must be a reasonable balance of favourable conditions. The most important of these conditions are the following:
1. For the growth and development of corals the surface temperature of the ocean must be above 20°C. Corals cannot live if the temperature of the sea water falls below this.
2. The water must be shallow, with a depth not exceeding 25-30 fathoms. As we know, with increasing depth, the amount of calcium as well as the temperature of water goes on decreasing, so that the coral polyps and other sea creatures which live on calcium carbonate do not thrive. That is why the reef building corals live in shallow waters and on the surface of the seas.
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3. The water must be normally saline, and, therefore, along the ocean margins where the water becomes fresh by the inflow of rivers, reef building corals do not grow and develop.
4. Corals need clear water that is free from abundant sediments. That is why coral reefs cannot develop where rivers enter the sea or where wave-erosion causes muddy coastal water.
5. There must be adequate food supply to nourish the abundant life of the coral reef. The most favourable condition for this purpose is the presence of continuously flowing ocean currents which provide to the stationary reef building organisms the much needed food supply.
For example, extensive coral reefs are found on the east coasts of Australia, Central America, and Africa, which are washed by warm ocean currents flowing along them. On the contrary, corals are found only in scattered patches on the west coasts of these continents.
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6. If the salinity of the ocean water is very high, the lime content is bound to be low, which is not favourable for the growth of corals. Therefore the average salinity ranging from 27% to 40% is ideal for the proper growth of corals and other reef forming creatures.
7. The most essential prerequisite for the formation of coral reefs is the presence of submarine platforms which must lie near the sea shore, or should be attached to some islands. The depth of water on such platforms should not exceed 50 fathoms. Remember that the corals build their permanent colonies only on these platforms.
8. Another point to remember is that corals cannot live for long out of water, and are therefore, rarely found above the low-tide level. On the other hand, their growth is retarded at depths much exceeding 25 or 30 fathoms.
9. Corals need clear oxygenated water with sufficient supplies of microscopic life as food.
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10. Since food supplies are plentiful on the seaside of a growing reef, the corals tend to grow rather at a fast rate outwards.
As the reefs grow in size, waves wash much broken corals in the form of boulders and sand over the crest, thus building up a reef-flat, often with sand dunes, upon which vegetation may grow. Coconut palms add special beauty to these ‘Coral Islands’.
It should be borne in mind that “the reefs and atolls are easily ‘drowned’ if their upward growth cannot keep place with any submergence (eustatic or tectonic) that may be in progress.”
As regards the rate of growth of corals, it depends on the species, the range being 6 to 45 mm per year. However, according to Arthur Holmes, the average rate of reef growth is about 14 mm a year.