A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Most reefs grow best in warm, shallow, clear, sunny and agitated water.

They occupy less than 0.1% of the world’s ocean area, yet they provide a home for at least 25% of all marine species. They are also known as “rainforests of the ocean”.

The subject of origin of coral reefs has been studied and debated for over one and half centuries.

Darwin’s theory of subsidence:

Figure 1: Transition from fringing to Barrier Reef

Figure 2: Transition of an island from barrier reef to atoll.

According to Darwin, coral would grow along the edges of a newly formed volcanic island, thereby forming a fringing reef (Figure 1).  Due to a general down warping of earth’s crust, the islands gradually subsided. The corals continued to grow upwards to keep pace with the subsidence. The growth was more vigorous at the outward edge than the landward edge because of the more favourable living conditions for the corals, so the encircling reef widened. It then formed a barrier reef, with a lagoon between the island and the reef. Eventually, when the land completely submerged, only the outer rims of the reef were seen, forming an atoll (Figure 2). The submerged island was covered by a layer of sediment so that the characteristic circular lagoon is generally shallow. Thus atolls mark the position of the former islands.

R.A Daly’s glacial control theory:

Daly noticed a close relationship between glaciations and the development of coral reefs. He believed that during the heights of the Ice Ages, the water was too cold for any coral growth to take place. With the absence of a coral barrier, marine erosion was able to attack and lower the islands. With the return of the warmer climate, the water that was locked up in the ice sheets melted. Consequently, there was a rise in the sea level which in some cases submerged these lower islands. On these wave-planed platforms, corals began to grow upwards at the rate of a foot in a decade to keep pace with the rising water level. Coral Reefs, where islands still project above sea level, and atolls were thus formed.

Conclusion:

Recent evidences of boring through Coral formations seem to favour Daly’s explanation of a change in sea level and consequent erosion of the Islands. However the deepest borings reveal basaltic rocks this corresponds to the subsided islands envisage by Darwin. Thus, a combination of the two theories accounts for all the important features of coral reefs and atolls.