atoll

atoll, a word derived from the Maldivian word atollon used to describe a ring of coral islets and reefs enclosing a shallow lagoon. They occur exclusively in the tropical seas of the Pacific and Indian Oceans. Charles Darwin first postulated that atolls form initially on the summits of volcanoes as fringing coral reefs. The ocean crust then subsides under the weight of the new volcano and the waves wash away the ash of the central cone leaving the ring of coral. As the volcano continues to subside the reefs keep above sea level by continual growth. Islands form in the lee of the reefs through the accumulation of coralline sand voided by fish that graze on the coral. Deep boreholes drilled by the Americans into Eniwetak Atoll during preparations for nuclear weapon testing found hundreds of metres of coralline rocks sitting atop the remains of a volcano, thus demonstrating Darwin's theory was correct.

Most atolls are only a few metres above sea level, and are threatened by any rapid rise in sea level. Increases in the concentration of carbon dioxide in the atmosphere will inhibit the ability of the corals to lay down calcium carbonate, so the corals may no longer grow fast enough to keep pace with future rises in sea level. Climate change may also result in an increased frequency and ferocity of tropical storms. So, many atolls may be inundated within the next century.

Bibliography

Nunn, P. , Oceanic Islands (1994).

M. V. Angel