what is coral island
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Coral island, tropical island built of organic material derived from skeletons of corals and numerous other animals and plants associated with corals. Coral islands consist of low land perhaps only a few metres above sea level, generally with coconut palms and surrounded by white coral sand beaches. They may extend dozens of kilometres and include almost any tropical limestone island whose structure is integrally part of a living or relatively recent coral reef. Reef building takes place mostly below high-tide level, and a typical coral island or cay usually surmounts the relatively flat top of the whole reef system. Geologically, the island is just one small part of the whole coral reef.Coral reef islands occur in association with all categories of reef but especially on reefs whose flat tops are well developed, perhaps a kilometre or more in width. Reef islands may occur in isolation or in a chain along the length of the reef. Sometimes they take the form of long strips of land occupying most of the length of the central area of a reef top.There are two completely different kinds of origin of reef islands: uplift and accretion. In the first, part or all of a reef system may become land as a result of crustal movements raising it above sea level (e.g., the Aldabra Islands in the western Indian Ocean). The previously submarine reef top becomes a low plateaulike feature, and such islands are typically rocky, with cliffs, and with land surfaces pitted and sculptured by solution weathering (karst). They are often still recognizable as atolls with a lagoon, now much shallower or even completely dry, as an interior basin. If present sea levels were to fall again, as they have done in the recent geological past due to an increase in polar ice, most of the world’s coral reefs would, in effect, become raised features. It is only because present sea levels are the highest for many thousands of years that there are no raised reef islands of this kind now.
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