Reference work entry
Synonym: Ring reef landscape
This is a type of coral reef landscape consisting of circular coral reefs in the ocean. An atoll contains an enclosed or semi-enclosed lagoon in the middle but no central islands; this is the main difference between an atoll and a barrier reef. An atoll consists of a coral reef several metres above sea level, and it is circular, elliptical or horseshoe-shaped. It has a diameter of 1–130 km and a depth of several metres to hundreds of metres. The seaward side is steeper, with a gradient up to 45°, and the upper part can reach 90°. In general, an atoll consists of a layer of limestone approximately 1,000 m thick that sits on an underwater basalt volcano. Atolls are mainly located in the tropical and subtropical seas of the Pacific and Indian Oceans, where corals grow easily. Typical examples are the atolls of the Marshall Islands and the Maldives Islands (Fig. 25).
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