The northernmost atoll of the world is Kure Atoll at 28☂4′ N, along with other atolls of the Northwestern Hawaiian Islands. Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are found only in the tropics and subtropics. The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean. Most of the world's atolls are in the Pacific Ocean (with concentrations in the Caroline Islands, the Coral Sea Islands, the Marshall Islands, the Tuamotu Islands, Kiribati, Tokelau, and Tuvalu) and the Indian Ocean (the Chagos Archipelago, Lakshadweep, the atolls of the Maldives, and the Outer Islands of Seychelles). There are approximately 440 atolls in the world. When relative sea level submerges the island again, the rim provides a rocky core on which coral grow again to form the islands of an atoll and the flooded bottom of the saucer forms the lagoon within them. As a result, a saucer shaped island with a raised rim forms. Because of hydrologic properties of this karst, the rate of dissolution of the exposed coral is lowest along its rim and the rate of dissolution increases inward to its maximum at the center of the island. Then, when relative sea level drops below the level of the flat surface of coral reef, it is exposed to the atmosphere as a flat topped island which is dissolved by rainfall to form limestone karst. In the antecedent karst model, the first step in the formation of an atoll is the development of a flat top, mound-like coral reef during the subsidence of an oceanic island of either volcanic or nonvolcanic origin below sea level. Prudy, and evaluated and modified using a compilation of seismic reflection and drillhole data from various atolls by A. Īn alternative model for the origin of atolls, called the antecedent karst model, was first proposed by J.
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For the atoll to persist, the coral reef must be maintained at the sea surface, with coral growth matching any relative change in sea level (subsidence of the island or rising oceans). The lagoon is not the former volcanic crater. Eventually, reef and the small coral islets on top of it are all that is left of the original island, and a lagoon has taken the place of the former volcano. As the volcanic island subsides, the coral fringing reef becomes a barrier reef that is detached from the island. Over geologic time, the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean.
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According to Charles Darwin's subsidence model, the formation of an atoll is explained by the subsidence of a volcanic island around which a coral fringing reef has formed. Two different, well-cited models, the subsidence and antecedent karst models, have been used to explain the development of atolls. Most of the approximately 440 atolls in the world are in the Pacific Ocean. : 60 Atolls are located in warm tropical or subtropical oceans and seas where corals can grow. There may be coral islands or cays on the rim. ɒ l, - ɔː l, - oʊ l, ə ˈ t ɒ l, - ˈ t ɔː l, - ˈ t oʊ l/) is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely.
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Satellite picture of the Atafu atoll in Tokelau in the Pacific OceanĪn atoll ( / ˈ æ t.