Atoll

[2][3] Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop.

[4] According to Charles Darwin's subsidence model,[5] the formation of an atoll is explained by the sinking of a volcanic island around which a coral fringing reef has formed.

Over geologic time, the volcanic island becomes extinct and eroded as it subsides completely beneath the surface of the ocean.

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.

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.

He recognized the word's indigenous origin and defined it as a "circular group of coral islets", synonymously with "lagoon-island".

The geological formation known as a reef knoll refers to the elevated remains of an ancient atoll within a limestone region, appearing as a hill.

In 1842, Charles Darwin[5] explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard HMS Beagle from 1831 to 1836.

As formulated by J. E. Hoffmeister,[13] F. S. McNeil,[14] E. G. Prudy,[6] and others, the antecedent karst model argues that atolls are Pleistocene features that are the direct result of the interaction between subsidence and preferential karst dissolution that occurred in the interior of flat topped coral reefs during exposure during glacial lowstands of sea level.

The research of A. W. Droxler, Stéphan J Jorry and others[4] supports the antecedent karst model as they found that the morphology of modern atolls are independent of any influence of an underlying submerged and buried island and are not rooted to an initial fringing reef/barrier reef attached to a slowly subsiding volcanic edifice.

Volcanic islands located beyond the warm water temperature requirements of hermatypic (reef-building) organisms become seamounts as they subside, and are eroded away at the surface.

An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point.

[17][18][19] In 1896, 1897 and 1898, the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs.

They wanted to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls.