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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Atoll | 2/2 | https://en.wikipedia.org/wiki/Atoll | reference | science, encyclopedia | 2026-05-05T07:34:19.794424+00:00 | kb-cron |
=== List of atolls ===
=== Gallery ===
== Formation ==
In 1842, Charles Darwin 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. Darwin's explanation suggests that several tropical island types: from high volcanic island, through barrier reef island, to atoll, represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upward as the island subsides (sinks), becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, and Bora Bora and others in the Society Islands. The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll. As formulated by J. E. Hoffmeister, F. S. McNeil, E. G. Prudy, 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 elevated rims along an island created by this preferential karst dissolution become the sites of coral growth and islands of atolls when flooded during interglacial highstands. The research of A. W. Droxler, Stéphan J Jorry and others 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. In fact, the Neogene reefs underlying the studied modern atolls overlie and completely bury the subsided island are all non-atoll, flat-topped reefs. In fact, they found that atolls did not form doing the subsidence of an island until MIS-11, Mid-Brunhes, long after the many the former islands had been completely submerged and buried by flat topped reefs during the Neogene. Atolls are the product of the growth of tropical marine organisms, and so these islands are found only in warm tropical waters. 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. Islands in colder, more polar regions evolve toward seamounts or guyots; warmer, more equatorial islands evolve toward atolls, for example Kure Atoll. However, ancient atolls during the Mesozoic appear to exhibit different growth and evolution patterns.
Coral atolls are important as sites where dolomitization of calcite occurs. Several models have been proposed for the dolomitization of calcite and aragonite within them. They are the evaporative, seepage-reflux, mixing-zone, burial, and seawater models. Although the origin of replacement dolomites remains problematic and controversial, it is generally accepted that seawater was the source of magnesium for dolomitization and the fluid in which calcite was dolomitized to form the dolomites found within atolls. Various processes have been invoked to drive large amounts of seawater through an atoll in order for dolomitization to occur.
== Royal Society expeditions 1896–98 ==
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. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific. The first expedition in 1896 was led by William Johnson Sollas of the University of Oxford. Geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. David led the expedition in 1897. The third expedition in 1898 was led by Alfred Edmund Finckh.
== See also ==
Baratal limestone, sometimes described as the oldest known atoll Coral island
== References ==
=== Inline citations ===
=== Sources === Dobbs, David (2005). Reef Madness: Charles Darwin, Alexander Agassiz, and the Meaning of Coral. Pantheon. ISBN 0-375-42161-0. Fairbridge, R. W. (July 1950). "Recent and Pleistocene Coral Reefs of Australia". J. Geol., 58(4: Reef Issue): 330–401. Bibcode:1950JG.....58..330F. doi:10.1086/625751. JSTOR 30070464. McNeil, F. S. (July 1954). "Organic Reefs and Banks and Associated Detrital Sediments". Amer. J. Sci., 252(7): 385–401. doi:10.2475/ajs.252.7.385.
== External links ==
Formation of Bermuda reefs Darwin's Volcano – A short video discussing Darwin and Agassiz' coral reef formation debate NOAA National Ocean Service Education – Coral Atoll Animation NOAA National Ocean Service – What are the three main types of coral reefs? Research Article: Predicting Coral Recruitment in Palau's Complex Reef Archipelago; Archived 2021-09-20 at the Wayback Machine World Atolls, Goldberg 2016: A global map containing all atolls