Editing Atoll (section)

== Formation<span class="anchor" id="Darwin Point"></span> ==
{{See also|Coral reef#Formation}}
[[File:Bora Bora (16542797633).jpg|thumb|Aerial view of [[Bora Bora]], French Polynesia]]
[[File:South Tarawa from the air.jpg|thumb|[[Tarawa Atoll]], Republic of Kiribati]]
[[File:Bikini Atoll.png|thumb|[[Bikini Atoll]], Marshall Islands]]

In 1842, Charles Darwin<ref name="darwincr1" /> 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,<ref name="Hoffmeister1930a">Hoffmeister, J.E., 1930. "Erosion of elevated fringing coral reefs". ''Geological Magazine'', 67(12), pp. 549–554.</ref> F. S. McNeil,<ref name="MacNeil1954a">MacNeil, F.S., 1954. [https://ajsonline.org/article/58503 "The Shape of Atolls; an Inheritance from Subaerial Erosion Forms"]. ''American Journal of Science'', 252(7), pp. 402–427. {{Doi|10.2475/ajs.252.7.402}}.</ref> E. G. Prudy,<ref name="Purdy1974a"/> 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<ref name="DroxlerOthers2021a"/> 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 [[seamount]]s 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 [[guyot]]s; 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.<ref>{{cite journal |last1=Bialik |first1=Or M. |last2=Samankassou |first2=Elias |last3=Meilijson |first3=Aaron |last4=Waldmann |first4=Nicolas D. |last5=Steinberg |first5=Josh |last6=Karcz |first6=Kul |last7=Makovsky |first7=Yizhaq |title=Short-lived early Cenomanian volcanic atolls of Mt. Carmel, northern Israel |journal=Sedimentary Geology |date=January 2021 |volume=411 |pages=105805 |doi=10.1016/j.sedgeo.2020.105805|bibcode=2021SedG..41105805B |s2cid=228873675 }}</ref><ref>{{cite journal |last1=El-Yamani |first1=Mahmoud S. |last2=John |first2=Cédric M. |last3=Bell |first3=Rebecca |title=Stratigraphic evolution and karstification of a Cretaceous Mid-Pacific atoll (Resolution Guyot) resolved from core-log-seismic integration and comparison with modern and ancient analogues |journal=Basin Research |date=16 May 2022 |volume=34 |issue=5 |pages=1536–1566 |doi=10.1111/bre.12670|bibcode=2022BasR...34.1536E |hdl=10044/1/98098 |s2cid=248223664 |hdl-access=free }}</ref>

<gallery widths="200" heights="160">
Atoll forming-volcano.png|Darwin's theory starts with a [[volcanic island]] which becomes extinct
Atoll forming-Fringing reef.png|As the island and ocean floor subside, coral growth builds a [[fringing reef]], often including a shallow lagoon between the land and the main reef
Atoll forming-Barrier reef.png|As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeper [[lagoon]] inside
Atoll forming-Atoll.png|Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon
</gallery>

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 [[Dolomite (rock)|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.<ref name="Budd1997a">Budd, D.A. (March 1997). [https://www.sciencedirect.com/science/article/abs/pii/S0012825296000517 "Cenozoic dolomites of carbonate islands: their attributes and origin"] {{subscription}}. ''Earth-Science Reviews'', 42(1–2), pp. 1–47. {{Doi|10.1016/S0012-8252(96)00051-7}}.</ref><ref name="WheelerOthers1999a">Wheeler, C.W., Aharon, P. and Ferrell, R.E. (1 January 1999). [https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/69/1/239/99004/Successions-of-late-Cenozoic-platform-dolomites "Successions of late Cenozoic platform dolomites distinguished by texture, geochemistry, and crystal chemistry; Niue, South Pacific"]. ''Journal of Sedimentary Research'', 69(1), pp. 239–255. {{Doi|10.2110/jsr.69.239}}.</ref><ref name="SuzkiOthers2006a">Suzuki, Y., Iryu, Y., Inagaki, S., Yamada, T., Aizawa, S. and Budd, D.A. (15 January 2006). [https://www.sciencedirect.com/science/article/abs/pii/S0037073805003167 "Origin of Atoll Dolomites Distinguished by Geochemistry and Crystal Chemistry: Kita-Daito-Jima, Northern Philippine Sea"]. ''Sedimentary Geology'', 183(3–4), pp. 181–202. {{Doi|10.1016/j.sedgeo.2005.09.016}}.</ref>