Two different, well-cited models, the subsidence model and the antecedentkarst model, have been used to explain the development of atolls.[4] According toCharles Darwin's subsidence model,[5] the formation of an atoll is explained by the sinking of avolcanic 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. As the volcanic island subsides, the coral fringing reef becomes abarrier reef that is detached from the island. Eventually, the 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. The lagoon is not the former volcanic crater. 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 (sinking of the island or rising oceans).[4]
An alternative model for the origin of atolls is called the antecedent karst model. 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. 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 limestonekarst. 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. As a result, a saucer shaped island with a raised rim forms. 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.[4][6]
The wordatoll comes from theDhivehi wordatholhu (އަތޮޅު,pronounced[ˈat̪oɭu]). Dhivehi is anIndo-Aryan language spoken in theMaldives. The word's first recorded English use was in 1625 asatollon.Charles Darwin coined the term in his monograph,The Structure and Distribution of Coral Reefs. He recognized the word's indigenous origin and defined it as a "circular group of coral islets", synonymously with "lagoon-island".[5]: 2
More modern definitions ofatoll describe them as "annular reefs enclosing a lagoon in which there are nopromontories other than reefs andislets composed of reefdetritus"[7] or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".[8]
Reef-building corals will thrive only in warmtropical andsubtropical waters of oceans and seas, and therefore atolls are found only in the tropics and subtropics. The northernmost atoll in the world isKure Atoll at 28°25′ N, along with other atolls of theNorthwestern Hawaiian Islands. The southernmost atolls in the world areElizabeth Reef at 29°57′ S, and nearbyMiddleton Reef at 29°27′ S, in theTasman Sea, both of which are part of theCoral Sea Islands Territory. The next southerly atoll isDucie Island in thePitcairn Islands Group, at 24°41′ S. The atoll closest to the Equator isAranuka of Kiribati. Its southern tip is just 13 km (8 mi) north of the Equator.
Bermuda is sometimes claimed as the "northernmost atoll" at alatitude of 32°18′ N. At this latitude, coral reefs would not develop without the warming waters of theGulf Stream. However, Bermuda is termed apseudo-atoll because its general form, while resembling that of an atoll, has a very different origin of formation.
In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 metres (16 ft). Measured by total area,Lifou (1,146 km2, 442 sq mi) is the largestraised coral atoll of the world, followed byRennell Island (660 km2, 250 sq mi).[10] More sources, however, listKiritimati as the largest atoll in the world in terms of land area. It is also a raised coral atoll (321 km2, 124 sq mi land area; according to other sources even 575 km2, 222 sq mi), 160 km2 (62 sq mi) main lagoon, 168 km2 (65 sq mi) other lagoons (according to other sources 319 km2, 123 sq mi total lagoon size).
The geological formation known as areef knoll refers to the elevated remains of an ancient atoll within a limestone region, appearing as a hill. The second largest atoll by dry land area isAldabra, with 155 km2 (60 sq mi).Huvadhu Atoll, situated in the southern region of the Maldives, holds the distinction of being the largest atoll based on the sheer number of islands it comprises, with a total of 255 individual islands.
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. 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,[13] F. S. McNeil,[14] E. G. Prudy,[6] and others, the antecedent karst model argues that atolls arePleistocene 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[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. 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 ofhermatypic (reef-building) organisms becomeseamounts 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 theDarwin Point. Islands in colder, more polar regions evolve toward seamounts orguyots; warmer, more equatorial islands evolve toward atolls, for exampleKure Atoll. However, ancient atolls during theMesozoic appear to exhibit different growth and evolution patterns.[15][16]
Darwin's theory starts with avolcanic island which becomes extinct
As the island and ocean floor subside, coral growth builds afringing reef, often including a shallow lagoon between the land and the main reef
As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeperlagoon inside
Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon
Coral atolls are important as sites wheredolomitization of calcite occurs. Several models have been proposed for the dolomitization ofcalcite andaragonite within them. They are the evaporative, seepage-reflux, mixing-zone, burial, and seawater models. Although the origin of replacementdolomites 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.[17][18][19]
^abcdDroxler, A.W. and Jorry, S.J., 2021. "The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory".Annual Review of Marine Science, 13, pp. 537–573.
^abPurdy, E. G., 1974. "Reef configurations, cause and effect". In Laporte, L. F. (ed.),Reefs in Time and Space.Society of Economic Palaeontologists and Mineralogists Special Publication 18, pp. 9–76.
^"Archipiélago de Los Roques" (in Spanish). Caracas, Venezuela: Instituto Nacional de Parques (INPARQUES). 2007. Archived fromthe original on 2008-04-24. Retrieved27 February 2013.
^Hoffmeister, J.E., 1930. "Erosion of elevated fringing coral reefs".Geological Magazine, 67(12), pp. 549–554.
^Bialik, Or M.; Samankassou, Elias; Meilijson, Aaron; Waldmann, Nicolas D.; Steinberg, Josh; Karcz, Kul; Makovsky, Yizhaq (January 2021). "Short-lived early Cenomanian volcanic atolls of Mt. Carmel, northern Israel".Sedimentary Geology.411 105805.Bibcode:2021SedG..41105805B.doi:10.1016/j.sedgeo.2020.105805.S2CID228873675.
^El-Yamani, Mahmoud S.; John, Cédric M.; Bell, Rebecca (16 May 2022). "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".Basin Research.34 (5):1536–1566.Bibcode:2022BasR...34.1536E.doi:10.1111/bre.12670.hdl:10044/1/98098.S2CID248223664.
^David, Cara (Caroline Martha) (1899).Funafuti or Three Months on a Coral Atoll: An Unscientific Account of a Scientific Expedition. London:John Murray.ISBN978-1-151-25616-4.{{cite book}}:ISBN / Date incompatibility (help)
^Rodgers, K A; Cantrell, Carol (1987). "Alfred Edmund Finckh 1866–1961: Leader of the 1898 Coral Reef Boring Expedition to Funafuti".Historical Records of Australian Science.7 (4):393–403.doi:10.1071/HR9890740393.PMID11617111.
![Los Roques Archipelago in Venezuela, the largest marine national park in Latin America,[12] from space. Courtesy NASA](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aLos_Roques.png)
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