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An atoll is a ring-shaped island featuring a coral rim that surrounds a central lagoon, often dotted with coral islands or cays. Found mainly in warm tropical and subtropical ocean regions, especially the Pacific Ocean, atolls are formed through processes explained by two major models. Charles Darwin's subsidence model describes atolls as the remnants of sinking volcanic islands where coral reefs become barrier reefs enclosing lagoons. Alternatively, the antecedent karst model attributes atoll formation to dissolution of exposed coral reefs during sea level changes, creating a saucer-shaped island with a raised rim that later floods, forming the lagoon.

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Usage

The word atoll comes from the Dhivehi word atholhu (އަތޮޅު, pronounced [ˈat̪oɭu]). Dhivehi is an Indo-Aryan language spoken in the Maldives. The word's first recorded English use was in 1625 as atollon. 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".9: 2 

More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus"10 or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".11

Distribution and size

There are approximately 440 atolls in the world.12 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). In addition, Indonesia also has several atolls spread across the archipelago, such as in the Thousand Islands, Taka Bonerate Islands, and atolls in the Raja Ampat Islands. 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.

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 northernmost atoll in the world is Kure Atoll at 28°25′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls in the world are Elizabeth Reef at 29°57′ S, and nearby Middleton Reef at 29°27′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands Territory. The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°41′ S. The atoll closest to the Equator is Aranuka of Kiribati. Its southern tip is just 13 km (8 mi) north of the Equator.

Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°18′ N. At this latitude, coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-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 largest raised coral atoll of the world, followed by Rennell Island (660 km2, 250 sq mi).13 More sources, however, list Kiritimati 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 a reef 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 is Aldabra, 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.

List of atolls

Largest atolls by total area (lagoon plus reef and dry land)14
NamePositionLocationLand area (km2)Total area (km2)Notes
Great Chagos Bank6°10′S 72°00′E / 6.17°S 72.00°E / -6.17; 72.00Indian Ocean4.512,642
Reed Bank11°27′N 116°54′E / 11.45°N 116.90°E / 11.45; 116.90Spratly Islands8,866Submerged, at shallowest 9 m
Macclesfield Bank16°00′N 114°30′E / 16.00°N 114.50°E / 16.00; 114.50South China Sea6,448Submerged, at shallowest 9.2 m
North Bank9°04′S 60°12′E / 9.07°S 60.20°E / -9.07; 60.20North of Saya de Malha Bank5,800Submerged, at shallowest <10 m
Rosalind Bank16°26′N 80°31′W / 16.43°N 80.52°W / 16.43; -80.52Caribbean4,500Submerged, at shallowest 7.3 m
Thiladhunmathi6°44′N 73°02′E / 6.73°N 73.04°E / 6.73; 73.04Maldives513,850
Chesterfield Islands19°21′S 158°40′E / 19.35°S 158.66°E / -19.35; 158.66New Caledonia<103,500
Huvadhu Atoll0°30′N 73°18′E / 0.50°N 73.30°E / 0.50; 73.30Maldives38.53,152
Chuuk Lagoon7°25′N 151°47′E / 7.42°N 151.78°E / 7.42; 151.78Chuuk, Micronesia3,152
Sabalana Islands6°45′S 118°50′E / 6.75°S 118.83°E / -6.75; 118.83Indonesia2,694
Lihou Reef17°25′S 151°40′E / 17.42°S 151.67°E / -17.42; 151.67Coral Sea12,529
Bassas de Pedro13°05′N 72°25′E / 13.08°N 72.42°E / 13.08; 72.42Lakshadweep, India2,474Submerged, at shallowest 16.4 m
Ardasier Bank7°43′N 114°15′E / 7.71°N 114.25°E / 7.71; 114.25Spratly Islands2,347
Kwajalein Atoll9°11′N 167°28′E / 9.19°N 167.47°E / 9.19; 167.47Marshall Islands16.42,304
Diamond Islets Bank17°25′S 150°58′E / 17.42°S 150.96°E / -17.42; 150.96Coral Sea<12,282
Namonuito Atoll8°40′N 150°00′E / 8.67°N 150.00°E / 8.67; 150.00Chuuk, Micronesia4.42,267
Ari Atoll3°52′N 72°50′E / 3.86°N 72.83°E / 3.86; 72.83Maldives692,252
Maro Reef25°25′N 170°35′W / 25.42°N 170.59°W / 25.42; -170.59Northwestern Hawaiian Islands1,934
Rangiroa15°08′S 147°39′W / 15.13°S 147.65°W / -15.13; -147.65Tuamotus791,762
Kolhumadulu Atoll2°22′N 73°07′E / 2.37°N 73.12°E / 2.37; 73.12Maldives791,617
Kaafu Atoll4°25′N 73°30′E / 4.42°N 73.50°E / 4.42; 73.50Maldives691,565
Ontong Java Atoll5°16′S 159°21′E / 5.27°S 159.35°E / -5.27; 159.35Solomon Islands121500
Lifou20°58′S 167°14′E / 20.97°S 167.23°E / -20.97; 167.23New Caledonia1146Raised atoll with no lagoon
Rennell11°40′S 160°10′E / 11.67°S 160.17°E / -11.67; 160.17Solomon Islands660Raised atoll with no lagoon
Kiritimati1°51′N 157°24′W / 1.85°N 157.4°W / 1.85; -157.4Kiribati312640

Gallery

Formation

See also: Coral reef § Formation

In 1842, Charles Darwin15 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,16 F. S. McNeil,17 E. G. Prudy,18 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 others19 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.2021

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.222324

Investigation by the Royal Society of London

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 Professor William Johnson Sollas of the University of Oxford. Geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. Professor Edgeworth David led the expedition in 1897.25 The third expedition in 1898 was led by Alfred Edmund Finckh.262728

See also

  • Islands portal

Inline citations

Sources

Wikimedia Commons has media related to Atoll. Look up atoll in Wiktionary, the free dictionary.

References

  1. "atoll". Dictionary.com Unabridged (Online). n.d. https://www.dictionary.com/browse/atoll ↩

  2. Migoń, Piotr, ed. (2010). Geomorphological Landscapes of the World. Springer. p. 349. ISBN 978-90-481-3055-9. Retrieved 12 February 2013. 978-90-481-3055-9 ↩

  3. Blake, Gerald Henry, ed. (1994). World Boundary Series. Vol. 5 Maritime Boundaries. Routledge. ISBN 978-0-415-08835-0. Retrieved 12 February 2013. 978-0-415-08835-0 ↩

  4. Droxler, 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. ↩

  5. Darwin, Charles R (1842). The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. London: Smith Elder and Co – via Darwin Online. /wiki/Charles_Darwin ↩

  6. Droxler, 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. ↩

  7. Droxler, 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. ↩

  8. Purdy, 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. ↩

  9. Darwin, Charles R (1842). The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. London: Smith Elder and Co – via Darwin Online. /wiki/Charles_Darwin ↩

  10. McNeil (1954, p. 396). ↩

  11. Fairbridge (1950, p. 341). ↩

  12. Watts, T. (2019). "Science, Seamounts and Society". Geoscientist. August 2019: 10–16. ↩

  13. "Misinformation about Islands". worldislandinfo.com. http://www.worldislandinfo.com/MISINFORMATION.htm ↩

  14. "Atoll Area, Depth and Rainfall" (spreadsheet). The Geological Society of America. 2001. ftp://rock.geosociety.org/pub/reposit/2001/2001075.pdf ↩

  15. Darwin, Charles R (1842). The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. London: Smith Elder and Co – via Darwin Online. /wiki/Charles_Darwin ↩

  16. Hoffmeister, J.E., 1930. "Erosion of elevated fringing coral reefs". Geological Magazine, 67(12), pp. 549–554. ↩

  17. MacNeil, F.S., 1954. "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. https://ajsonline.org/article/58503 ↩

  18. Purdy, 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. ↩

  19. Droxler, 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. ↩

  20. 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. S2CID 228873675. /wiki/Bibcode_(identifier) ↩

  21. 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. S2CID 248223664. /wiki/Bibcode_(identifier) ↩

  22. Budd, D.A. (March 1997). "Cenozoic dolomites of carbonate islands: their attributes and origin" (subscription required). Earth-Science Reviews, 42(1–2), pp. 1–47. doi:10.1016/S0012-8252(96)00051-7. https://www.sciencedirect.com/science/article/abs/pii/S0012825296000517 ↩

  23. Wheeler, C.W., Aharon, P. and Ferrell, R.E. (1 January 1999). "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. https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/69/1/239/99004/Successions-of-late-Cenozoic-platform-dolomites ↩

  24. Suzuki, Y., Iryu, Y., Inagaki, S., Yamada, T., Aizawa, S. and Budd, D.A. (15 January 2006). "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. https://www.sciencedirect.com/science/article/abs/pii/S0037073805003167 ↩

  25. David, Cara (Caroline Martha) (1899). Funafuti or Three Months on a Coral Atoll: An Unscientific Account of a Scientific Expedition. London: John Murray. ISBN 978-1-151-25616-4. 978-1-151-25616-4 ↩

  26. Finckh, Dr. Alfred Edmund (11 September 1934). "To the Editor of the Herald". The Sydney Morning Herald. NSW: National Library of Australia. p. 6. Retrieved 20 June 2012. http://nla.gov.au/nla.news-article17092086 ↩

  27. Cantrell, Carol (1996). "Alfred Edmund Finckh (1866–1961)". Australian Dictionary of Biography. National Centre of Biography, Australian National University. Retrieved 23 December 2012. http://adb.anu.edu.au/biography/finckh-alfred-edmund-10180/text17987 ↩

  28. 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. PMID 11617111. /wiki/Doi_(identifier) ↩