An atoll (/ˈæt.ɒl,-ɔːl,-oʊl,əˈtɒl,-ˈtɔːl,-ˈtoʊl/)[1] is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim.[2][3]: 60 Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most of the approximately 440 atolls in the world are in the Pacific Ocean.
Two different, well-cited models, the subsidence model and the antecedent karst model, have been used to explain the development of atolls.[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. As the volcanic island subsides, the coral fringing reef becomes a barrier reef that is detached from the island. 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. 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, called the antecedent karst model, was first proposed by J. E. Hoffmeister,[6][7] later revised by E. G. Prudy,[8] and evaluated and modified using a compilation of seismic reflection and drillhole data from various atolls by A. W. Droxler and others.[4] 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 limestone karst. 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][8]
^Migoń, Piotr, ed. (2010). Geomorphological Landscapes of the World. Springer. p. 349. ISBN 978-90-481-3055-9. Retrieved 12 February 2013.
^Cite error: The named reference blakegh1 was invoked but never defined (see the help page).
^ 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.
^Cite error: The named reference darwincr1 was invoked but never defined (see the help page).
^Hoffmeister, J.E., and Ladd, H.S., 1935. "The foundations of atolls: a discussion." The Journal of Geology, 43, no. 6, 653–665.
^ 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.
Look up atoll in Wiktionary, the free dictionary. An atoll ( /ˈæt.ɒl, -ɔːl, -oʊl, əˈtɒl, -ˈtɔːl, -ˈtoʊl/) is a ring-shaped island, including a coral rim...
Johnston Atoll is an unincorporated territory of the United States, under the jurisdiction of the United States Air Force (USAF). The island is closed...
Bikini Bikini Atoll (/ˈbɪkɪˌniː/ or /bɪˈkiːni/; Marshallese: Pikinni, [pʲiɡinnʲi], lit. 'coconut place'), known as Eschscholtz Atoll between the 19th century...
Midway Atoll (colloquial: Midway Islands; Hawaiian: Kuaihelani, lit. 'the backbone of heaven'; Pihemanu, 'the loud din of birds') is a 2.4 sq mi (6.2 km2)...
Palmyra Atoll (/pælˈmaɪrə/), also referred to as Palmyra Island, is one of the Northern Line Islands (southeast of Kingman Reef and north of Kiribati)...
South. The largest of these atolls is Boduthiladhunmathi, while the atoll containing the most islands is Huvadhu. Some atolls are in the form of a number...
Kwajalein Atoll (/ˈkwɑːdʒəlɪn/; Marshallese: Kuwajleen [kʷuwɑzʲ(ɛ)lʲɛːnʲ]) is part of the Republic of the Marshall Islands (RMI). The southernmost and...
Enewetak Atoll (/ɛˈniːwəˌtɔːk, ˌɛnɪˈwiːtɔːk/; also spelled Eniwetok Atoll or sometimes Eniewetok; Marshallese: Ānewetak, [ænʲeːwɛːdˠɑk], or Āne-wātak,...
North Malé Atoll and South Malé Atoll. As the two Malé Atolls are the main islands of the administrative district, the entire Kaafu Atoll administrative...
flower'; also known as Wake Atoll) is a coral atoll in the Micronesia subregion of the northwestern Pacific Ocean. The atoll is composed of three islets...
(Baker Island, Howland Island, Jarvis Island, Johnston Atoll, Kingman Reef, Midway Atoll, Palmyra Atoll, and Wake Island) and one in the Caribbean Sea (Navassa...
Lagoons are commonly divided into coastal lagoons (or barrier lagoons) and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel...
The Rocas Atoll (Portuguese: Atol das Rocas [aˈtɔw dɐz ˈʁɔkɐs]) is the only atoll in the South Atlantic Ocean. It belongs to the Brazilian State of Rio...
Kure Atoll (/ˈkʊəriː/; Hawaiian: Hōlanikū, lit. 'bringing forth heaven'; Mokupāpapa, 'flat island') or Ocean Island is an atoll in the Pacific Ocean 48...
Baa Atoll (includes Southern Maalhosmadulu Atoll or Maalhosmadulu Dhekunuburi, Fasdhūtherē Atoll, and Goifulhafehendhu Atoll) is an administrative division...
Moruroa (Mururoa, Mururura), also historically known as Aopuni, is an atoll which forms part of the Tuamotu Archipelago in French Polynesia in the southern...
Rongerik Atoll or Rongdrik Atoll (Marshallese: Ron̄dik, [rʷɔŋʷ(ɯ)rʲik]) is an unpopulated coral atoll of 17 islands in the Pacific Ocean, and is located...
Ari Atoll (also called Alif or Alifu Atoll) is one of the natural atolls of the Maldives. It is one of the biggest atolls and is located in the west of...
Haddhunmathi or Laamu Atoll (Dhivehi: ހައްދުންމަތި އަތޮޅު) is an administrative division of the Maldives. The administrative capital is Fonadhoo Island...
mainland. The Maldives' chain of 26 atolls stretches across the equator from Ihavandhippolhu Atoll in the north to Addu Atoll in the south. The Maldives is...
Faadhippolhu Atoll (code name Lhaviyani) is an administrative division of the Maldives. It corresponds to the natural atoll of the same name. Faadhippolhu...
of which 187 are inhabited. They are listed by administrative division/atoll. The islands are divided into: Inhabited islands- those officially recognized...
Atoll, also known as Seenu Atoll, is the southernmost atoll of the Maldives. Addu Atoll, together with Fuvahmulah, located 40 km north of Addu Atoll,...
Dhaalu Atoll (also known as Southern Nilandhe Atoll or Dhivehi: ނިލަންދެ އަތޮޅު ދެކުނުބުރި, Nilandhe Atholhu Dhekunuburi) is one of the atolls of the...
The flag of Bikini Atoll, a member of the Marshall Islands, closely resembles the flag of the United States and was adopted in 1987. The flag is symbolic...
Wotje Atoll (Marshallese: Wōjjā, [wʌttʲæ]) is a coral atoll of 75 islands in the Pacific Ocean, and forms a legislative district of the Ratak Chain of...
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