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Encyclopedia > Atoll
Portion of a Pacific atoll showing two islets on the ribbon or barrier reef separated by a deep pass between the ocean and the lagoon.
Portion of a Pacific atoll showing two islets on the ribbon or barrier reef separated by a deep pass between the ocean and the lagoon.

An atoll (pronounced /ˈætʌl/) is an island of coral that encircles a lagoon partially or completely. Image File history File links Download high-resolution version (1808x1184, 1377 KB) Portion of a Pacific atoll (Yap) showing two islets on the ribbon or barrier reef separated by a deep pass betwen the ocean and the lagoon. ... Image File history File links Download high-resolution version (1808x1184, 1377 KB) Portion of a Pacific atoll (Yap) showing two islets on the ribbon or barrier reef separated by a deep pass betwen the ocean and the lagoon. ... Extant Subclasses and Orders Alcyonaria    Alcyonacea    Helioporacea Zoantharia    Antipatharia    Corallimorpharia    Scleractinia    Zoanthidea [1][2]  See Anthozoa for details For other uses, see Coral (disambiguation). ... This mid bay barrier in Narrabeen, a suburb of Sydney (Australia), has blocked what used to be a bay to form a lagoon. ...

Contents

Usage

The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު).[citation needed] Its first recorded use in English was in 1625. However, the term was popularised by Charles Darwin (1842, p. 2), who described atolls as a subset in a special class of islands, the unique property of which is the presence of an organic reef. More modern definitions of atoll are those of McNeil (1954, p. 396) as "...an annular reef enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus" and Fairbridge (1950, p. 341) "...in an exclusively morphological sense, [as] ...a ring-shaped ribbon reef enclosing a lagoon in the center." Dhivehi or Divehi is an Indo-Aryan language spoken by about 300,000 people in the Republic of Maldives where it is the official language of the country and in the island of Minicoy (Maliku) in neighbouring India where it is known as Mahl. ... The Indo-Aryan languages form a subgroup of the Indo-Iranian languages, thus belonging to the Indo-European family of languages. ... Motto vision 2020 Anthem Gavmii mi ekuverikan matii tibegen kuriime salaam In National Unity Do We Salute Our Nation Capital Malé Largest city Hulhumalé Official languages Dhivehi Government Republic  -  President Maumoon Abdul Gayyoom Independence  -  from the UK 26 July 1965  Area  -  Total 298 km² (204th) 115 sq mi   -  Water (%) negligible... Dhivehi or Divehi is an Indo-Aryan language spoken by about 300,000 people in the Republic of Maldives where it is the official language of the country and in the island of Minicoy (Maliku) in neighbouring India where it is known as Mahl. ... The English language is a West Germanic language that originates in England. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Mōkōlea Rock in Kailua Bay, O‘ahu, Hawai‘i, 2. ... In biology, detritus is organic waste material from decomposing dead plants or animals. ...


Formation

This animation shows the dynamic process of coral atoll formation. Corals (represented in tan and purple) settle and grow around an oceanic island, forming a fringing reef. In favorable conditions, the reef will expand, and the interior island will subside. Eventually the island completely subsides beneath the water, leaving a ring of growing coral with an open lagoon in its center. The process of atoll formation may take as long as 30,000,000 years to occur.
This animation shows the dynamic process of coral atoll formation. Corals (represented in tan and purple) settle and grow around an oceanic island, forming a fringing reef. In favorable conditions, the reef will expand, and the interior island will subside. Eventually the island completely subsides beneath the water, leaving a ring of growing coral with an open lagoon in its center. The process of atoll formation may take as long as 30,000,000 years to occur.

Darwin wrote an explanation for the creation of coral atolls in the South Pacific (1842) based upon observations made during a five-year voyage aboard the HMS Beagle (1831–1836). His explanation, which is accepted as basically correct, involved considering 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 upwards as the island subsides (sinks), becoming an "almost atoll" (barrier reef island) (as typified by an island such as Aitutaki, 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 corals and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface, but the barrier reef remains. At this point, the island has become an atoll. Image File history File links Coral_atoll_formation_animation. ... Image File history File links Coral_atoll_formation_animation. ... The bouncing ball animation (below) consists of these 6 frames. ... Pacific redirects here. ... HMS Beagle was a Cherokee class 10-gun brig of the Royal Navy, named after the beagle, a breed of dog. ... A small island in the Adriatic sea An island is any piece of land smaller than a continent and larger than a rock, that is completely surrounded by water. ... A road destroyed by subsidence and shear. ... Cleveland Volcano in the Aleutian Islands of Alaska photographed from the International Space Station For other uses, see Volcano (disambiguation). ... Aerial view of Aitutaki Aitutaki, traditionally known as Araura, is one of the Cook Islands, north of Rarotonga. ... Mount Otemanu, Bora Bora Bora Bora Pearl Beach Resort, Bora Bora Frigate Flor al, stationned in Bora-Bora lagoon Bora Bora is an atoll in French Polynesia, about 260 km northwest of the capital, Papeete. ... The Society Islands (French: ÃŽles de la Société or offically Archipel de la Société) are a group of islands in the south Pacific, administratively part of French Polynesia. ... For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ... Look up biotic in Wiktionary, the free dictionary. ... Osborne (talk) 20:17, 5 December 2007 (UTC):For the programming language, see algae (programming language) Laurencia, a marine red alga from Hawaii. ...


Atolls are the product of the growth of tropical marine organisms, so these islands are only found in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of reef building (hermatypic) 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 more polar evolve towards seamounts or guyots; islands more equatorial evolve towards atolls (see Kure Atoll). A seamount is a mountain rising from the ocean seafloor that does not reach to the waters surface (sea level), and thus is not an island. ... A guyot is a flat-topped seamount. ... Satellite image of Kure Atoll Kure Atoll or Ocean Island (Hawaiian: Kānemiloha‘i) lies some 55 miles beyond Midway Atoll in the Northwestern Hawaiian Islands at . ...


Reginald Aldworth Daly offered a somewhat different explanation for atoll formation: islands worn away by erosion (ocean waves and streams) during the last glacial stand of the sea of some 900 feet (270 m) below present sea level, developed as coral islands (atolls) (or barrier reefs on a platform surrounding a volcanic island not completely worn away) as sea level gradually rose from melting of the glaciers. Discovery of the great depth of the volcanic remnant beneath many atolls (see Midway Atoll), favors the Darwin explanation, although there can be little doubt that fluctuating sea level has had considerable influence on atoll and other reefs. Reginald Aldworth Daly (March 18, 1871 – September 19, 1957) was a Canadian geologist. ... For morphological image processing operations, see Erosion (morphology). ... Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ... This article is about the geological formation. ... Orthographic projection centred over Midway. ...


Distribution and size

An atoll in the western Pacific Ocean.

The distribution of atolls around the globe is instructive: most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Laccadive Islands, the Chagos Archipelago and the Outer Islands of the Seychelles). 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. An Atoll in the Western Pacific This image has been cropped and sharpened from an original image from http://www. ... An Atoll in the Western Pacific This image has been cropped and sharpened from an original image from http://www. ... Categories: Stub | Polynesia ... Sunset at Colonia on Yap The Caroline Islands form a large archipelago of widely scattered islands in the western Pacific Ocean, northeast of New Guinea. ... Geographic distribution of the Natural Atolls of the Maldives Geographically, the Maldives are formed by a number of natural atolls plus a few islands and isolated reefs which form a pattern stretching from 7 degrees 10 North to 0 degrees 45 South. ... Lakshadweep is a Union Territory of India. ... // The Chagos Archipelago. ... The Outer Islands are a collective term for those islands of the Seychelles that are not on the shallow Seychelles Bank (Seychelles Plateau), which defines the location of the Inner Islands. ... San Andrés and Providencia (Spanish: San Andrés y Providencia) is a department of Colombia. ...


As noted above, reef-building corals can thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are only found in the tropics and subtropics. The northernmost atoll of the world is Kure Atoll at 28°24' N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef at 29°58' S, and nearby Middleton Reef at 29°29' 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°40' S. Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°24' N. At this latitude coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is what is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different mode of formation. While there is no atoll directly on the Equator, the closest atoll to the Equator is Aranuka of Kiribati, with its southern tip just 12 km North of the Equator. The Hawaiian island chain. ... The Elizabeth and Middleton Reefs are two reefs located in the Tasman Sea. ... The Elizabeth and Middleton Reefs are two reefs located in the Tasman Sea. ... fuck you Map of the Tasman Sea Satellite photo of the Tasman Sea The Tasman Sea is the large body of water between Australia and New Zealand, some 2000 kilometres (1250 miles) across. ... Map of Pitcairn Islands, with inset of Ducie Island Ducie Island, a rarely visited island atoll, has been part of the Pitcairn Islands since 1902. ... This article is about the geographical term. ... For the album by Ocean Colour Scene, see North Atlantic Drift (album) The Gulf Stream is orange and yellow in this representation of water temperatures of the Atlantic. ... World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe. ... Categories: Possible copyright violations ...


The largest atolls by total area (lagoon plus reef and dry land) are (information mostly from [1]):

In most cases, the land area of an atoll is very small in comparison to the total area. According to [4], Lifou (land area 1146 km²) is the largest raised coral atoll of the world, followed by Rennell Island (660 km²). More sources however list as the largest atoll in the world in terms of land area Kiritimati, which is also a raised coral atoll (321.37 km² land area; according to other sources even 575 km²), 160 km² main lagoon, 168 km² other lagoons (according to other sources 319 km² total lagoon size). The remains of an ancient atoll as a hill in a limestone area is called a reef knoll. The second largest atoll by dry land area is Aldabra with 155 km². The Saya de Malha Bank (also Sahia de Malha Bank) is a large undersea bank in the Indian Ocean, part of the vast undersea Mascarene Plateau. ... The Great Chagos Bank, in the Chagos Archipelago, about 500 km South of the Maldives, is the largest atoll structure in the world, with a total area of roughly 13 000 km2. ... MacClesfield Bank or Zhongsha Islands (Chinese 中沙群島 Pinyin: Zhongsha Qundao, literally Central Sand Islands) is an elongated atoll of underwater reefs and shoals in South China Sea and part of the disputed South China Sea Islands. ... Filipino name Tagalog: Timog Dagat Tsina (Dagat Luzon for the portion within Philippine waters) Malay name Malay: Laut China Selatan Portuguese name Portuguese: Mar da China Meridional Vietnamese name Vietnamese: The South China Sea is a marginal sea south of China. ... The Saya de Malha Bank (also Sahia de Malha Bank) is a large undersea bank in the Indian Ocean, part of the vast undersea Mascarene Plateau. ... Bahama banks, with Cay Sal Bank below center of left margin Cay Sal Bank overview map Cay Sal, with Lavanderas Rocks below right center Cay Sal Bank is the third largest (after Great Bahama Bank and Little Bahama Bank) and the westernmost of the Bahama Banks. ... [--168. ... Rosalind Bank, also called Rosalinda or Rosa Linda Bank, is a large, completely submerged atoll that lies with its southern extremity 269 km ENE of Cabo Gracias a Dios, at . ... Map of Central America and the Caribbean The Caribbean Sea (pronounced or ) is a tropical sea in the Western Hemisphere, part of the Atlantic Ocean, southeast of the Gulf of Mexico. ... Chesterfield Islands from space Chesterfield Islands (îles Chesterfield in french) is an archipelago of New Caledonia located in the Coral Sea, 550 km North East of Grande Terre the main island of New Caledonia. ... Huvadhoo Atoll is divided into two administrative atolls, Gaafu Alif and Gaafu Dhaalu Huvadu (H.C.P. Bell) or Huvadhoo is the largest Atolls of the Maldives and one of the largest atolls in the world, larger than Ontong Java Atoll and Lihou Reef in the SW Pacific area. ... Truk Lagoon is a sheltered body of water almost fifty miles long by thirty miles wide surrounded by a protective reef. ... This article or section does not cite any references or sources. ... Lihou Reef and Cays is the largest atoll by total size in the Coral Sea. ... Map of the Coral Sea Islands A political map of the South Pacific. ... Bassas de Pedro, also known as Munyal Par or Padua Bank, is a submerged bank (a sunken atoll) in the Union Territory of Lakshadweep, India, at . ... Kwajalein Atoll - NASA NLT Landsat 7 (Visible Color) Satellite Image Kwajalein Atoll (Marshallese: Kuwajleen ; common English pronunciation , often nicknamed Kwaj by English-speaking residents of the U.S. facilities) is part of the Republic of the Marshall Islands (RMI). ... Map of the Coral Sea Islands A political map of the South Pacific. ... This article or section does not cite any references or sources. ... Ari Atoll is one of natural atolls of the Maldives. ... Maro Reef (Hawaiian: Nalukakala) is a largely submerged coral atoll located in the Northwestern Hawaiian Islands. ... The Hawaiian island chain. ... Rangiroa, which means Far Sky in Tuamotuan, is the largest atoll in the Tuamotus, and one of the largest in the world (larger than Kwajalein in the Marshall Islands). ... Categories: Stub | Polynesia ... Kolhumadulhu Atoll or Thaa is an is an administrative division of the Maldives. ... Kaafu Atoll is an atoll from the Maldives. ... Ontong Java Atoll is the northernmost tract of land in the Solomon Islands and an outlying part of the province of Malaita. ... Lifou from space, November 1990 Lifou is the largest and most important island of the Loyalty Islands. ... Rennell Island is the largest island of Rennell and Bellona Province. ... For the island in the Indian Ocean, see Christmas Island. ... A reef knoll is an immense pile of calcareous material which has accumulated on the sea floor . ... Map of Aldabra Atoll Aldabra and Assumption Island Aldabra Atoll Aldabra is a raised coral atoll in the Indian Ocean virtually untouched by humans, with distinctive island fauna, including the Aldabra Giant Tortoise. ...


See also

Some of the biodiversity of a coral reef, in this case the Great Barrier Reef, Australia. ... Islomania is a craze for or a strong attraction to islands. ... A peninsula in Croatia A peninsula is a piece of land that is bordered on three or more sides by water. ... Animated, colour-coded map showing the various continents. ...

References

Wikimedia Commons has media related to:
Atoll
  • Darwin, C. 1842. The structure and distribution of coral reefs. London.
  • Dobbs, David. 2005. Reef Madness : Charles Darwin, Alexander Agassiz, and the Meaning of Coral. Pantheon. ISBN 0-375-42161-0
  • Fairbridge, R. W. 1950. Recent and Pleistocene coral reefs of Australia. J. Geol., 58(4): 330–401.
  • McNeil, F. S. 1954. Organic reefs and banks and associated detrital sediments. Amer. J. Sci., 252(7): 385–401.

Image File history File links Commons-logo. ...

External links


  Results from FactBites:
 
Atoll - Wikipedia, the free encyclopedia (1184 words)
An atoll is a type of low, coral island found in tropical oceans and consisting of a coral-algal reef surrounding a central depression.
The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°40' S. Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°24' N. At this latitude coral reefs would not develop without the warming waters of the Gulf Stream.
The remains of an ancient atoll as a hill in a limestone area is called a reef knoll.
The Ultimate Atoll Dog Breeds Information Guide and Reference (977 words)
One of the largest atolls is also Lihou Reef in the Coral Sea, with a lagoon of 2500 km² (which contrasts to a minuscule land area of 0.91 km²).
The southernmost atolls of the world are Elizabeth Reef at 29°58' South, and nearby Middleton Reef at 29°29', in the Tasman Sea, which both belong to the Coral Sea Islands Territory.
Concentrations of atolls in the Indian Ocean are the Maldives, the Laccadive Islands, the Chagos Archipelago and the Outer Islands of the Seychelles.
  More results at FactBites »

 
 

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