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1.an island consisting of a circular coral reef surrounding a lagoon
AtollA*toll" (�), n. [The native name in the Indian Ocean.] A coral island or islands, consisting of a belt of coral reef, partly submerged, surrounding a central lagoon or depression; a lagoon island.
ATOLL (programming language) • Ailinginae Atoll • Ailuk Atoll • Ainglaplap Atoll • Alif Alif Atoll • Alif Dhaal Atoll • Alifushi (Raa Atoll) • Alphonse Atoll • Amo Atoll • Angolhitheemu (Raa Atoll) • Atoll missile • Aur Atoll • Baa Atoll • Bandidhoo (Dhaalu Atoll) • Bikar Atoll • Bileddhoo (Faafu Atoll) • Bileffahi (Shaviyani Atoll) • Bodufulhadhoo (Alif Alif Atoll) • Burunee (Thaa Atoll) • Butaritari Atoll • Dhaalu Atoll • Dhaandhoo (Gaafu Alif Atoll) • Dhanbidhoo (Laamu Atoll) • Dhangethi (Alif Dhaal Atoll) • Dharanboodhoo (Faafu Atoll) • Dharavandhoo (Baa Atoll) • Dhevvadhoo (Gaafu Alif Atoll) • Dhiddhoo (Alif Dhaal Atoll) • Dhiffushi (Kaafu Atoll) • Dhiggaru (Meemu Atoll) • Dhigurah (Alif Dhaal Atoll) • Dhiyadhoo (Gaafu Alif Atoll) • Dhiyamingili (Thaa Atoll) • Dhonfanu (Baa Atoll) • Ebon Atoll • Enewetak Atoll • Faafu Atoll • Fainu (Raa Atoll) • Fares-Maathodaa (Gaafu Dhaalu Atoll) • Faridhoo (Haa Dhaalu Atoll) • Farquhar Atoll • Feeali (Faafu Atoll) • Feevah (Shaviyani Atoll) • Fehendhoo (Baa Atoll) • Fenfushi (Alif Dhaal Atoll) • Feridhoo (Alif Alif Atoll) • Feydhoo (Seenu Atoll) • Feydhoo (Shaviyani Atoll) • Fiyoaree (Gaafu Dhaalu Atoll) • Foakaidhoo (Shaviyani Atoll) • Foddhoo (Noonu Atoll) • Fonadhoo (Laamu Atoll) • Fulhadhoo (Baa Atoll) • Fulidhoo (Vaavu Atoll) • Funadhoo (Shaviyani Atoll) • Gaadhiffushi (Thaa Atoll) • Gaadhoo (Laamu Atoll) • Gaafaru (Kaafu Atoll) • Gaafu Alif Atoll • Gaafu Dhaalu Atoll • Gaddhoo (Gaafu Dhaalu Atoll) • Gan (Laamu Atoll) • Gemanafushi (Gaafu Alif Atoll) • Gemendhoo (Dhaalu Atoll) • Gnaviyani Atoll • Goidhoo (Baa Atoll) • Goidhoo (Shaviyani Atoll) • Gulhi (Kaafu Atoll) • Guraidhoo (Kaafu Atoll) • Guraidhoo (Thaa Atoll) • Haa Alif Atoll • Haa Dhaalu Atoll • Haggnaameedhoo (Alif Dhaal Atoll) • Henbandhoo (Noonu Atoll) • Himandhoo (Alif Alif Atoll) • Himmafushi (Kaafu Atoll) • Hirilandhoo (Thaa Atoll) • Hithaadhoo (Baa Atoll) • Hithadhoo (Laamu Atoll) • Hoandeddhoo (Gaafu Dhaalu Atoll) • Holhudhoo (Noonu Atoll) • Hulhudheli (Dhaalu Atoll) • Hulhudhoo (Seenu Atoll) • Hulhudhuffaaru (Raa Atoll) • Huraa (Kaafu Atoll) • Inguraidhoo (Raa Atoll) • Innamaadhoo (Raa Atoll) • Isdhoo (Laamu Atoll) • Johnston Atoll • Kaafu Atoll • Kaafu Atoll, Maldives • Kaashidhoo (Kaafu Atoll) • Kalaidhoo (Laamu Atoll) • Kamadhoo (Baa Atoll) • Kandholhudhoo (Raa Atoll) • Kanditheemu (Shaviyani Atoll) • Kandoodhoo (Thaa Atoll) • Kanduhulhudhoo (Gaafu Alif Atoll) • Kendhikolhudhoo (Noonu Atoll) • Kendhoo (Baa Atoll) • Keyodhoo (Vaavu Atoll) • Kihaadhoo (Baa Atoll) • Kinbidhoo (Thaa Atoll) • Kinolhas (Raa Atoll) • Kolamaafushi (Gaafu Alif Atoll) • Kolhufushi (Meemu Atoll) • Komandoo (Shaviyani Atoll) • Kondey (Gaafu Alif Atoll) • Kudafaree (Noonu Atoll) • Kudarikilu (Baa Atoll) • Kunahandhoo (Laamu Atoll) • Kunburudhoo (Alif Dhaal Atoll) • Kure Atoll • Kurendhoo (Lhaviyani Atoll) • Kwajalein Atoll • Laamu Atoll • Lae Atoll • Landhoo (Noonu Atoll) • Lhaimagu (Shaviyani Atoll) • Lhaviyani Atoll • Lhohi (Noonu Atoll) • Likiep Atoll • Maabaidhoo (Laamu Atoll) • Maaenboodhoo (Dhaalu Atoll) • Maafaru (Noonu Atoll) • Maafilaafushi (Lhaviyani Atoll) • Maafushi (Kaafu Atoll) • Maakurathu (Raa Atoll) • Maalhendhoo (Noonu Atoll) • Maalhos (Alif Alif Atoll) • Maalhos (Baa Atoll) • Maamendhoo (Gaafu Alif Atoll) • Maamendhoo (Laamu Atoll) • Maamingili (Alif Dhaal Atoll) • Maaungoodhoo (Shaviyani Atoll) • Maavah (Laamu Atoll) • Madaveli (Gaafu Dhaalu Atoll) • Madifushi (Meemu Atoll) • Madifushi (Thaa Atoll) • Maduvvaree (Meemu Atoll) • Maduvvaree (Raa Atoll) • Magoodhoo (Faafu Atoll) • Magoodhoo (Noonu Atoll) • Mandhoo (Alif Dhaal Atoll) • Maradhoo (Seenu Atoll) • Maradhoo-Feydhoo (Seenu Atoll) • Maroshi (Shaviyani Atoll) • Mathiveri (Alif Alif Atoll) • Meedhoo (Dhaalu Atoll) • Meedhoo (Raa Atoll) • Meemu Atoll • Miladhoo (Noonu Atoll) • Milandhoo (Shaviyani Atoll) • Muli (Meemu Atoll) • Mundoo (Laamu Atoll) • Muroroa Atoll • Naalaafushi (Meemu Atoll) • Nadellaa (Gaafu Dhaalu Atoll) • Namdrik Atoll • Namu Atoll • Narudhoo (Shaviyani Atoll) • Nilandhoo (Gaafu Alif Atoll) • Noomaraa (Shaviyani Atoll) • Noonu Atoll • Olhuvelifushi (Lhaviyani Atoll) • Omadhoo (Alif Dhaal Atoll) • Omadhoo (Thaa Atoll) • Providence Atoll • Raa Atoll • Raimmandhoo (Meemu Atoll) • Rakeedhoo (Vaavu Atoll) • Rasgetheemu (Raa Atoll) • Rathafandhoo (Gaafu Dhaalu Atoll) • Rinbudhoo (Dhaalu Atoll) • Rose Atoll • Ross Atoll • Seenu Atoll • Seenu Atoll, Maldives • Shaviyani Atoll • Thaa Atoll • Thimarafushi (Thaa Atoll) • Thinadhoo (Gaafu Dhaalu Atoll) • Thinadhoo (Vaavu Atoll) • Thoddoo (Alif Alif Atoll) • Thulhaadhoo (Baa Atoll) • Thulusdhoo (Kaafu Atoll) • Ujae Atoll • Ujelang Atoll • Ukulhas (Alif Alif Atoll) • Utirik Atoll • Vaadhoo (Gaafu Dhaalu Atoll) • Vaadhoo (Raa Atoll) • Vaanee (Dhaalu Atoll) • Vaavu Atoll • Vandhoo (Thaa Atoll) • Velidhoo (Noonu Atoll) • Veyvah (Meemu Atoll) • Villingili (Gaafu Alif Atoll) • Vilufushi (Thaa Atoll)
ledge; shelf; reef[Classe]
(coral island; atoll)[Thème]
corail (fr)[termes liés]
The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު, [ˈət̪ɔɭu])OED. Its first recorded use in English was in 1625 as atollon - Charles Darwin recognized its indigenous origin and coined, in his The Structure and Distribution of Coral Reefs, the definition of atolls as "..circular groups of coral islets [...] and [the word] is synonymous with 'lagoon-island'." (1842, p. 2). 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."
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 in the Caribbean Sea.
Reef-building corals will 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 largest atolls by total area (lagoon plus reef and dry land) are listed below:
In most cases, the land area of an atoll is very small in comparison to the total area. Measured by total area, Lifou (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 largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives with 255 islands.
In 1842 Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard the HMS Beagle from 1831 to 1836. Accepted as basically correct, his explanation 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", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, 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.
Atolls are the product of the growth of tropical marine organisms, and so these islands are only found in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of reef building, or 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 in colder, more polar regions evolve towards seamounts or guyots; warmer, more equatorial islands evolve towards atolls, for example Kure Atoll.
Darwin’s theory starts with a volcanic island which becomes extinct
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
As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeper lagoon inside
Reginald Aldworth Daly offered a somewhat different explanation for atoll formation: islands worn away by erosion, by 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 such as at Midway Atoll favors the Darwin explanation, although there can be little doubt that fluctuating sea level has had considerable influence on atolls and other reefs.
Coral atolls are also an important place where dolomitization of calcite occurs. At certain depths water is undersaturated in calcium carbonate but saturated in dolomite. Convection created by tides and sea currents enhance this change. Hydrothermal currents created by volcanoes under the atoll may also play an important role.
As of January 6, 2009 U.S. President Bush announced that several remote Pacific islands under U.S. jurisdiction are now national monuments, protecting coral reefs.
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