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For other senses of this word, see atoll (disambiguation).
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. Corals are Marine organisms from the class Anthozoa and exist as small Sea anemone –like Polyps typically in colonies of many A lagoon is a body of comparatively shallow salt or Brackish water separated from the deeper Sea by a shallow or exposed sandbank, coral

Contents

Usage

The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު). Dhivehi Writing Systems Dhivehi (or Divehi) is an Indo-Aryan language spoken by more than about 300000 people in the Republic of Maldives where it is the The Indo-Aryan languages (within the context of Indo-European studies also Indic) are a branch of the Indo-European language family The Maldives ( or, or Maldive Islands) officially the Republic of Maldives, is an Island nation consisting of a group of atolls stretching Dhivehi Writing Systems Dhivehi (or Divehi) is an Indo-Aryan language spoken by more than about 300000 people in the Republic of Maldives where it is the Its first recorded use in English was in 1625. English is a West Germanic language originating in England and is the First language for most people in the United Kingdom, the United States However, the term was popularised by Charles Darwin (1842, p. Charles Robert Darwin (February 12 1809 &ndash April 19 1882 was an English naturalist, who realised and demonstrated that all Species of life 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. For human anatomy see Islets of Langerhans An islet is a small Island. In Biology, detritus is non-living particulate organic material (as opposed to dissolved organic material 341) ". . . in an exclusively morphological sense, [as] . . . a ring-shaped ribbon reef enclosing a lagoon in the center. "

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. The bouncing ball animation (below consists of these 6 frames 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). The Pacific Ocean is the largest of the Earth 's Oceanic divisions First Voyage On 27 September 1825 Beagle docked at Woolwich for repairs and fitted out for her new duties at a total cost of £5913 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. An island (ˈaɪlənd or isle (/ˈaɪl/ is any piece of land that is completely surrounded by water in two dimensions above high tide and isolated from other significant In Geology, Engineering, and Surveying, subsidence is the motion of a surface (usually the Earth's surface as it shifts downward relative to Plate tectonics and hotspots Divergent plate boundaries At the 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). Aitutaki, also traditionally known as Araura Ararau and Utataki, is one of the Cook Islands, north of Rarotonga. Bora Bora is an island in the Leeward group of the Society Islands of French Polynesia, an overseas territory of France in the Pacific The Society Islands ( French: Îles de la Société or officially Archipel de la Société) are a group of Islands in the south Pacific 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. Mean sea level (MSL is the average (mean height of the Sea, with reference to a suitable reference surface Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms 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.

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. A seamount is a Mountain rising from the Ocean Seafloor that does not reach to the water's surface ( Sea level) and thus is not an Island 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). For the French viticulturist see Jules Guyot. For the Old French name see Guiot. Kure Atoll or Ocean Island ( Hawaiian: Mokupāpapa) lies some 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. Reginald Aldworth Daly ( March 18, 1871 &ndash September 19, 1957) was a Canadian Geologist. Erosion is the carrying away or displacement of solids ( Sediment, Soil, rock and other particles usually by the agents of currents such as wind An ice age is a period of long-term reduction in the Temperature of the Earth 's surface and atmosphere resulting in an expansion of continental Ice sheets "Glacial" and "Glaciation" redirect here For the geological periods see Glacial period. 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. Midway Atoll (or Midway Island or Midway Islands; Hawaiian: Pihemanu Kauihelani) is a 2

Distribution and size

An atoll in the western Pacific Ocean.
An atoll in the western Pacific Ocean. The Pacific Ocean is the largest of the Earth 's Oceanic divisions

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 Tuamotus or the Tuamotu Archipelago ( French: Îles Tuamotu officially Archipel des Tuamotu) are a chain of atolls in French Polynesia The Caroline Islands form a large Archipelago of widely scattered islands in the western Pacific Ocean, northeast of New Guinea. The Marshall Islands, officially the Republic of the Marshall Islands (RMI is a Micronesian nation of islands in the middle of the Pacific Ocean The Coral Sea Islands Territory includes a group of small and mostly uninhabited tropical islands and reefs in the Coral Sea, northeast of Queensland, Australia Kiribati or ( kirr-i-bas or KEE-ree-buhss ˈkiɾibas in Gilbertese) officially the Republic of Kiribati, is an Island nation located in Tuvalu, formerly known as the Ellice Islands, is a Polynesian Island nation located in the Pacific Ocean midway between Hawaii and Tokelau (ˈtoʊkəlaʊ is a territory of New Zealand that consists of three tropical coral Atolls in the South Pacific Ocean. The Indian Ocean is the third largest of the world's Oceanic divisions covering about 20% of the water on the Earth 's surface This article discusses the geographical atolls of the Maldives Lakshadweep ( (ലക്ഷദ്വീപ്,) the smallest Union territory of India, is a group of islands 200 to 300 km off of the coast of The Chagos Archipelago (formerly Oil Islands) is a group of seven Atolls comprising more than 60 individual tropical Islands roughly in the centre of The Outer Islands or Coralline Seychelles is a collective term for those Islands of the Seychelles that are not on the shallow Seychelles Bank Seychelles (seɪˈʃɛl or /seɪˈʃɛlz/ in English and seʃɛl in French) officially the Republic of Seychelles (République des Seychelles Creole 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. Nicaragua (ˌnɪkəˈrɑgwə officially the Republic of Nicaragua () is a representative democratic republic and the largest nation in Central America Colombia (kəˈlʌmbɪə officially the Republic of Colombia () is a country in northwestern South America. Archipelago of San Andres Providencia and Santa Catalina or colloquially San Andres y Providencia is one of the departments 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 Northwestern Hawaiian Islands or the Leeward Islands are the small Islands and Atolls in the Hawaiian island chain located northwest 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. Elizabeth and Middleton Reefs form the Elizabeth and Middleton Reefs Marine National Park Reserve managed by the Government of Australia under the Elizabeth and Middleton Reefs form the Elizabeth and Middleton Reefs Marine National Park Reserve managed by the Government of Australia under the The Tasman Sea is the large body of water between Australia and New Zealand, some 2000 Kilometres (1250 Miles across The next southerly atoll is Ducie Island in the Pitcairn Islands Group, at 24°40' S. Ducie Island, a rarely visited Island Atoll, has been part of the Pitcairn Islands since 1902 Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°24' N. Ba (officially The Bermuda Islands or The Somers Isles) is a British overseas territory in the North Atlantic Ocean. Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the At this latitude coral reefs would not develop without the warming waters of the Gulf Stream. The Gulf Stream, together with its northern extension towards Europe the North Atlantic Drift, is a powerful warm and swift Atlantic Ocean current that 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 equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the Aranuka (also previously known as Henderville Nonouki or Starbuck is an Atoll of Kiribati, located just north of the Equator, in the Gilbert Islands

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. Ari Atoll (also called Alif or Alufu Atoll is one of natural Atolls of the Maldives. The Maldives ( or, or Maldive Islands) officially the Republic of Maldives, is an Island nation consisting of a group of atolls stretching Maro Reef ( Hawaiian: Nalukākala) is a largely submerged Coral Atoll located in the Northwestern Hawaiian Islands. The Northwestern Hawaiian Islands or the Leeward Islands are the small Islands and Atolls in the Hawaiian island chain located northwest Rangiroa (meaning "Vast Sky" in Tuamotuan) or Te Kokota, is the largest Atoll in the Tuamotus, and one of the largest in the world The Tuamotus or the Tuamotu Archipelago ( French: Îles Tuamotu officially Archipel des Tuamotu) are a chain of atolls in French Polynesia Kolhumadulhu Atoll, letter code Thaa is an is an administrative division of the Maldives. The Maldives ( or, or Maldive Islands) officially the Republic of Maldives, is an Island nation consisting of a group of atolls stretching Kaafu Atoll is an administrative division of the Maldives. It consists of Kaashidhoo Island, Gaafaru, and Malé Atoll (which The Maldives ( or, or Maldive Islands) officially the Republic of Maldives, is an Island nation consisting of a group of atolls stretching Ontong Java redirects here For the undersea plateau see Ontong Java Plateau. The Solomon Islands is a country in Melanesia, east of Papua New Guinea, consisting of nearly one thousand islands According to [4], Lifou (land area 1146 km²) is the largest raised coral atoll of the world, followed by Rennell Island (660 km²). Lifou is a commune in the Loyalty Islands Province of New Caledonia, an overseas territory of France in the Pacific Ocean. The local name for Rennell Island is Mungava and is the main island of two inhabited islands that make up the Rennell and Bellona Province in the Solomon Islands 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. Kiritimati or Christmas Island is a Pacific Ocean Atoll in the northern Line Islands and part of the Republic of Kiribati. 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. A reef knoll is an immense pile of Calcareous material on land that accumulated on the ancient sea floor The second largest atoll by dry land area is Aldabra with 155 km². Aldabra is a raised Coral Atoll in the Aldabra Group of islands in the Indian Ocean that form part of the Seychelles.

See also

References

External links

Dictionary

atoll

-noun

  1. A type of island consisting of a ribbon reef that nearly or entirely surrounds a lagoon and supports, in most cases, one to many islets on the reef platform. Atolls have a unique geology, so not all islands with a reef and a lagoon are atolls.
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