Atoll


Atoll
Satellite picture of the Atafu atoll in Tokelau in the Pacific Ocean.

An atoll (play /ˈætɒl/ or /æˈtɒl/)[1] is a coral island (or islands) that encircles a lagoon partially or completely.

Contents

Usage

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."

Satellite Image of some of the atolls of the Maldives by NASA. A total of 1322 islands arranged into 26 atolls make up the country.

Distribution and size

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.

Nukuoro from space. Courtesy NASA.

The largest atolls by total area (lagoon plus reef and dry land)[2] are listed below:

In most cases, the land area of an atoll is very small in comparison to the total area. According to,[5] 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 largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives with 255 islands.

Formation

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.

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.

United States national monuments

Aerial overview of the Wake Island atoll, part of the Pacific Remote Islands Marine National Monument.

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.[6][7]

See also

References

  1. ^ pronunciation in old video on youtube, Bikini Atoll video
  2. ^ ftp://rock.geosociety.org/pub/reposit/2001/2001075.pdf Atoll Area, Depth and Rainfall] (2001) spreadsheet from The Geologicl Society of America. Retrieved d.d. October 1, 2009.
  3. ^ Lansdowne Bank "21 000" - Meine Bibliothek - Google Bücher
  4. ^ http://conserveonline.org/workspaces/pacific.island.countries.publications/fsm/fsmblueprint
  5. ^ Misinformation about Islands
  6. ^ Presidential Proclamation 8336
  7. ^ Weekly Compilation of Presidential Documents: Monday, January 12, 2009 Volume 45—Number 1, Page 14

Further reading

  • 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.

External links


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Look at other dictionaries:

  • atoll — atoll …   Dictionnaire des rimes

  • atoll — [ atɔl ] n. m. • attôle 1773; atolon 1611; mot des îles Maldives , par l angl. ♦ Récif corallien annulaire des mers chaudes, enfermant un lagon communiquant avec la haute mer. Des atolls. ● atoll nom masculin (maldive atolu, du cinghalais ätul, à …   Encyclopédie Universelle

  • Atoll — Sn Koralleninsel per. Wortschatz exot. (19. Jh.) Entlehnung. Entlehnt aus ne. atoll, das seinerseits aus einheimischen Bezeichnungen wie atollon und atoll für die Malediven übernommen ist. Im Englischen ist das Wort seit dem 16. Jh. als Exotismus …   Etymologisches Wörterbuch der deutschen sprache

  • Atoll — Жанры прогрессив рок Годы 1972 1976 1989 2003 …   Википедия

  • Atoll (BD) — Atoll (revue) Pour les articles homonymes, voir Atoll (homonymie). Atoll est une revue de Petit format de l éditeur Jeunesse et Vacances qui a eu 121 numéros de mars 1967 à novembre 1981. Ce fascicule est l un des plus gros succès de l éditeur… …   Wikipédia en Français

  • Atoll K — ((it) Atollo K) est un film franco italien réalisé par Léo Joannon, sorti en 1951, avec dans les rôles principaux le duo comique Laurel et Hardy dont c est le dernier film tourné en commun. Sommaire 1 Synopsis 2 Fiche technique …   Wikipédia en Français

  • Atoll 66 — 20e album de la série Natacha Scénario Guy d Artet Dessin François Walthéry Bruno Di Sano Couleurs Cerise Éditeur Marsu Productions …   Wikipédia en Français

  • Atoll — das; s, e <über gleichbed. engl. atoll aus Malayalam aḍal »verbindend«> ringförmige Koralleninsel …   Das große Fremdwörterbuch

  • Atoll — Atoll: Die im Deutschen seit dem 19./20. Jh. übliche Bezeichnung für eine ringförmige Koralleninsel stammt vermutlich aus der südwestindischen Drawidasprache Malayalam, wo aḍal »verbindend« bedeutet. Ins Dt. gelangte das Wort durch Vermittlung… …   Das Herkunftswörterbuch

  • Atoll — A*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. [1913 Webster] …   The Collaborative International Dictionary of English

  • atoll — 1620s, atollon, from Malayam atolu reef, probably from adal closing, uniting. Popularized in present form by Darwin s writings …   Etymology dictionary


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