Atoll: Wikis

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

An atoll (pronounced /ˈætɒl/[1] or English pronunciation: /æˈtɒl/) is an island of coral 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. However, the term was created by Charles Darwin (1842, p. 2), who described atolls as the final stage in oceanic island development. 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. 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 [3], 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

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 explained 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 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 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.

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).

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.

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.

National Monuments

As of January 6, 2009 President Bush announced that some remote islands in the Pacific ocean are now national monuments, protecting coral reefs. [3]

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 The Geologicl Society of America. Retrieved d.d. October 1, 2009.
  3. ^ Federal Register, Presidential Proclamation 8336

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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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ATOLL (native name atollon in the Maldive Islands), a horseshoe or ring shaped coral reef enclosing a lagoon. The usual shape is that of a partly submerged dish with a broken edge, forming the ring of islands, standing upon a conical pedestal. The dish is formed of coral rock and the shells of various reefdwelling mollusca, covered, especially at the seaward edges, with a film of living coral polyps that continually extend the fringe, and enlarge the diameter of the atoll. The lagoon tends to deepen when the land is stationary by the death of the coral animals in the still water, and the patchy disintegration of the "hard" coral, while waves and storms tear off blocks of rock and pile them up at the margin, increasing the height of the islands, which become covered by vegetation. The lagoon entrance in the open part of the horse-shoe is always to leeward of prevailing winds, since the coral growth is there slower than where the waves constantly renew the polyps' food supply. The conical pedestal rising from the depths is frequently a submarine volcanic cone or island, though any submerged peak may be crowned by an atoll. For the theory of atoll formation see Coral-Reefs.


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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also atoll

German

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Noun

Atoll n. (genitive Atolls, plural Atolle)

  1. atoll

Simple English

File:Nukuoro
The atoll of Nukuoro in the Federated States of Micronesia.

An atoll is a kind of island. It is made when a coral reef forms around an island that sinks over many years. In the end, the land is gone, and only the coral reef continues to grow until it becomes an atoll, an island shaped like a doughnut. [1]

Charles Darwin, who is most famous for his theory of evolution, was the first person to find out how atolls form. He said that volcanoes in the ocean sometimes wear away or sink deeper. Coral growing on a volcano likes to be near the surface, and it keeps growing to stay there.

Most atolls are in the warm parts of the Pacific Ocean or the Indian Ocean.

References

  • Darwin, C. 1842. The structure and distribution of coral reefs, being the first part of the voyage of the Beagle under the command of Capt. Vitzroy R.N. during the years 1832 to 1834. Smith, Elder and Co.: London. 214 pp.
  1. History and Geography. LIFEPAC. Alpha Omega Publications. ISBN 978-1-58095-155-5. 

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