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An atoll ( /ˈætɒl/, /ˈætɔːl/, /ˈætoʊl/, /əˈtɒl/, /əˈtɔːl/ or /əˈtoʊl/),[1][2] sometimes called a coral atoll, is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim.[3](p60)[4] The coral of the atoll often sits atop the rim of an extinct seamount or volcano which has eroded or subsided partially beneath the water. The lagoon forms over the volcanic crater or caldera while the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upwards and outwards to replace the lost height.[5]

Contents

1 Usage 2 Distribution and size 3 Formation 4 Investigation by the Royal Society of London
Royal Society of London
into the formation of coral reefs 5 United States national monuments 6 See also 7 References 8 External links

Usage[edit] The word atoll comes from the Dhivehi (an Indo-Aryan language
Indo-Aryan language
spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު, [ˈət̪ɔɭu]), meaning an administrative subdivision. OED
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
Coral
Reefs, the definition of atolls as "circular groups of coral islets" that is synonymous with "lagoon-island".[6](p2) More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus"[7] or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".[8] Distribution and size[edit]

Penrhyn atoll

NASA
NASA
satellite image of some of the atolls of the Maldives, which consists of 1,322 islands arranged into 26 atolls.

Nukuoro
Nukuoro
from space. Courtesy NASA.

Los Roques Archipelago
Los Roques Archipelago
in Venezuela, the largest marine National Park in Latin America,[9] from space. Courtesy NASA.

View of the coast of Bikini Atoll
Bikini Atoll
from above

Most of the world's atolls are in the Pacific Ocean
Ocean
(with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea
Coral Sea
Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean
Indian Ocean
(the Atolls of the Maldives, the Lakshadweep
Lakshadweep
Islands, the Chagos Archipelago
Chagos Archipelago
and the Outer Islands of the Seychelles). The Atlantic Ocean
Ocean
has no large groups of atolls, other than eight atolls east of Nicaragua
Nicaragua
that belong to the Colombian department of San Andres and Providencia
San Andres and Providencia
in the Caribbean. 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
Atoll
at 28°24′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef
Reef
at 29°58′ S, and nearby Middleton Reef
Middleton Reef
at 29°29′ S, in the Tasman Sea, both of which are part of the Coral Sea Islands
Coral Sea Islands
Territory. The next southerly atoll is Ducie Island
Ducie Island
in the Pitcairn Islands Group, at 24°40′ S. Bermuda
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
Bermuda
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
Aranuka
of Kiribati, with its southern tip just 12 km north of the equator.

Largest atolls by total area (lagoon plus reef and dry land)[10]

Name

Position

Location

Area (km²)

Remarks

Great Chagos Bank 6°10′S 72°00′E / 6.17°S 72.00°E / -6.17; 72.00

Indian Ocean

12,642

Land area 4.5 km²

Reed Bank 11°27′N 116°54′E / 11.45°N 116.90°E / 11.45; 116.90

Spratly Islands

8,866

Submerged, at shallowest 9 m

Macclesfield Bank 16°00′N 114°30′E / 16.00°N 114.50°E / 16.00; 114.50

South China Sea

6,448

Submerged, at shallowest 9.2 m

North Bank (Ritchie Bank, north of Saya de Malha Bank) 9°04′S 60°12′E / 9.07°S 60.20°E / -9.07; 60.20

North of Saya de Malha Bank

5,800

Submerged, at shallowest <10 m

Cay
Cay
Sal Bank (Placer de los Roques) 23°50′N 80°05′W / 23.83°N 80.08°W / 23.83; -80.08

Straits of Florida

5,226.73

Land area 14.87 km²

Rosalind Bank 16°26′N 80°31′W / 16.43°N 80.52°W / 16.43; -80.52

Caribbean

4,500

Submerged, at shallowest 7.3 m

Boduthiladhunmathi (Thiladhunmathi- Miladhunmadulu) Atoll 6°44′N 73°02′E / 6.73°N 73.04°E / 6.73; 73.04

Maldives

3,850

Two names, but a single atoll structure; land area 51 km²

Chesterfield Islands 19°21′S 158°40′E / 19.35°S 158.66°E / -19.35; 158.66

New Caledonia

3,500

Land area <10 km²

Huvadhu
Huvadhu
Atoll 0°30′N 73°18′E / 0.50°N 73.30°E / 0.50; 73.30

Maldives

3,152

Land area 38.5 km²

Truk Lagoon 7°25′N 151°47′E / 7.42°N 151.78°E / 7.42; 151.78

Chuuk, FSA

3,152

[11]

Sabalana Islands 6°45′S 118°50′E / 6.75°S 118.83°E / -6.75; 118.83

Indonesia

2,694

Nukuoro
Nukuoro
atoll 3°51′N 154°56′E / 3.85°N 154.94°E / 3.85; 154.94

FSA

40

Land area 1.7 km², divided among more than 40 islets that lie on the northern, eastern and southern sides of the lagoon

Lihou Reef 17°25′S 151°40′E / 17.42°S 151.67°E / -17.42; 151.67

Coral
Coral
Sea

2,529

Land area 1 km²

Bassas de Pedro 13°05′N 72°25′E / 13.08°N 72.42°E / 13.08; 72.42

Lakshadweep, India

2,474.33

Submerged, at shallowest 16.4 m

Ardasier Bank 7°43′N 114°15′E / 7.71°N 114.25°E / 7.71; 114.25

Spratly Islands

2,347

Cay
Cay
on the south side?

Kwajalein 9°11′N 167°28′E / 9.19°N 167.47°E / 9.19; 167.47

Marshall Islands

2,304

Land area 16.4 km²

Diamond Islets Bank 17°25′S 150°58′E / 17.42°S 150.96°E / -17.42; 150.96

Coral
Coral
Sea

2,282

Land area <1 km²

Namonuito Atoll 8°40′N 150°00′E / 8.67°N 150.00°E / 8.67; 150.00

Chuuk, FSA

2,267

Land area 4.4 km²

Ari Atoll 3°52′N 72°50′E / 3.86°N 72.83°E / 3.86; 72.83

Maldives

2,252

Land area 69 km²

Maro Reef 25°25′N 170°35′W / 25.42°N 170.59°W / 25.42; -170.59

Northwestern Hawaiian Islands

1,934

Rangiroa 15°08′S 147°39′W / 15.13°S 147.65°W / -15.13; -147.65

Tuamotu Islands

1,762

Land area 79 km²

Kolhumadulhu Atoll 2°22′N 73°07′E / 2.37°N 73.12°E / 2.37; 73.12

Maldives

1,617

Land area 79 km²

Kaafu Atoll
Kaafu Atoll
(North Malé Atoll) 4°25′N 73°30′E / 4.42°N 73.50°E / 4.42; 73.50

Maldives

1,565

Land area 69 km²

Ontong Java 5°16′S 159°21′E / 5.27°S 159.35°E / -5.27; 159.35

Solomon Islands

1500

Land area 12 km²

Raa Atoll
Raa Atoll
in Maldives

In most cases, the land area of an atoll is very small in comparison to the total area. Atoll
Atoll
islands are low lying, with their elevations less than 5 meters (9). Measured by total area, Lifou
Lifou
(1146 km²) is the largest raised coral atoll of the world, followed by Rennell Island
Island
(660 km²).[12] 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
Aldabra
with 155 km². The largest atoll in terms of island numbers is Huvadhu Atoll
Huvadhu Atoll
in the south of the Maldives
Maldives
with 255 islands.

Map from Charles Darwin’s 1842 The Structure and Distribution of Coral
Coral
Reefs showing the world’s major groups of atolls and coral reefs.

Formation[edit] See also: Formation of coral reefs

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Aerial view of Bora Bora

Tarawa Atoll

In 1842, Charles Darwin
Charles Darwin
explained the creation of coral atolls in the southern Pacific Ocean
Ocean
based upon observations made during a five-year voyage aboard 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
Aitutaki
in the Cook Islands, Bora Bora
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 hermatypic (reef-building) 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

Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon

Reginald Aldworth Daly
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
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
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. Investigation by the Royal Society of London
Royal Society of London
into the formation of coral reefs[edit] In 1896, 1897 and 1898, the Royal Society of London
Royal Society of London
carried out drilling on Funafuti
Funafuti
atoll in Tuvalu
Tuvalu
for the purpose of investigating the formation of coral reefs to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin
Charles Darwin
in the Pacific. The first expedition in 1896 was led by Professor William Johnson Sollas of the University of Oxford. The geologists included Walter George Woolnough and Edgeworth David
Edgeworth David
of the University of Sydney. Professor Edgeworth David
Edgeworth David
led the expedition in 1897.[13] The third expedition in 1898 was led by Alfred Edmund Finckh.[14][15][16] United States national monuments[edit]

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

On January 6, 2009, U.S. President George W. Bush
George W. Bush
announced the creation of the Pacific Remote Islands Marine National Monument, covering several islands and atolls under U.S. jurisdiction.[17][18](Number 1, page 14) See also[edit]

Islands portal Low island

References[edit]

^ pronunciation in old video on youtube, Bikini Atoll
Bikini Atoll
video ^ "the definition of atoll". reference.com.  ^ Blake, Gerald Henry, ed. (1994). World Boundary Series. Google books. 5 Maritime Boundaries. Routledge. ISBN 978-0-415-08835-0. Retrieved 12 February 2013.  ^ Migoń, Piotr, ed. (2010). Geomorphological Landscapes of the World. Springer. p. 349. ISBN 978-90-481-3055-9. Retrieved 12 February 2013.  ^ Erickson, Jon (2003), Marine Geology, Facts on File, pp. 126–8, ISBN 0-8160-4874-6  ^ Darwin, Charles R (1842). The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. Darwin Online. London: Smith Elder and Co.  ^ McNeil (1954, p. 396). ^ Fairbridge (1950, p. 341). ^ "Archipiélago de Los Roques" (in Spanish). Caracas, Venezuela: Instituto Nacional de Parques (INPARQUES). 2007. Archived from the original on 2008-04-24. Retrieved 27 February 2013.  ^ Atoll
Atoll
Area, Depth and Rainfall (2001) spreadsheet from The Geological Society of America. Retrieved d.d. October 1, 2009. ^ http://conserveonline.org/workspaces/pacific.island.countries.publications/fsm/fsmblueprint ^ "Misinformation about Islands". worldislandinfo.com.  ^ David, Cara (Caroline Martha) (1899). Funafuti
Funafuti
or Three Months On A Coral
Coral
Atoll: an unscientific account of a scientific expedition. London: John Murray. ASIN B00A1YMOFO. ISBN 978-1-151-25616-4.  ^ Finckh, Dr. Alfred Edmund (11 September 1934). "TO THE EDITOR OF THE HERALD". The Sydney Morning Herald. NSW: National Library of Australia. p. 6. Retrieved 20 June 2012.  ^ Cantrell, Carol (1996). "Finckh, Alfred Edmund (1866–1961)". Australian Dictionary of Biography at Australian National University. National Centre of Biography. Retrieved 23 December 2012.  ^ Rodgers, K A; Cantrell, Carol. "Alfred Edmund Finckh 1866–1961: Leader of the 1898 Coral
Coral
Reef
Reef
Boring Expedition to Funafuti" (PDF). Historical Records of Australian Science. pp. 393–403. doi:10.1071/HR9890740393. Retrieved 23 December 2012.  ^ "Presidential Proclamation 8336" (PDF). The White House. 6 January 2009.  ^ "Weekly Compilation of Presidential Documents" (PDF). 12 January 2009. Archived from the original (PDF) on March 1, 2009. 

Dobbs, David. 2005. Reef
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[edit]

Wikimedia Commons has media related to Atoll.

Nukuoro
Nukuoro
Atoll
Atoll
Nukuoro Kapingamarangi Atoll
Kapingamarangi Atoll
Kapingamarangi Formation of Bermuda
Bermuda
reefs Darwin's Volcano
Volcano
– a short video discussing Darwin and Agassiz' coral reef formation debate

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