thumb|The atoll of [[Tetiʻaroa in French Polynesia]]
An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. According to Charles Darwin's subsidence model,
Usage
The word atoll comes from the Dhivehi word (, ). Dhivehi is an Indo-Aryan language spoken in the Maldives. The word's first recorded English use was in 1625 as atollon. Charles Darwin coined the term in his monograph, The Structure and Distribution of Coral Reefs. He recognized the word's indigenous origin and defined it as a "circular group of coral islets", synonymously with "lagoon-island". or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".
Distribution and size
There are approximately 440 atolls in the world. Most of the world's atolls are in the Pacific Ocean (with concentrations in the Caroline Islands, the Coral Sea Islands, the Marshall Islands, the Tuamotu Islands, Kiribati, Tokelau, and Tuvalu) and the Indian Ocean (the Chagos Archipelago, Lakshadweep, the atolls of the Maldives, and the Outer Islands of Seychelles). In addition, Indonesia also has several atolls spread across the archipelago, such as in the Thousand Islands, Taka Bonerate Islands, and atolls in the Raja Ampat Islands. 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.
Reef-building corals will thrive only in warm tropical and subtropical waters of oceans and seas, and therefore atolls are found only in the tropics and subtropics. The northernmost atoll in the world is Kure Atoll at 28°25′ N, along with other atolls of the Northwestern Hawaiian Islands. The southernmost atolls in the world are Elizabeth Reef at 29°57′ S, and nearby Middleton Reef at 29°27′ 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°41′ S.
The atoll closest to the Equator is Aranuka of Kiribati. Its southern tip is just north of the Equator.
Bermuda is sometimes claimed as the "northernmost atoll" at a latitude of 32°18′ N. At this latitude, coral reefs would not develop without the warming waters of the Gulf Stream. However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different origin of formation.
In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than . Measured by total area, Lifou () is the largest raised coral atoll of the world, followed by Rennell Island (). More sources, however, list Kiritimati as the largest atoll in the world in terms of land area. It is also a raised coral atoll ( land area; according to other sources even ), main lagoon, other lagoons (according to other sources total lagoon size).
The geological formation known as a reef knoll refers to the elevated remains of an ancient atoll within a limestone region, appearing as a hill. The second largest atoll by dry land area is Aldabra, with . Huvadhu Atoll, situated in the southern region of the Maldives, holds the distinction of being the largest atoll based on the sheer number of islands it comprises, with a total of 255 individual islands.
thumb|upright=3|center|Map from [[Charles Darwin's 1842 The Structure and Distribution of Coral Reefs showing the world's major groups of atolls and coral reefs]]
List of atolls
{| class="wikitable plainrowheaders sortable static-row-numbers sort-under col1right"
|+ style="padding-bottom: 0.5em" | Largest atolls by total area (lagoon plus reef and dry land)
|-
! scope="col" | Name
! scope="col" | Position
! scope="col" | Location
! scope="col" style="max-width: 4em;" | Land area (km<sup>2</sup>)
! scope="col" style="max-width: 4em;" | Total area (km<sup>2</sup>)
! scope="col" | Notes
|-
! scope="row" | Great Chagos Bank
|
| Indian Ocean
| style="text-align: right;" | 4.5
| style="text-align: right;" | 12,642
|
|-
! scope="row" | Reed Bank
|
| Spratly Islands
| style="text-align: right;" |
| style="text-align: right;" | 8,866
| Submerged, at shallowest 9 m
|-
! scope="row" | Macclesfield Bank
|
| South China Sea
| style="text-align: right;" |
| style="text-align: right;" | 6,448
| Submerged, at shallowest 9.2 m
|-
! scope="row" | North Bank
|
| North of Saya de Malha Bank
| style="text-align: right;" |
| style="text-align: right;" | 5,800
| Submerged, at shallowest <10 m
|-
! scope="row" | Rosalind Bank
|
| Caribbean
| style="text-align: right;" |
| style="text-align: right;" | 4,500
| Submerged, at shallowest 7.3 m
|-
! scope="row" | Thiladhunmathi
|
| Maldives
| style="text-align: right;" | 51
| style="text-align: right;" | 3,850
|
|-
! scope="row" | Chesterfield Islands
|
| New Caledonia
| style="text-align: right;" | <10
| style="text-align: right;" | 3,500
|
|-
! scope="row" | Huvadhu Atoll
|
| Maldives
| style="text-align: right;" | 38.5
| style="text-align: right;" | 3,152
|
|-
! scope="row" | Chuuk Lagoon
|
| Chuuk, Micronesia
| style="text-align: right;" |
| style="text-align: right;" | 3,152
|
|-
! scope="row" | Sabalana Islands
|
| Indonesia
| style="text-align: right;" |
| style="text-align: right;" | 2,694
|
|-
! scope="row" | Lihou Reef
|
| Coral Sea
| style="text-align: right;" | 1
| style="text-align: right;" | 2,529
|
|-
! scope="row" | Bassas de Pedro
|
| Lakshadweep, India
| style="text-align: right;" |
| style="text-align: right;" | 2,474
| Submerged, at shallowest 16.4 m
|-
! scope="row" | Ardasier Bank
|
| Spratly Islands
| style="text-align: right;" |
| style="text-align: right;" | 2,347
|
|-
! scope="row" | Kwajalein Atoll
|
| Marshall Islands
| style="text-align: right;" | 16.4
| style="text-align: right;" | 2,304
|
|-
! scope="row" | Diamond Islets Bank
|
| Coral Sea
| style="text-align: right;" | <1
| style="text-align: right;" | 2,282
|
|-
! scope="row" | Namonuito Atoll
|
| Chuuk, Micronesia
| style="text-align: right;" | 4.4
| style="text-align: right;" | 2,267
|
|-
! scope="row" | Ari Atoll
|
| Maldives
| style="text-align: right;" | 69
| style="text-align: right;" | 2,252
|
|-
! scope="row" | Maro Reef
|
| Northwestern Hawaiian Islands
| style="text-align: right;" |
| style="text-align: right;" | 1,934
|
|-
! scope="row" | Rangiroa
|
| Tuamotus
| style="text-align: right;" | 79
| style="text-align: right;" | 1,762
|
|-
! scope="row" | Kolhumadulu Atoll
|
| Maldives
| style="text-align: right;" | 79
| style="text-align: right;" | 1,617
|
|-
! scope="row" | Kaafu Atoll
|
| Maldives
| style="text-align: right;" | 69
| style="text-align: right;" | 1,565
|
|-
! scope="row" | Ontong Java Atoll
|
| Solomon Islands
| style="text-align: right;" | 12
| style="text-align: right;" | 1500
|
|-
! scope="row" | Lifou
|
| New Caledonia
| style="text-align: right;" |
| style="text-align: right;" | 1146
| Raised atoll with no lagoon
|-
! scope="row" | Rennell
|
| Solomon Islands
| style="text-align: right;" |
| style="text-align: right;" | 660
| Raised atoll with no lagoon
|-
! scope="row" | Kiritimati
|
| Kiribati
| style="text-align: right;" | 312
| style="text-align: right;" | 640
|
|}
Gallery
<gallery widths="200" heights="200">
Penrhyn Aerial EFS 1280.jpg|Penrhyn atoll
Maldives.visibleearth.nasa.jpg|upright|NASA satellite image of some of the atolls of the Maldives, which consists of 1,322 islands arranged into 26 atolls
Nukuoro ISS013-E-28610.jpg|Nukuoro from space. Courtesy NASA
Los Roques.png|Los Roques Archipelago in Venezuela, the largest marine national park in Latin America, F. S. McNeil, E. G. Prudy,
<gallery widths="200" heights="160">
Atoll forming-volcano.png|Darwin's theory starts with a volcanic island which becomes extinct
Atoll forming-Fringing reef.png|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
Atoll forming-Barrier reef.png|As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeper lagoon inside
Atoll forming-Atoll.png|Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon
</gallery>
Coral atolls are important as sites where dolomitization of calcite occurs. Several models have been proposed for the dolomitization of calcite and aragonite within them. They are the evaporative, seepage-reflux, mixing-zone, burial, and seawater models. Although the origin of replacement dolomites remains problematic and controversial, it is generally accepted that seawater was the source of magnesium for dolomitization and the fluid in which calcite was dolomitized to form the dolomites found within atolls. Various processes have been invoked to drive large amounts of seawater through an atoll in order for dolomitization to occur.
Royal Society expeditions 1896–98
thumb|The [[Wake Island atoll in the Pacific Islands Heritage Marine National Monument]]
In 1896, 1897 and 1898, the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs. They wanted 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 in the Pacific.
The first expedition in 1896 was led by William Johnson Sollas of the University of Oxford. Geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. David led the expedition in 1897.
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Sources
- Dobbs, David (2005). Reef Madness: Charles Darwin, Alexander Agassiz, and the Meaning of Coral. Pantheon. .
- Fairbridge, R. W. (July 1950). "Recent and Pleistocene Coral Reefs of Australia". J. Geol., 58(4: Reef Issue): 330–401. . . .
- McNeil, F. S. (July 1954). "Organic Reefs and Banks and Associated Detrital Sediments". Amer. J. Sci., 252(7): 385–401. .
External links
- Formation of Bermuda reefs
- Darwin's Volcano – A short video discussing Darwin and Agassiz' coral reef formation debate
- NOAA National Ocean Service Education – Coral Atoll Animation
- NOAA National Ocean Service – What are the three main types of coral reefs?
- Research Article: Predicting Coral Recruitment in Palau's Complex Reef Archipelago;
- World Atolls, Goldberg 2016: A global map containing all atolls