An atoll ( /ˈætɒl/, /ˈætɔːl/, /ˈætoʊl/, /əˈtɒl/,
/əˈtɔːl/ or /əˈtoʊl/), 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.(p60) 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
2 Distribution and size
4 Investigation by the
Royal Society of London
Royal Society of London into the formation of
5 United States national monuments
6 See also
8 External links
The word atoll comes from the Dhivehi (an
Indo-Aryan language spoken
on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު,
[ˈət̪ɔɭu]), meaning an administrative subdivision.
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
Coral Reefs, the definition of atolls as "circular
groups of coral islets" that is synonymous with
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" or "in an exclusively
morphological sense, [as] a ring-shaped ribbon reef enclosing a
Distribution and size
NASA satellite image of some of the atolls of the Maldives, which
consists of 1,322 islands arranged into 26 atolls.
Nukuoro from space. Courtesy NASA.
Los Roques Archipelago
Los Roques Archipelago in Venezuela, the largest marine National Park
in Latin America, from space. Courtesy NASA.
View of the coast of
Bikini Atoll from above
Most of the world's atolls are in the Pacific
concentrations in the Tuamotu Islands, Caroline Islands, Marshall
Coral Sea Islands, and the island groups of Kiribati, Tuvalu
and Tokelau) and
Indian Ocean (the Atolls of the Maldives, the
Lakshadweep 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
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 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
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
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 of Kiribati, with its southern
tip just 12 km north of the equator.
Largest atolls by total area (lagoon plus reef and dry land)
Great Chagos Bank
6°10′S 72°00′E / 6.17°S 72.00°E / -6.17; 72.00
Land area 4.5 km²
11°27′N 116°54′E / 11.45°N 116.90°E / 11.45; 116.90
Submerged, at shallowest 9 m
16°00′N 114°30′E / 16.00°N 114.50°E / 16.00; 114.50
South China Sea
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
Submerged, at shallowest <10 m
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
Land area 14.87 km²
16°26′N 80°31′W / 16.43°N 80.52°W / 16.43; -80.52
Submerged, at shallowest 7.3 m
6°44′N 73°02′E / 6.73°N 73.04°E / 6.73; 73.04
Two names, but a single atoll structure;
land area 51 km²
19°21′S 158°40′E / 19.35°S 158.66°E / -19.35; 158.66
Land area <10 km²
0°30′N 73°18′E / 0.50°N 73.30°E / 0.50; 73.30
Land area 38.5 km²
7°25′N 151°47′E / 7.42°N 151.78°E / 7.42; 151.78
6°45′S 118°50′E / 6.75°S 118.83°E / -6.75; 118.83
3°51′N 154°56′E / 3.85°N 154.94°E / 3.85; 154.94
Land area 1.7 km², divided among more
than 40 islets that lie on the northern, eastern
and southern sides of the lagoon
17°25′S 151°40′E / 17.42°S 151.67°E / -17.42; 151.67
Land area 1 km²
Bassas de Pedro
13°05′N 72°25′E / 13.08°N 72.42°E / 13.08; 72.42
Submerged, at shallowest 16.4 m
7°43′N 114°15′E / 7.71°N 114.25°E / 7.71; 114.25
Cay on the south side?
9°11′N 167°28′E / 9.19°N 167.47°E / 9.19; 167.47
Land area 16.4 km²
Diamond Islets Bank
17°25′S 150°58′E / 17.42°S 150.96°E / -17.42; 150.96
Land area <1 km²
8°40′N 150°00′E / 8.67°N 150.00°E / 8.67; 150.00
Land area 4.4 km²
3°52′N 72°50′E / 3.86°N 72.83°E / 3.86; 72.83
Land area 69 km²
25°25′N 170°35′W / 25.42°N 170.59°W / 25.42; -170.59
Northwestern Hawaiian Islands
15°08′S 147°39′W / 15.13°S 147.65°W / -15.13; -147.65
Land area 79 km²
2°22′N 73°07′E / 2.37°N 73.12°E / 2.37; 73.12
Land area 79 km²
Kaafu Atoll (North Malé Atoll)
4°25′N 73°30′E / 4.42°N 73.50°E / 4.42; 73.50
Land area 69 km²
5°16′S 159°21′E / 5.27°S 159.35°E / -5.27; 159.35
Land area 12 km²
Raa Atoll in Maldives
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 5 meters (9). Measured by total area,
Lifou (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
Huvadhu Atoll in the south of the
Maldives with 255
Map from Charles Darwin’s 1842 The Structure and Distribution of
Coral Reefs showing the world’s major groups of atolls and coral
See also: Formation of coral reefs
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Aerial view of Bora Bora
Charles Darwin explained the creation of coral atolls in the
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 in the Cook
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
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 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.
Investigation by the
Royal Society of London
Royal Society of London into the formation of
In 1896, 1897 and 1898, the
Royal Society of London
Royal Society of London carried out
Funafuti atoll in
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 in the
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 of the University of Sydney.
Edgeworth David led the expedition in 1897. The third
expedition in 1898 was led by Alfred Edmund Finckh.
United States national monuments
Aerial overview of the
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.(Number 1, page 14)
^ pronunciation in old video on youtube,
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
^ 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 Area, Depth and Rainfall (2001) spreadsheet from The
Geological Society of America. Retrieved d.d. October 1, 2009.
^ "Misinformation about Islands". worldislandinfo.com.
^ David, Cara (Caroline Martha) (1899).
Funafuti or Three Months On A
Coral Atoll: an unscientific account of a scientific expedition.
London: John Murray. ASIN B00A1YMOFO.
^ 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
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
^ "Weekly Compilation of Presidential Documents" (PDF). 12 January
2009. Archived from the original (PDF) on March 1, 2009.
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.
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