Atoll
For other senses of this word, see atoll (disambiguation).An
atoll is a type of low,
coral island found in
tropical oceans and consisting of a
coral-algal reef surrounding a central depression. The depression may be part of the emergent island, but more typically is a part of the sea (that is, a
lagoon), or very rarely is an enclosed body of
fresh,
brackish, or highly salinated water.
The word
atoll comes from the
Dhivehi (an
Indo-Aryan language spoken on the
Maldive Islands) word
atholhu (
Dhivehi: އަތޮޅު). Its first recorded use in
English was in
1625. However, the term was popularised by
Charles Darwin (1842, p. 2), who described atolls as a subset in a special class of islands, the unique property of which is the presence of an organic reef. 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 in the centre."
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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 published an explanation for 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), eventually becoming a barrier reef island (as typified by an island such as
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 300 feet 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.
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Portion of a Pacific atoll showing two islets on the ribbon or barrier reef separated by a deep pass between the ocean and the lagoon. |
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 and
Tuvalu) and
Indian Ocean (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.
As noted above, reef-building corals can 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.
In terms of total area (lagoon plus reef), the largest atolls are found in the Maldives:
Huvadhoo Atoll, having an area of 2800 (or 3200?) km²; the area of Thiladhunmathi and Miladhunmadulu Atolls (two names, but a single atoll structure) is even larger at 3680 km². Another large atoll is
Lihou Reef in the
Coral Sea, with a lagoon of 2,500 km². However, by far the largest atoll structure of the world is the
Great Chagos Bank in the Indian Ocean, a mostly submerged atoll, part of the Chagos Islands, with an area of roughly 13,000 km². If
Saya de Malha Bank were to be recognized as a wholly submerged atoll structure, it would be the world's largest, at 40,000 km².
Kwajalein in the Marshall Islands, which is sometimes listed as the largest atoll of the world (largest in the Pacific), measures only 846 km², by contrast. Large atolls are also found in the Tuamotu Archipelago, the largest being
Rangiroa, with a lagoon area of 1018 km².
In most cases, the land area of an atoll is very small in comparison to the total area. The largest atoll in the world in terms of "land area" (land "permanently" above sea level) is
Kiritimati (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). If the
Caicos Islands are to be considered a huge coral atoll, with the
Caicos Bank as a lagoon, this complex would be the largest atoll in land area (460.2 km²) and second largest in total area (lagoon size roughly 3700 km²).
The remains of an ancient atoll as a hill in a limestone area is called a
reef knoll.
*Darwin, C. 1842.
The structure and distribution of coral reefs. London. This book is
available online.
*Dobbs, David. 2005.
Reef Madness : Charles Darwin, Alexander Agassiz, and the Meaning of Coral. Pantheon. ISBN 0375421610.
*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.
*
Formation of Bermuda reefs
