Atoll in the western pacific ocean

For the programming language, see ATOLL programming language.

An atoll is a type of low, coral island found in the tropical ocean consisting of a coral-algal reef surrounding a central depression. The depression may be part of the emergent island or part of the sea (that is, a lagoon), or more rarely an enclosed body of fresh, brackish, or highly saline water.

Definition and etymology

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."

The word atoll comes from the Dhivehi (Indo-Aryan language of the Maldive Islands) word atolu. Its first recorded use in English was in 1625.

Size: largest atolls

In terms of total area (lagoon plus land), some of the largest atolls are found in the Maldives, such as Huvadhoo Atoll , with a lagoon covering an area of 2238 km². According to other sources, Huvadhoo measures 2800 or even 3200 km² (the difference cannot be explained by including the land area, since the total land area of the Maldives is only 298 km²). The area of Thiladhunmathi and Miladhunmadulu Atolls (with two names, but a single atoll structure) is even larger, some 3680 km².

One of the largest atolls is also Lihou Reef in the Coral Sea, with a lagoon of 2500 km² (which contrasts to a minuscule land area of 0.91 km²). However, by far the largest atoll structure of the world is the Great Chagos Bank in the Indian Ocean, a mostly submerged atoll, with some islands only on the Western and Northern rim, part of the Chagos Islands, with an area of roughly 13000 km² (the land area of the islands measuring less than 10 km²). Kwajalein in the Marshall Islands, which is sometimes listed as the largest atoll of the world, 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 (land area 79 km²).

While in the above examples, the land area of the atolls is very small in comparison to the total area, the largest atolls of the world in terms of land area are 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) and Aldabra (155.4 km² land, 224 km² lagoon). If the Caicos Islands, however, are to be considered a huge coral atoll, with the Caicos Bank as lagoon, this complex would be the the largest atoll in land area (460.2 km²) and second largest in total area (lagoon size roughly 3700 km²).

Geographic distribution

Since reef-building corals can thrive only in the warm tropical and subtropical waters of the oceans, most atolls can be found in the tropics and subtropics. The northernmost atoll of the world is Bermuda, at a latitude of 32°24' North. At this latitude no coral atoll would be possible without the gulf stream. Next to Bermuda, the northernmost atolls are Kure Atoll at 28°24' North, and the other atolls of the Northwestern Hawaiian Islands. The southernmost atolls of the world are Elizabeth Reef at 29°58' South, and nearby Middleton Reef at 29°29', in the Tasman Sea, which both belong to the Coral Sea Islands Territory. Those two atolls are submerged except at low tide. The Southernmost coral reef of the world, although not forming a true atoll, stretches along the Western shores of Lord Howe Island down to 31°34' South. The southermost atoll with a permanent land area is Ducie Island at 24°40' South.

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.

Concentrations of atolls in the Indian Ocean are the Maldives, the Laccadive Islands, the Chagos Archipelago and the Outer Islands of the Seychelles.

The Atlantic Ocean ocean has no big groups of atolls, except perhaps the group of eight atolls East of Nicaragua that belong to the Colombian department of San Andres and Providencia, and a comparatively small number of atolls.

Mode of formation

Portion of a Pacific atoll

Charles Darwin published an explanation for the creation of coral atolls in the South Pacific (Darwin, 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 an original 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 where conditions are less favorable for the 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 is an atoll. Because atolls are the product of the growth of tropical marine organisms, these islands are only found in the tropical ocean. Volcanic islands located beyond the warm water temperature requirements of reef bulding (hermatypic) organisms become seamounts as they subside and are eroded away.

Reginald Aldworth Daly offered a somewhat different explanation: 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, favored the Darwin explanation, although there can be little doubt that fluctuating sea level has had considerable influence on atoll and other reefs.

The distribution of atolls around the globe is instructive: they are mostly limited to the oceanic basins of the Pacific and Indian oceans. Very few atolls are found in the Atlantic.

References

• Darwin, C. 1842. The structure and distribution of coral reefs. London. This book is available online at [1].
• 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.