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. In terms of physical dimensions, the largest atolls are found in the Maldives.

Mode of formation

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 &#151 from high volcanic island, through barrier reef island, to atoll &#151 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] http://www.lifelineaquarium.com/charles_darwin/charles_darwin_index.htm .
• 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.