Atoll: Encyclopedia II - Atoll - Distribution and size

Atoll - Distribution and size

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 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).

Adapted from the Wikipedia article "Distribution and size", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki