Atolls are coral reefs found in the open ocean consisting of an annular rim surrounding a central lagoon. There is a general presumption that atolls have volcanic foundations and some 425 have been recognized around the world (Wiens 1962; McLean and Woodroffe 1994). However, because of similar superficial morphology many shallower water coral reefs have been termed “shelf atolls” (Ladd 1977). In Indonesian waters 55 such reefs have been recognized though many will not have the volcanic foundations of the majority of atolls found in the Pacific and Indian Oceans.
The largest atoll is Kwajalein in the Marshall Islands (120 × 32 km2) followed by Rangiroa in the Tuamotus (79 × 34 km2), though many smaller atolls are only a few kilometers in diameter. In a sample of 99 atolls, Stoddart (1965) indicated a mean area of 272.5 km2, and atolls worldwide are considered to have a total area of 115,000 km2.
The Contribution of Atoll Studies to Marine Geology
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