Atolls are circular, oval, or horseshoe-shaped arrays of coral reef islands that are perched around an oceanic volcanic seamount and encircle a shallow central lagoon. The small islands are separated from each other by channels that lead from the sea into the central lagoon.
Distribution of Atolls
Because atoll formation requires coral reef building, atolls are limited to tropical waters. Atolls are most commonly found in the Pacific and Indian Oceans.
Size of Atolls
Atolls can vary greatly in size, ranging from quite small to over 130 kilometers (km) long and 32 km wide (Kwajalein in the Marshal Islands). The reef and lagoon area in some of the atolls of the Maldives are larger than 3000 km sq. Kiribati has a land area of about 390 km sq. Atoll islands are typically low lying, with elevations of less than 5 meters (m).
Formation of Atolls
Charles Darwin was one of the first people to think about the origins of coral atolls and his “subsidence theory of coral reef formation” has received empirical support. An atoll is thought to begin as living corals colonize and build a fringing reef on the flanks of a seamount or volcano. Over time, as the volcano cools and becomes denser, its gradually sinks below the sea surface. The corals, whose symbiotic algae require light to grow, continue to build the reef upward towards the sea surface, maintaining the top of the reef in the photic zone. The reef gradually becomes separated from the subsiding island by a lagoon, thus forming a barrier reef. Eventually the cold volcano sinks so far that it disappears beneath the surface, leaving behind the characteristic ring-shaped reef surrounding a central lagoon. Thus, atolls are built on thick layers of dead coral reefs. Scientists have drilled over 1400 m through coral limestone before striking volcanic rock on Pacific atolls, supporting Darwin’s hypothesis that atolls develop as fringing reefs subside. Raised atolls, such as Nauru and Niue, were produced when geologic action elevated the coral limestone above the surrounding sea.