An atoll is a ring-shaped island that features a coral rim encircling a lagoon
. There may be coral islands or cays on the rim, and atolls are typically located in warm tropical or subtropical parts of the oceans and seas where corals can develop
. Most of the approximately 440 atolls in the world are in the Pacific Ocean
. The formation of an atoll is explained by two well-cited models: the subsidence and antecedent karst models
. According to Charles Darwin's subsidence model, an atoll forms around a sinking volcanic island, with a coral fringing reef developing around it
. Over geologic time, the volcanic island subsides, becoming extinct and eroded, while the coral fringing reef becomes a barrier reef detached from the island
. Eventually, only the reef and small coral islets on top of it remain, forming the atoll
. Key features of atolls include:
- Coral Rim : A ring-shaped coral reef that encircles a lagoon
- Lagoon : A body of water enclosed by the coral rim, which may be approximately 50 meters (160 feet) deep or more
- Barrier Reef : A stage in the development of an atoll where the reef structure becomes separated from the volcanic shoreline by an intervening stretch of lagoon water
Atolls have fascinated sailors and naturalists for centuries, and their formation process is still studied today
