atolls

An atoll is a ring-shaped island formed from coral reefs that encloses a central lagoon. Most atolls develop as a fringing reef around a volcanic island; as the island subsides, reef growth keeps pace with sea level, producing a circular or oval reef rim with a central lagoon. This Darwinian model explains many atolls, though variations exist, including reefs built around seamounts and alternative subsidence histories.

Structure: The reef rim often hosts sand cay islets. A central lagoon is usually shallow and braided, with passes allowing tidal exchange to the open sea. Atoll size varies, typically a few to several tens of kilometers in diameter. The interior is often low-lying, with limited freshwater and soils, supporting sparse human settlement.

Distribution and ecology: Atolls are common in the tropical Indian and Pacific Oceans, with notable examples in the Maldives, Kiribati, Marshall Islands, and Tuamotu Archipelago. They host coral communities that form the reef framework and support diverse fish, invertebrates, seabirds, and sea turtles. Lagoons may contain brackish pools that harbor specialised organisms.

Human use and threats: Some atolls are inhabited and used for fishing or tourism; freshwater scarcity and limited land constrain agriculture. They are highly vulnerable to sea-level rise, ocean warming and bleaching, storms, and overfishing. Conservation efforts focus on protecting reefs, promoting sustainable tourism, and implementing climate adaptation strategies.