atoll

An atoll is a ring-shaped coral reef that encloses a central lagoon and rises from the seabed. It typically consists of a continuous or broken reef crest with a surrounding ring of sand cays or motu along the rim, and a shallow interior lagoon. Atolls occur mainly in warm, sunlit tropical seas and can range from a few to several dozen kilometers in circumference.

Most atolls form through the subsidence of an volcanic island. Corals colonize the shallow subsiding platform, building outward as the island sinks. Over time the island becomes submerged, leaving a circular or oval reef that remains as a ring surrounding a central lagoon. In some areas a barrier reef or a raised platform can create a similar ring structure without a single prominent island. The term atoll derives from a Dhivehi word, atholhu, and was popularized in English by Charles Darwin.

Ecologically, atolls host diverse coral communities and associated organisms. The reef crests endure wave energy, while the interior lagoon supports a variety of fishes, invertebrates, and juvenile organisms that depend on the calmer, brackish to marine waters. Water conditions are typically warm, clear, and shallow, favoring calcareous reef-building corals.

Distribution and significance: atolls are widespread in the Indo-Pacific and parts of the Indian Ocean and central Pacific, with notable examples in the Maldives, Kiribati, the Marshall Islands, Tuamotu Archipelago, and Chagos. They support fisheries and, in many cases, tourism, but are vulnerable to threats such as climate change, ocean warming, acidification, sea-level rise, and human impacts on reef health.

History and naming: the word atoll comes from the Dhivehi word atholhu, describing reef islands, and entered scientific usage in the 19th century.