How do coral atolls form?
An atoll is a ring-shaped coral reef surrounding a lagoon. In the typical sequence, a fringing reef around a volcanic island continues growing upward while the island subsides relative to sea level.
From reef to ring
Skeletons of hard corals accumulate in shallow, sunlit water. As the island sinks, the reef's living upper part can remain near the surface under suitable conditions. Once the island is submerged, the ring enclosing the lagoon becomes distinct. This takes a long time, and rates of subsidence and reef growth matter.
Lagoon and limits
Passages may connect the lagoon to the ocean, and sandy islets may develop on the reef. Not every ring-shaped island is an atoll, and not every volcanic island becomes one. Temperature, light, turbidity and water chemistry constrain coral growth. Reef structure, underlying rock and sea-level records help reconstruct an atoll's history.