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How does a coral atoll form?

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Map of the Pacific Islands region showing political boundaries, major island groups, and countries such as Papua New Guinea, Fiji, Samoa, French Polynesia, and Kiribati. The map uses color shading to indicate elevation and ocean depth, with key islands and capitals labeled throughout the central and southwestern Pacific Ocean.
Pacific Islands

A coral atoll forms through a process of continuous growth around a sinking foundation. The most widely accepted explanation, originally proposed by Charles Darwin, describes this as the final stage of reef development.12

The Subsidence Theory

  1. Fringing Reef: Initially, coral reefs grow in shallow waters fringing a volcanic island.12
  2. Barrier Reef: As the volcanic island gradually subsides or the sea level rises, the reef continues to grow upward and outward to remain in the shallow, sunlit waters it requires. This creates a barrier reef separated from the island by a lagoon.12
  3. Atoll: Eventually, the volcanic island sinks completely below the surface, leaving behind a ring-shaped reef that encloses a central lagoon.12

Alternative Processes

While subsidence is a primary driver, atolls can also form through other mechanisms. For example, eustatic sea-level changes—often linked to cycles of glaciation—can influence reef growth. Additionally, some annular reefs develop when uplifted limestone platforms undergo subaerial erosion, creating a saucer-shaped interior that becomes a lagoon upon subsequent submergence.12