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How does an atoll form?

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Quick answer

An atoll forms in three stages. First, coral grows as a fringing reef around a tropical volcanic island. Next, the extinct volcano slowly sinks, while the coral grows upward and becomes a barrier reef around a lagoon. Finally, the island disappears below the sea, leaving a ring of reef and sandy islets around a lagoon.

An atoll forms when a coral reef grows around a volcanic island that slowly sinks, so the reef keeps building upward while the island disappears beneath the sea. What remains is a ring of coral reef and low sandy islands surrounding a shallow central lagoon.

Step by step

  1. An undersea volcano erupts and builds an island in warm tropical ocean water.
  2. Corals settle in the shallow water around its shore and grow into a fringing reef.
  3. The extinct volcano and the cooling crust beneath it slowly sink, opening a lagoon between island and reef.
  4. The reef keeps growing upward toward the light, becoming a barrier reef as the island shrinks.
  5. The volcanic peak sinks out of sight, leaving a ring-shaped reef and sandy islets around a central lagoon.

Key things to understand

  1. Reef-building corals grow in warm, clear, sunlit tropical water, forming a fringing reef around a new volcanic island.
  2. Once the volcano becomes extinct, the ocean crust beneath it cools, contracts and slowly sinks.
  3. Corals keep growing upward to stay in shallow, sunlit water, while the island shrinks inside a widening lagoon.
  4. When the volcanic peak finally sinks below sea level, only a ring-shaped reef surrounding a lagoon is left.
  5. Waves pile broken coral and shell into sandy islets on the reef, which people and plants can then settle.

Labelled diagram

  1. Fringing reef

    Corals grow in shallow water hugging the shore of a young volcanic island.

  2. Barrier reef

    The island sinks and a lagoon opens between the shrinking island and reef.

  3. Atoll

    The volcano is submerged, leaving a ring of reef around a lagoon.

  4. Motu islets

    Waves heap coral sand and rubble into low islands on the reef rim.

Three stages from volcanic island to atoll

Real-world examples

  • India's Lakshadweep islands are mostly coral atolls and reefs in the Arabian Sea.
  • The Maldives is a chain of roughly two dozen natural atolls, a word taken from the Dhivehi language.
  • Enewetak Atoll in the Marshall Islands is where deep drilling confirmed volcanic rock beneath the reef.
  • Aldabra in the Seychelles is a raised atoll famous for its giant tortoises.

Common mistake

✗ Myth: An atoll is the rim of a sunken volcanic crater filled with seawater.

✓ Fact: The ring shape comes from coral growing around the edge of a sinking island, not from a crater. The lagoon sits over the submerged volcano, buried under hundreds of metres of reef limestone.

People also ask

How are coral atolls formed?
Coral atolls form when a fringing reef surrounds a volcanic island in warm tropical water. As the extinct volcano and the ocean floor slowly subside, corals build upward to stay near the sunlit surface. Eventually the island sinks completely, leaving a circular reef enclosing a lagoon, topped with sandy islets made of broken coral.
What is the difference between an atoll and an island?
An ordinary island is usually land rising from bedrock, such as a volcano or a piece of continent. An atoll is a ring of coral reef with low sandy islets around a lagoon, built by living organisms on top of a sunken volcano. Atoll islands are generally just a few metres high.
Why do atolls have a lagoon in the middle?
The lagoon marks where the volcanic island used to stand. Corals grow fastest on the outer edge, where waves bring nutrients and oxygen-rich water, so the reef rim keeps pace with sinking. The centre, once land, sank below the waves and became a sheltered, shallow lagoon.
Who first explained how atolls form?
Charles Darwin proposed the subsidence theory after his voyage on HMS Beagle in the 1830s. Drilling on Enewetak Atoll in the 1950s confirmed it, striking volcanic rock beneath more than 1,200 metres of reef limestone.
Why do corals need shallow water?
Reef-building corals host tiny algae inside their tissues that photosynthesise and supply much of the coral's food. Those algae need sunlight, which fades quickly with depth, so corals thrive only in shallow, clear water.
How long does an atoll take to form?
Typically millions of years, because the sinking of the volcanic island and the upward growth of the reef are both very slow, usually a few millimetres per year or less on average.
Why are atolls at risk from rising seas?
Atoll islands are mostly only a few metres above sea level, built of loose coral sand. Rising seas, stronger storm waves and coral damage from warming water threaten flooding, erosion and freshwater supplies.

Part of the guide: Nature & Earth: Living Things and Rocks →