An atoll is a ring-shaped island made of a coral rim that encircles a lagoon. It forms in warm tropical seas, usually on top of a volcano that has slowly sunk beneath the waves.
Key takeaways
- An atoll is a coral rim around a lagoon, and it only forms where the water is warm enough for corals to grow.
- Charles Darwin's explanation is that a volcanic island sinks while the reef around it keeps growing upwards.
- About 440 atolls exist worldwide, and most are in the Pacific Ocean, which has an average depth of 4,315 m.
- If the corals cannot keep pace with the sinking, the volcano becomes a drowned, flat-topped guyot instead.
How an atoll forms
An atoll is a ring of coral, sometimes with small islands called cays on it, that surrounds a lagoon. Atolls form only in warm tropical or subtropical water, where corals can grow [1].
The usual explanation was proposed by Charles Darwin and is called the subsidence model. It has four steps [1]:
1. A volcanic island forms in warm tropical water. 2. Coral grows around the edge of the island as a fringing reef. 3. The island sinks gradually. The reef keeps growing upwards near the sea surface and separates from the shore, leaving a widening lagoon. 4. The island eventually sinks below the surface and erodes away. The reef and its small islets remain, enclosing the lagoon.
The key point is that coral must keep pace with the sinking. If the reef cannot grow upwards fast enough, it drowns [1]. Corals only live close to the surface, so the reef is effectively a living platform that stays at sea level while the volcano below it goes down.
The lagoon is not the old volcanic crater. It forms because the inner part of the reef falls behind the outer reef, where conditions are less favourable for coral growth [1]. Wikipedia also notes a competing explanation, the antecedent karst model, which its authors say challenges Darwin's model for modern atolls [1].
About 440 atolls exist worldwide, and most are in the Pacific Ocean. Major groups include the Caroline Islands, Marshall Islands, Tuamotu Islands, Kiribati, Tokelau and Tuvalu. In the Indian Ocean they include the Maldives and the Chagos Archipelago [1].
An example from our data
The map draws the seabed from a grid and does not mark coral reefs, but the undersea half of the story is easy to see. Limalok Guyot has a summit 1,172 m below the surface, only 61 km from the Marshall Islands, a group where atolls are concentrated [1]. Sylvania Tablemount has a summit 1,296 m down, 51 km from the same group. A flat-topped, sunk volcano like these is the other possible outcome of the same story: corals that could not keep up [2].
All of this happens in the Pacific Ocean, which covers 139.1 million km² and has a mean depth of 4,315 m in the grid. Atolls sit on top of volcanoes that rise from that deep seabed. You can tap any part of the map for the depth, and the homepage explains how deep the sea is on average.
The thing people get wrong
Many people think an atoll's lagoon is the crater of a volcano. In fact the volcano has sunk well below the surface and eroded away, and the lagoon is just the water enclosed by the reef [1]. The volcanic rock is still there, but it is buried far beneath the reef.
A second misconception is that atolls are flat, simple rings sitting on the sea floor. Each one is the top of a much larger structure built on a sunken volcano, so the true size of an atoll is mostly below the water.
Atoll vs guyot
| Atoll | Guyot | |
|---|---|---|
| What you see | A ring of coral at the surface | Nothing at the surface |
| Corals | Alive, keeping pace with the sinking | Sank too deep to grow |
| Top | Reef rim around a lagoon | Flat summit more than 200 m below the surface [2] |
Frequently asked questions
Is an atoll the crater of a volcano?
No. The lagoon is not the old volcanic crater. It forms because the inner part of the reef grows more slowly than the outer part, and the volcano itself has sunk below the surface.
Where are most atolls?
Most are in the Pacific Ocean, in groups such as the Marshall Islands. Others are in the Indian Ocean, including the Maldives.
Sources
- Atoll, Wikipedia.
- Guyot, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).