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Coral atolls

Bora Bora, a coral atoll in the Pacific Ocean Layers of lava from eruptions eventually build underwater volcanoes high enough for them to rise above sea level as islands. In tropical waters, coral reefs sometimes form in the warm shallows around these volcanic islands. After they stop erupting, the volcanoes will, over thousands of years, sink back into the Earth's crust. But the reef keeps growing upwards so that when the islands eventually disappear below water, ring-shaped reefs, called atolls, remain. Most atolls are in the South Pacific Ocean and Indian Ocean. Many are inhabited.



Stages in the formation of an atoll

How an atoll forms

When magma stops rising up through a volcano—as when the volcanic island no longer lies above a hot spot—the volcano will sink back into the ocean floor. If it sinks very gradually, coral that formed a fringing reef around it will carry on growing upwards (1). Coral animals must stay near the sunlight, so they will build up the reef in order to remain close to the surface.
Eventually the coral will form a circular barrier reef, enclosing a lagoon around the dwindling volcanic island (2). By the time the volcanic island disappears completely, there is a ring of low-lying coral reefs, called an atoll (3). The Society Islands in the South Pacific include examples of a fringing reef (Tahiti), an “almost-atoll” (Bora Bora) and an atoll (Tetiaroa).
As a result of waves piling up sediment and reef rubble on top of the reef itself, some parts of an atoll may emerge above sea level as low-lying islands. In most cases, the land area of an atoll is tiny in comparison to the undersea portion.
An aerial view of Atafu atoll in Tokelau
An atoll's coral reef, teeming with life

Coral 

Atolls do not usually form complete circles around sunken volcanoes. They are frequently broken up in places by storms.

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