Atoll facts for kids

The atoll of Nukuoro in the Federated States of Micronesia.

An atoll is a special type of island. It looks like a ring or a doughnut shape. Atolls are made of coral reefs. They form around an island that slowly sinks into the ocean. Over many years, the original land disappears. Only the coral reef keeps growing upwards. This creates the unique atoll shape.

Most atolls are found in warm waters. You can see them in the Pacific Ocean and the Indian Ocean. Some countries are made entirely or partly of atolls. Examples include Kiribati, Tuvalu, and the Federated States of Micronesia.

How Atolls Form

Darwin's Theory of Atoll Formation

The famous scientist Charles Darwin was the first to explain how atolls are made. He is best known for his theory of evolution. Darwin studied how these ring-shaped islands came to be.

Volcanoes and Coral Reefs

Darwin noticed that atolls often start with an underwater volcano. This volcano forms an island in the ocean. Tiny sea animals called polyps build coral reefs. These reefs grow around the edges of the volcanic island. Coral needs sunlight, so it grows close to the water's surface.

Island Sinking and Reef Growth

Over a very long time, the volcanic island slowly sinks. This can happen due to movements in the Earth's crust. As the island sinks, the coral reef keeps growing upwards. It grows to stay near the sunlight. The coral builds on top of itself, forming a thick ring.

From Reef to Atoll

Eventually, the original volcanic island sinks completely below the water. The coral reef continues to grow. It forms a circular or oval shape. This ring of coral creates a shallow body of water in the middle. This water is called a lagoon. The ring of coral and the lagoon together form an atoll.

How an atoll forms.

Images for kids

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  Satellite picture of the Atafu atoll in Tokelau in the Pacific Ocean

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  Penrhyn atoll

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  NASA satellite image of some of the atolls of the Maldives, which consists of 1,322 islands arranged into 26 atolls

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  Los Roques Archipelago in Venezuela, the largest marine national park in Latin America, from space. Courtesy NASA

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  View of the coast of Bikini Atoll from above

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  Raa Atoll in Maldives

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  Kaafu Atoll in Maldives

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  Map from Charles Darwin’s 1842 The Structure and Distribution of Coral Reefs showing the world’s major groups of atolls and coral reefs

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  Aerial view of Bora Bora

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  Tarawa Atoll

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  Bikini Atoll

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  Aerial overview of the Wake Island atoll, part of the Pacific Remote Islands Marine National Monument

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  Darwin's theory starts with a volcanic island which becomes extinct

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  As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef

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  As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeper lagoon inside

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  Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon

See also

In Spanish: Atolón para niños