Brian Knutsen courtesy MTPB
Atolls are often called islands but they aren't the same thing at all. Islands are pieces of land surrounded by water. Atolls are ring-shaped coral reefs with a lagoon in the center. And while some islands are the tips of underwater volcanoes, all atolls get their start from this landform.
How is an atoll formed? Corals grow on underwater volcanoes just below the surface of the water. As a volcano cools it becomes less dense and sinks. But because coral reefs need light to survive they build themselves upward as the volcano descends. When the volcano sinks so far that it disappears the only thing left is the round coral ring.
Why do some atolls have trees? Soil often forms on the top of the coral reef enabling plants to grow and giving atolls an island-like look.
How long does it take for an atoll to form? It can take as much as 30 million years.
Where can I see one? Atolls are found in the warm waters of the Pacific and Indian oceans. Coral reefs can't grow in cold water after all.
