An atoll is a reef or island indicative of an undersea volcano.

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An atoll is actually an island formed from coral reefs surrounding a central lagoon, usually without any sign of a volcano. The reefs are formed around a sinking island or seamount, and as the island sinks, the coral continues to grow upward, forming a ring-shaped reef. Over time, sand and sediment accumulate on the reef, creating a small island in the center. However, some atolls may be situated over a volcanic caldera which has subsided and produced an atoll (known as a volcano-seamount-atoll).

An Atoll Is a Reef or Island Indicative of an Undersea