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How did Charles Darwin explain the formation of atolls?
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coral reef
The Great Barrier Reef, off the coast of Queensland, Australia, is the world's largest coral reef.
In 1842, English naturalist Charles Darwin proposed a theory to explain the formation of atolls that remains fundamentally valid today.1
Darwin hypothesized that atolls represent the final stage of a continuous process of reef development around a sinking volcanic island. His model follows these stages:2
- Fringing Reef: Initially, a reef grows in shallow water directly around the perimeter of a volcanic island.12
- Barrier Reef: As the volcanic foundation gradually undergoes subsidence, the reef continues to grow upward and outward to remain in the sunlit, shallow waters necessary for coral survival. This upward growth creates a lagoon between the reef and the receding shoreline.12
- Atoll: Eventually, the volcanic cone subsides completely beneath the sea level. The remaining reef structure forms a circular or annular shape, enclosing a central lagoon where the island once stood.12
Geologic evidence, such as drilling at Enewetak atoll, has confirmed this theory by revealing volcanic rock deep beneath the modern reef surface.2