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How does a barrier reef become an atoll?

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coral reef The Great Barrier Reef, off the coast of Queensland, Australia, is the world's largest coral reef.

According to the subsidence theory proposed by Charles Darwin, a barrier reef evolves into an atoll through the continued sinking of the underlying volcanic island.12

As the volcanic foundation subsides, the coral reef continues to grow upward and outward to maintain its position near the sea level, where conditions are optimal for growth. During the barrier-reef stage, the reef is separated from the volcanic shoreline by a lagoon. As the volcanic peak eventually sinks completely beneath the surface, the reef remains as a ring-shaped structure enclosing a central lagoon, marking the final stage of development.13

While subsidence is a primary driver, modern geological understanding also incorporates the effects of sea level fluctuations, such as those caused by Pleistocene Epoch glaciation cycles, which can influence the upward growth and submergence of these structures.13