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Why are atolls usually ring-shaped?

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

Atolls are typically ring-shaped because they represent the final stage of a long-term developmental process involving the growth of coral around a sinking volcanic island.12

As the volcanic island subsides or the sea level rises, the reef grows upward and outward to remain in the shallow, sunlit waters necessary for survival. Over time, the reef structure becomes separated from the volcanic shoreline by a lagoon. Once the volcanic cone is completely submerged, only the reef remains, forming a circular or annular rim around the central lagoon.12

Alternatively, some ring-shaped reefs develop through the erosion of uplifted limestone platforms. In this scenario, subaerial erosion creates a prominent rim and a saucer-shaped interior. When the platform subsequently submerges, coral and algal growth resumes most rapidly along the outer rim, maintaining the characteristic ring shape.1