Which process best explains the formation of an atoll?

Which process best explains the formation of an atoll?

• A. Growth of a coral reef around a sinking volcanic island, leaving a ring-shaped reef.
• B. Submergence of a volcanic island without reef formation.
• C. A barrier island migrating landward.
• D. A shoal formed by wind-blown sand accumulating near shore.

Answer: (A)

The process tested is how a coral reef can keep pace with the sinking of a volcanic island to form a circular reef around a central lagoon. As a volcanic island subsides, corals continue to grow upward into shallow, sunlit waters. Over time the island may disappear below the surface, but the reef persists, expanding outward and creating a ring-shaped formation with a central lagoon—the atoll. This sequence—reef growth around a sinking island leading to a ring around a lagoon—is why the described process best explains atoll formation. Submerging volcanic islands without reef growth, migrating barrier islands, or wind-blown sand shoals do not produce the same circular reef with a central lagoon.

The process tested is how a coral reef can keep pace with the sinking of a volcanic island to form a circular reef around a central lagoon. As a volcanic island subsides, corals continue to grow upward into shallow, sunlit waters. Over time the island may disappear below the surface, but the reef persists, expanding outward and creating a ring-shaped formation with a central lagoon—the atoll. This sequence—reef growth around a sinking island leading to a ring around a lagoon—is why the described process best explains atoll formation. Submerging volcanic islands without reef growth, migrating barrier islands, or wind-blown sand shoals do not produce the same circular reef with a central lagoon.