In Darwin's model, atolls begin around what kind of island?

In Darwin’s model, an atoll begins around a volcanic island. He proposed that a fringing coral reef first grows around a tropical oceanic volcano, while the volcanic island gradually subsides or sinks.

As the island descends, the reef continues growing upward toward sunlight and sea level. The original fringing reef therefore becomes separated from the shrinking island, forming a barrier reef with a lagoon between the reef and the land. Eventually, the volcanic island disappears beneath the sea, leaving a ring-shaped reef and lagoon: an atoll.

Charles Darwin developed this subsidence theory from observations made during the voyage of HMS Beagle and published it in 1842. Modern marine geology has identified additional formation processes and challenged parts of the classic model, but “volcanic island” remains the canonical answer to this question. Sandbars and coral debris can form islands on reefs, but they are not Darwin’s starting structure.