What theory did Darwin propose to explain coral reef formation?

Darwin proposed the subsidence theory to explain coral reef formation. He argued that coral reefs developed around volcanic islands as the islands and their underlying oceanic crust gradually sank.

Darwin’s model described three stages: a fringing reef formed close to an island, then became a barrier reef separated from land by a lagoon, and finally remained as an atoll after the island disappeared below sea level. Coral could continue growing upward because reef-building organisms lived only in shallow, sunlit water.

Darwin developed the idea during the Beagle voyage and published it in The Structure and Distribution of Coral Reefs in 1842. Observations from the Keeling Islands, now called the Cocos (Keeling) Islands, helped him test the model. Modern research supports the central role of subsidence, although sea-level changes and plate tectonics also affect reef development. A common mix-up is to call this simply a volcanic theory: volcanoes supplied the original island foundations, but subsidence explains the reef’s changing shape.