Updated by: Arabinda Naik
This theory is put forth by Darwin. According to this theory, all known coral reefs are found in regions where subsidence of land has taken place. Both Darwin and Dana recognized that many coral reefs were associated with volcanic islands. They proposed that these islands were of volcanic origin, formed by the accumulation of lava from underwater volcanic eruptions. The central idea of the theory is that after the initial formation of a volcanic island, the land gradually subsides or sinks over time. This subsidence is thought to be caused by various geological processes, such as cooling and contraction of the Earth's crust, tectonic plate movements, or the weight of the accumulated coral reef itself. He reasons that, initially the corals grew as fringing reef on the sloping shores of an island in a shallow tropical sea. Subsidence of the islands commenced so that the fringing reef turned into barrier reef, separated from the island by a wide, deep-water channel or the lagoon. The island, while sinking became smaller and smaller and finally disappeared entirely beneath the surface of the ocean. By gradual sinking the island ultimately vanished and the barrier reef became a coral atoll with a central lagoon.
In time it acquired a growth of vegetation. As the volcanic island subsides, coral organisms continue to grow upward toward the sunlight. Coral polyps secrete calcium carbonate skeletons, forming the structure of the reef. The upward growth of coral colonies keeps pace with the gradual sinking of the underlying land.
Depending on the rate of subsidence and other environmental factors, different types of coral reefs are formed. Fringing reefs are proposed to develop close to the shorelines of sinking volcanic islands, barrier reefs at a greater distance with a lagoon in between, and atolls in the final stages, where the original island has completely submerged, leaving a ring of coral with a central lagoon.