When a volcano emerges from tropical seas, it creates conditions favourable for the development of a fringe-like coral reef, or fringe reef, about its sloping coasts. A coral bank develops upward and outward, increasing in thickness and width.
After the phases of eruption and immersion, the third phase in that the volcano slowly sinks. This process can result in the formation of a barrier reef when a channel of deep water forms between the island and the fringe reef. The old fringe reef is often found to be detached from the coast because of the sinking.
When the volcano finally submerges below the surface, only the ring of coral around the old island remains, and an atoll is born.