How did Charles Darwin explain the formation of coral atolls?

English naturalist Charles Darwin proposed that coral atolls are the final stage of a continuous process of reef development around a sinking volcanic island.12

According to his theory, the process occurs in three progressive stages:

• Fringing Reef: Coral reefs initially grow in shallow waters directly fringing the coastline of a volcanic island.12
• Barrier Reef: As the volcanic island gradually subsides, the coral continues to grow upward to remain within the optimal heat and light conditions required for survival. This upward growth creates a barrier reef separated from the island by a growing lagoon.12
• Atoll: Eventually, the volcanic island sinks completely below the surface of the ocean. The remaining reef structure forms a circular or annular shape, enclosing a central lagoon where the island once stood.12

Darwin’s concept remains fundamentally valid today, though modern science also incorporates the effects of sea level changes and plate-tectonic theory, which explains how the ocean floor subsides as it spreads from oceanic ridges.13