How did Charles Darwin explain the formation of atolls and their lagoons?

In 1842, Charles Darwin proposed that atolls and their lagoons form through the gradual subsidence of volcanic islands. According to his theory, the process occurs in three primary stages:12

• Fringing Reef: Initially, coral reefs grow in shallow waters directly fringing a volcanic island.12
• Barrier Reef: As the volcanic island slowly sinks, the reef continues to grow upward and outward to remain near the surface. This creates an intervening stretch of water between the reef and the island, forming a barrier reef.12
• Atoll: Eventually, the volcanic cone becomes completely submerged. The reef remains at the surface as a ring-shaped structure, enclosing a central lagoon where the island once stood.12

While modern science acknowledges that sea level fluctuations—particularly those following the Pleistocene Epoch—also influence reef development, Darwin’s subsidence model remains a fundamental explanation for the origin of these structures.12