How do reef-building corals create reefs and atolls?

Reef-building corals, primarily stony corals, create reefs by constructing rigid, calcareous skeletons made of calcium carbonate. These colonial organisms grow in shallow, sunlit waters, where they form a symbiotic relationship with zooxanthellae—algae that accelerate skeletal formation through photosynthesis. As polyps divide and grow, they build upon one another, while other organisms like calcareous algae help cement the structure together.123

The development of atolls is often explained by the subsidence theory, famously proposed by Charles Darwin. According to this model:45

• Fringing Reefs: Corals initially grow as a reef fringing a volcanic island.45
• Barrier Reefs: As the island slowly subsides or sea levels rise, the reef continues to grow upward to remain in shallow, sunlit water, eventually becoming separated from the island by a lagoon.45
• Atolls: Once the volcanic cone is completely submerged, the remaining reef forms an annular shape around a central lagoon.45

This process relies on the corals' ability to maintain growth at the seaward edge, effectively keeping pace with environmental changes over thousands of years.45