How do atolls and lagoons form as volcanic islands subside?

The formation of atolls and their central lagoons is primarily explained by the subsidence theory proposed by Charles Darwin.12

The process follows a progressive sequence:

• Fringing Reef Stage: Coral reefs initially grow in shallow waters fringing a volcanic island.12
• Barrier Reef Stage: As the volcanic island gradually subsides, the reef continues to grow upward and outward to remain in optimal shallow-water conditions. This upward growth creates an intervening stretch of water between the reef and the sinking shoreline, forming a barrier reef and a lagoon.12
• Atoll Stage: Eventually, the volcanic island subsides completely below sea level. The reef structure remains as a circular or closed-shape rim reaching the surface, enclosing a deeper central area known as a lagoon.12

While subsidence is a primary driver, plate-tectonic theory and fluctuations in sea level—often linked to cycles of glaciation—also play significant roles in the development and current configuration of these structures.12