What is the term for a coral island or series of islands surrounding a lagoon?

The term for a coral island or series of islands surrounding a lagoon is atoll. Atolls are formed from coral reefs that develop on the rim of submerged volcanic islands. Over time, as the volcanic island erodes away and sinks, the coral continues to grow upwards, forming a ring-shaped island with a central lagoon. This unique formation creates an ecosystem that supports diverse marine life and often features shallow waters, making it an important area for biodiversity. The structure of atolls is significant in marine geography and helps in understanding how coral reefs contribute to coastal protection and marine habitats.