Why has Chuuk not become a classic coral atoll despite being surrounded by a reef?

The Chuuk Islands are considered an anomaly in the Pacific because, unlike many other volcanic islands that have followed the classic progression toward becoming an atoll, they have not yet subsided beneath sea level.4

According to the subsidence theory of reef development, an atoll is typically the final stage of a process where a volcanic island gradually sinks, allowing the surrounding reef to grow upward and eventually remain as a ring of coral enclosing a lagoon once the central landmass has disappeared. While the Chuuk Islands are encircled by a barrier reef, the volcanic peaks within the group remain elevated—some rising to more than 500 feet (150 metres)—meaning the islands have not yet completed the geological transition required to become a classic atoll.1234