An atoll is a ring of coral reef and small sandy
islands that encircles a shallow lagoon. Atolls are
common in the tropical Pacific Ocean, where such
groups of islands as the Marshall Islands and Kiribati are chains of atolls. They form when
volcanic islands move away from the heating
anomaly that creates them, as in the Hawaiian
island hot-spot trace (p. 98), and they begin to
subside beneath sea level. Reefs initially form as a
fringe in the shallow waters around a volcanic
island, as in Tahiti. With time, the island erodes
and subsides. However, the reef continues growing
upwards to create an offshore barrier reef separ -
ated from the main island by a lagoon, as in the
case of Bora Bora in the Society Islands. The lifting
of a reef above sea level creates a raised atoll. These
often have spectacular cave landscapes.