The Lesser Antilles, an island arc in the West Indies, were built up by volcanic activity about 50 million years ago. Coral islands are low, flat islands that consist chiefly of coral reef material. Coral reefs are limestone formations composed of tiny sea organisms and their remains. They form and grow in warm, shallow water. Many coral islands develop from reefs that grow up around volcanic islands.
Some volcanic islands sink because of the movement of oceanic plates.
Others become submerged following a rise in the sea level. As islands sink or as the sea level rises, the reefs grow upward until only atolls remain. An atoll is a circular reef that surrounds a central body of water called a lagoon. Ocean waves break away parts of the reefs and pile up this material into flat, sandy islands. The Pacific Ocean has many atolls, including Bikini Atoll and Enewetak.