<P> An atoll is an island formed from a coral reef that has grown on an eroded and submerged volcanic island . The reef rises to the surface of the water and forms a new island . Atolls are typically ring - shaped with a central lagoon . Examples are the Line Islands in the Pacific and the Maldives in the Indian Ocean . </P> <P> Approximately 45,000 tropical islands with an area of at least 5 hectares (12 acres) exist . Examples formed from coral reefs include Maldives, Tonga, Samoa, Nauru, and Polynesia . Granite islands include Seychelles and Tioman and volcanic islands such as Saint Helena . </P> <P> The socio - economic diversity of tropical islands ranges from the Stone Age societies in the interior of Madagascar, Borneo, and Papua New Guinea to the high - tech lifestyles of the city - islands of Singapore and Hong Kong . </P> <P> International tourism is a significant factor in the economy of many tropical islands including Seychelles, Sri Lanka, Mauritius, Réunion, Hawaii, and the Maldives . </P>

When does an island stop being an island