Ring shaped islands, called ‘Atolls’, are commonly found in the Pacific Ocean. They are formed when, millions of coral shells are piled on one another and are cemented together. When corals attach themselves to the coasts of a small submerged island, they build all around it a ring of corals.
As the island rises up, the ring of corals is lifted up until it is above water level. The wind and the waves bring to it some seeds and the atoll becomes full of vegetation.
Below Atolls info from Wikipedia (Read More):-
The distribution of atolls around the globe is instructive: most of the world's atolls are in the Pacific Ocean (with concentrations in the Tuamotu Islands, Caroline Islands, Marshall Islands, Coral Sea Islands, and the island groups of Kiribati, Tuvalu and Tokelau) and Indian Ocean (the Atolls of the Maldives, the Laccadive Islands, the Chagos Archipelago and the Outer Islands of the Seychelles). The Atlantic Ocean has no large groups of atolls, other than eight atolls east of Nicaragua that belong to the Colombian department of San Andres and Providencia in the Caribbean Sea.
http://en.wikipedia.org/wiki/Atolls
