A coral island is a type of island formed from coral detritus and associated organic material. They occur in tropical and sub-tropical areas, typically as part of coral reefs which have grown to cover a far larger area under the sea.
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Formation
Islands develop from coral reefs through one of two processes, uplift and accretion. In uplift, part or all of the coral reef becomes land as a result of the earth's crust rising above sea level. In accretion, rocks and sand are layered on top of coral reefs during cyclonic storms, and the gradual accumulation of other solid material through the action of wind and waves leads to the development of the island. The process is later enhanced with the remains of plant life which grows on the island.
Where coral islands form from atoll reefs, the result is an island or string of islands in a roughly circular form, surrounding a shallow lagoon.
Distribution
Most of the world's coral islands are in the Pacific Ocean. The American territories of Jarvis, Baker and Howland Islands are clear examples of coral islands. Also, some of the islands belonging to Kiribati are considered coral islands. The Maldives also consist of coral islands. St. Martin's Island is an 8 km2 coral island located in Bangladesh. Coral islands are also located near Pattaya and Koh Samui, Thailand.
Many coral islands are small with low elevation above sea level. Thus they are at threat from storms and rising sea levels.