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An atoll is a ring-shaped coral reef or group of coral islets that has grown around the rim of an extinct submerged volcano forming a central lagoon . Atoll formation is dependent on erosion of land and growth of coral reefs around the island . Coral reef atolls can only occur in tropical regions that are optimal for coral growth. The main Hawaiian Islands will all likely become coral atolls millions of years into the future. The older Northwestern Hawaiian Islands, many of which are now atolls, were formed by the same volcanic hot spot as the younger main Hawaiian Islands .
