Atolls are geological formations made up of :

An atoll is a ring-shaped coral reef, island, or series of islets. An atoll surrounds a body of water called a lagoon. ... The type of corals that build reefs are called hermatypic corals, or hard corals. Hermatypic corals create a hard exoskeleton of limestone (calcium carbonate).

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answer; i believe the correct answer is ; large pieces of pangaea drifted in different directions over millions of years;