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> Homepage > Welcome > Learn more > Nature > The world of the reef >   How does a reef form?
As the polyps grow they divide and form colonies. A reef is the gradual accumulation of coral colonies which grow on top of one another and it takes thousands of years to build. Coral growth varies from one species to another but it is painfully slow, some varieties grow between two and three millimeters a year.
In 1842 Charles Darwin identified three kinds of reef: fringing reefs which grow in shallow water, close to shore, barrier reefs which are much longer and continuous, and coral atolls which are circular or oval in shape and occur further out to sea. Atolls have a deep, central lagoon and in some areas of the tropics are associated with the peaks of extinct submarine volcanoes. Darwin's classification was later modified to include patch reefs.
Sunlight, water depth and temperature, sedimentation, wave action and ocean currents influence the growth of a reef, its shape, size and the varieties of coral and other marine creatures present.
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