How are coral reefs formed?

The calcium carbonate (chalk) skeletons of dead corals serve as the foundation upon which layers of successively younger animals attach themselves. The coral animal, or polyp, is shaped like a column. Its lower end is attached to the hard floor of the reef and its upper end extends into the water.

There are two kinds of corals—hard and soft—depending on the type of skeleton secreted. The hard-coral polyp deposits a cup-like solid skeleton of calcium carbonate around itself into which the polyp withdraws during the daytime. For this reason, deep sea divers see only the skeletons—and not the polyps—of hard corals.

A coral colony consists of thousands of individuals. Accumulations of dead and living coral polyps, combined with rising water levels, slowly lead to the formation of coral reefs that can be hundreds of meters deep and long. Coral reefs grow only in warm, shallow water.

Sources: Kaplan, Eugene H. Field Guide to Coral Reefs, pp. 78-79; The New Book of Popular Science, vol. 2, p. 270.