How are coral reefs formed?

Coral reefs are made up mostly of coral colonies like these.

Corals are live animals made up of a colony of individual "polyps." Each polyp produces a hard skeleton in a cuplike shape for protection and support, and links its skeleton to those of the polyps around it. Thus, the individual skeletons of hundreds of coral polyps live as one coral colony.

Polyps are continually creating new skeleton at their base and sides. As they do this, they extend upward and outward from the coral colony center, living atop the old, dead skeleton they have created. New polyps can be created, and old ones may die off, as the whole colony continues to grow. Thus, most of the structure of a single coral colony—and of the larger coral reef comprised of many coral colonies—is made up of dead skeletal material. A live, healthy coral has only a thin layer of living material that inhabits its surface.

Coral reefs are made up mostly of coral colonies, as well as other animals like starfish and clams. Reefs are also important habitats for fish. Coral reefs have existed on earth for about 450 million years. Australia's Great Barrier Reef is the largest in the world, stretching across 2,028 kilometers (1,260 miles).