How are coral reefs formed?
Located in warm, shallow seas, coral reefs are enormous, rocklike structures, but unlike mountains, deserts, and most other natural landscapes, a coral reef does not consist of rock and soil. Instead it is a huge city built by stony coral polyps. Each polyp uses minerals from seawater to build a limestone skeleton, which cements it to the seafloor. Polyps, like people, live close together in colonies. As polyps grow, they move up to build new skeletons on top of the old. And very slowly, over many years, the reef grows with them. Some colonies contain millions of individual polyps.
