Coral reefs start to form when coral larvae attach to rocks or hard surfaces that are submerged under water. Coral polyps secrete calcium carbonate, closing off each layer as they grow bigger. Each polyp bonds itself to other polyps, eventually forming reef structures.
Coral reefs form within 150 feet of the surface of the water since they require sunlight to survive. Reefs grow slowly, varying from 1/2 inch to 3 inches per year depending on the species. Corals and some types of algae have symbiotic relationships. The corals provide a home and carbon dioxide for the algae, and the algae provide food and oxygen to the corals. Coral reefs recycle carbon dioxide and absorb impacts from waves and storms. Corals are animals, with the living corals staying on the outer part of the reef and in deeper waters.
The first coral reefs formed about 500 million years ago. The Great Barrier Reef is 500,000 years old, with the existing reef structure less than eight thousand years old. Barrier reefs, atolls and fringing reefs are the three types of reef structures. Fringing reefs are found closer to land. Atolls are found around lagoons, and barrier reefs are found farther off-shore and most commonly in the Indo-Pacific and Caribbean.