Reefs are formed in warm, shallow waters that are able to receive lots of sunlight to supply food for a microscopic form of algae that is required to help coral form reefs. Reefs generally grow where water temperatures range between 77 and 88 degrees Fahrenheit, according to Texas A&M University. Australia's Great Barrier Reef, which stretches for 1,242 miles in length, survives in waters with a temperature range between 64 and 91 degrees Fahrenheit.
There are three common types of reefs found in the world that all grow in the same temperature of water, but grow under different land conditions in the ocean. A fringing reef grows along the shallow shorelines of the tropics. Fringing reefs grow as a shelf that extends from the shoreline into the ocean. A barrier reef is separated from the land by a deep lagoon. Barrier reefs are formed where the sea level changes in an area or where the shoreline is sinking faster than the water level is receding. An atoll is formed around a lagoon and is often created when an island sinks into the ocean. Reefs are made up of both living and dead coral polyp shells. Tan areas of a coral reef are made up of the dead coral polyp shells, while the colorful areas of a reef are the living coral polyps.
As reefs require sunlight to grow and survive in the ocean it is rare for a reef to grow more than 130 feet below the surface of the ocean. Each reef tolerates salt water better than the reef does fresh water; therefore areas with large amounts of freshwater river runoff are not good for supporting coral reef life. River water runoff can cover a reef in darker waters and mud that restrict the growth of coral reefs.
Coral reefs grow at a rate of around ½ inch annually. In areas with an appropriate environment, including water temperature and sunlight, with low levels of human intervention, reefs can grow to a large scale. The salinity of water is also important, with a level between 34 and 37 parts per thousand appropriate for coral growth. Reefs grow where warm waters flow along the eastern shores of land masses. For instance, the Great Barrier Reef grows along the northeastern coast of Australia.
