In some parts of the ocean, there are living organisiums called coral polyps. These are soft-bodied creatures that extract calcium carbonate from the sea to build protective limestone structures around themselves. These limestone structures are known as corals. When coral polyps die, the corals are left behind and over time build up to form coral reefs. One Example is the Great Barrier Reef in Australia.
Three conditions favourable for the growth of coral polyps are high water temperature, clear shallow seawater and a plentiful supply of oxygen and plankton. Planktons are the tiny sea animals and plants on which the coral polyps feed. Coral polyps grow well in water with temperature above 20 degrees Celcius. This explains why coral reefs are found in warm tropical waters. It also explains why they are found in abundance on the eastern coasts of islands and continents as these areas are washed by the warm currents. Coral polyps also survive well in seawater that is shallow and free from sediment. As coral polyps need sunlight to grow, deep or muddy water is unsuitable since it prevents the sunlight from reaching them. As a result, they are found in waters not more than 60m deep and far away from muddy coasts. Coral polyps need oxygen and planton to grow, which explains why they are found in the seaward side of the coral reefs. This part of the coast is washed by waves, currents and tides which maintain a fresh and plentiful supply of oxygen and planton for the coral polyps.
Types of Reefs
FRINGING REEFS are reefs that grow near the coast. They are found in waters around Malaysia, Singapore and Indonesia.
BARRIER REEFS are reefs that are situated far away form the coast in deeper waters and are seperated from the mainland by lagoons. One example is the Great Barrier Reef in Australia which is located 100km off the Australian mainland and stretches to about 2000km that it is also visible from outer space.
ATOLLS are coral reefs that encircle a lagoon within which there is no land at or above sea level. Atolls are circular in shape and are usually seperated from each other by narrow channels of water. An example in the Maldives in the Indian Ocean.