Corals are tiny living animals found in tropical marine waters across the world. They are made up of a colony of individual polyps that produce a hard skeleton in a cuplike shape, for protection and support. Every polyp links its skeleton to the nearby skeletons, thereby forming a coral colony consisting of individual skeletons of hundreds of coral polyps. A coral reef consists of numerous coral colonies as well as other animals like starfish and clams. Coral reefs have been in existence for about 500 million years. Australia’s Great Barrier Reef is the largest in the world, stretching across 2,300 kilometers and is relatively a younger coral reef of about 10,000 years old. Known as rainforests of the sea, coral reefs can be found in bright colors like orange, tan, yellow, purple and green. There are three types of coral reefs, namely, fringing reefs, barrier reefs and atoll reefs, each having its individual formation process. Let us look at the formation of the different kinds of coral reefs.
How Are Coral Reefs Formed
A fringing reef forms in the shallow waters, along the coast of mostly new tropical islands or continents. It is formed by the hard skeletons of corals and limestone deposits of coralline algae and other marine animals. The dead limestone skeletons become the base for a new generation and hence, start growing upwards. The fringing reef is the first step in the ‘classical’ formation of a coral reef. Fringing reefs are formed in the tropical zones of water, with a temperature warmer than 72 °F (22 °C) and a depth of more than 100 feet (30 meters).
A barrier reef forms parallel to the coastline separated by a lagoon. The lagoon develops between the fringing reef and the land. The reef, while growing further and further offshore, reaches the edge of the continental shelf. A barrier reef can originate offshore also, in case the depth of the seabed is shallow enough to allow corals to grow there. Barrier reef is formed when the oceanic island begins to sink into the Earth’s crust. This happens due to the absence of volcanic island building forces, added weight of the coral reef and the erosion at the surface of the island. The coral reef continues to grow upward, while the island sinks. The lower part of the reef consists of calcium carbonate skeletons left by the reef building coral. The upward growth of the reef creates a lagoon of water between the top of the reef (fringing reef) and the sinking crustal island. The famous barrier reef is the Great Barrier Reef, which lies between 24 and 240 km from the main continent of Australia.
Atolls are broken rings of reefs usually encircling an island, either sand or coral rubble. They have a shallow, sandy, sheltered lagoon in the middle. There are channels that allow access to the open sea, providing fresh and cold water to the lagoons. Atoll reefs are formed on the top of submarine mountains that are remnants of volcanoes. The atoll reef is formed when the oceanic island sinks below the surface of the ocean, while the coral reef surrounding it continues to grow upwards.