How does a coral reef form?

A coral reef is an accumulation of dead corals and other organisms with a limestone skeleton, cemented together by some algal species and by physical processes. The reef builds slowly towards the surface of the water, at the rate of a few millimetres per year. Once the reef reaches sea level, the corals cannot survive, and the reef grows horizontally. Reefs build as a result of the growth of corals and other biota, and also from accumulation of sand and rubble formed when the organisms are broken down by waves and animals, such as worms and sponges, that bore into the coral.

Reefs grow horizontally once they reach sea level Photo: GBRMPA

The world's first coral reefs occurred about 500 million years ago, and the first close relatives of modern corals developed in southern Europe about 230 million years ago. By comparison, the Great Barrier Reef is relatively young at just 500,000 years old. The existing reef's structure is even younger; less than about 8,000 years old.

Most modern reefs have formed on hard surfaces in the ocean, such as a base of an old reef that died during a period when sea level was lower, or the edge of a rocky island. Depending on how they start out, several types of reefs can form. Some coral reefs form in the deep ocean and are called atolls. The theories on how coral reefs form were first put forward by Charles Darwin (of The Origin of Species fame) who proposed that atolls form around the edges of high volcanic islands that gradually submerge beneath the sea with changes in sea level or subsidence of the land. Thus an atoll starts life as a fringing reef, then becomes more of a ring growing on the shrinking land-mass, until the land disappears and just the coral circle remains. In some cases, the coral growth is unable to keep pace with the sinking island, and sunken dead reefs have been found.