Coral reefs are calcium carbonate structures, made up of communities of reef-building stony or scleractinian corals.
The vertical distribution of corals is limited to the depth of light penetration and so reefs occur in shallow water, ranging to depths of 60 meters. This dependence on light also means reefs are only found where the surrounding waters contain relatively small amounts of suspended material (and are paradoxically of low productivity).
Coral reefs are further restricted by water temperature where the above average minimum temperature is not less than 20 degrees.
Three general types of reef can be recognized - fringing, barrier and atolls.
Fringing reefs project seaward directly from the shore. They surround islands as well as border continental masses and are the most common types of reef.
Barrier reef platforms are separated from the adjacent land masses by a lagoon. The Great Barrier Reef of Australia is the longest in the world, stretching for over 1000 miles along Australia's north-east coast. Another long barrier reef is located in the Caribbean off the coast of Belize between Mexico and Guatemala.