| Reefs and Atolls |
| A reef is a ridge of rock just beneath the sea surface, sometimes exposed at low tides. Reefs are common in coastal areas where wave action has eroded the land, but rocks below the wave zone remain relatively undisturbed. They pose a hazard to shipping since they lie at a level shallower than the drought of many vessels. Reefs are less common in the mid ocean, although they occur around oceanic islands in the tropical zone, where the top layer consists mainly of living coral rather than rock.
Ledges of rock, called reefs or shoals, can run hundreds of metres (yards) out to sea under the waves. Coral reefs are the result of the growth of marine organisms called polyps, which secrete a hard outer skeleton of calcium carbonate which anchors them to the sea bed. Living coral can survive only to a depth of about 60 metres, but a great mass of carbonate rock can build up below as polyps die and leave behind their skeletons. The reef may act to trap fragments of shells, coral and coral sand, which are piled up by wind and waves to form a coral island. Barrier reefs, such as Australia's Great Barrier Reef, occur some distance offshore, originating in shallow water at a time of lower sea levels. Fringing reefs lie nearer the shore. Many mid-ocean coral reefs have grown around volcanic islands. When volcanic activity ceases, a volcanic island may gradually sink back into the ocean crust, aided by erosive forces. As the island subsides, the living coral continues to grow upwards, maintaining its position near the sea surface. Eventually, the original island may have disappeared from view altogether, but a circle of fringing reefs remains, some of them well-established enough to be islands in their own right. These collections of islands, often almost completely enclosing the location of the submerged volcano, are called atolls. The Maldives in the Indian Ocean and the Tuamotu Islands in French Polynesia contain many examples of coral atolls. |
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