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Existing reefs are complex structures built of calcium carbonate taken from sea water by shallow-water organisms. They vary in size and shape depending on the nature of their foundations and on many ecologic factors. The conventional classification recognizes fringing reefs, barriers, atolls, and table reefs; a broader classification differentiates the reefs that lie in continental areas from those that rise from the deep ocean.
Living reef-building organisms are concentrated along the seaward margin where they form a rigid, wave-resisting framework. Most reef surfaces show a distinct zonation parallel with the reef front. Sedimentary materials increase landward or lagoonward, and where lagoons are present the sediments may form the major part of the reef. Extensive sedimentary deposits likewise accumulate off the seaward side of the reef.
The most influential control of reef growth is temperature, which in large part explains the marked development of reefs in the western parts of the oceans in tropical latitudes; reefs are largely absent from the eastern oceanic areas because of the upwelling of cold waters. In areas where surface temperatures are 70°F. or higher, reefs are developed if waters are relatively clear and of near-normal salinity, and if suitable foundations project into shallow water.
Many reefs are veneers that coat a variety of foundations, but others are known to have a thickness far in excess of the depth ranges of the organisms forming them. The origin of thick reefs is still a major problem, but it is certain that important changes in the relations of land and sea are involved. The problem is closely related to that of the seamounts and other enigmatic features of the ocean basins. Recent reefs differ from ancient reefs, but knowledge of their development will aid in understanding their ancient counterparts, some of which are oil reservoirs.
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