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coral reefs, limestone formations produced by living organisms, found in shallow, tropical marine waters. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). The accumulation of skeletal material, broken and piled up by wave action, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator; the reef-forming corals do not grow at depths of over 100 ft (30 m) or where the water temperature falls below 72℉ (22℃). Corals are not the only, and in some cases not even the major, reef-forming organisms. Calcium carbonate is also deposited by coralline algae, the protozoan foraminiferans, some mollusks, echinoderms, and tube-building annelid worms. However, any reef formed by a biological community is usually called a coral reef.
Geologically, coral reefs are classified into three main types. Fringing reefs are coral platforms that are more or less continuous with the shore and exposed at low tide. Barrier reefs are separated from the shore by a wide, deep lagoon or surround a lagoon that has a central island. An atoll is a reef surrounding a lagoon that has no central island, with passages through the reef to the sea. It is generally believed that fringing reefs formed as a result of upward and outward growth of corals that became established on rocks near shore; there is disagreement about the nature of barrier reef and atoll formation. Charles Darwin postulated a progression from fringing reef to barrier reef to atoll, as a result of a slow, steady sinking of the seafloor that creates a lagoon and a simultaneous upward and outward growth of coral. Where entire volcanic islands sink, only the reef remains above water, forming an atoll. Not all scientists accept Darwin's proposal, but most current theories involve subsidence of the seafloor, although changes of the ocean level may also be involved.
Sediments accumulate on the lagoon side of atolls and support vegetation; in time the entire lagoon may fill, creating an island. Many such atolls and islands, common in the Pacific and Indian oceans, are inhabited. The Great Barrier Reef of NE Australia is the largest known complex of coral reefs. It is 10 to 90 mi (16–145 km) wide and about 1250 mi (2010 km) long, and is separated from the shore by a lagoon 10 to 150 mi (16–240 km) wide.
Reefs are under numerous environmental pressures, including damage from increased coastal development, water pollution, tourism, runoff containing agricultural chemicals, abrasion by ships' hulls and anchors, and smothering by upstream sedimentation. Coral reefs are sometimes destroyed in fishing when poison or dynamite are used to catch fish and by the harvesting of coral for use in jewelry. During the 1990s, many previously unknown diseases began attacking coral reefs worldwide, causing rapidly spreading damage. Also since the 1990s increasingly warm ocean temperatures have led to recurring episodes of bleaching; caused by heat stress, bleaching results when coral polyps expel the colorful algae they host and depend on. If the water fails to cool in time for the algae to become reestablished, the coral polyps die. Even higher water temperatures resulting from global warming can kill the coral directly, severely damaging coral reefs.
See A. Emery, The Coral Reef (1981); J. A. Fagerstrom, The Evolution of Reef Communities (1987).
geological structures that form as a result of the vital activity of colonial coral polyps (primarily madrepore corals) and attendant organisms capable of producing lime from seawater.
Four types of coral reefs are distinguished: fringing, barrier, ring-shaped (atolls), and reefs inside lagoons.
Fringing coral reefs are formed as a result of the settlement of corals in outer areas of tectonically stable coastal shallow water (usually on an abrasion terrace). As the polyps die and as a result of the cementation of coral reef fragments, coral lime-stone is formed, which provides the foundation for coral reefs. Living colonies of corals settle on this substratum, which may be several dozen meters thick. Since the most active growth of the fringing coral reef occurs at its outer edge, the inner (coastal) zone gradually acquires the character of dead coral limestone, which is sometimes covered with a layer of loose disintegration products (sand, detritus).
As a result of the tectonic subsidence of a shore the fringing reef is gradually transformed into a barrier reef. The area of the outer edge of the reef grows upward at a brisk rate, and the space between the outer edge of the coral structure and the shore is gradually converted into a lagoon. Barrier coral reefs have developed off the northeast coast of Australia and the coasts of New Caledonia and the islands of Fiji and the Greater Antilles. Off the shores of small oceanic islands that are subsiding, barrier reefs are transformed over time into ring-shaped coral reefs with lagoons in the center (atolls). Coral islands are located on the underwater coral barrier of the atoll. Lagoon reefs in the form of steep peaks and ridges, with a relative altitude of 40–50 m, rise up from the floor of the lagoon. In atolls the base of coral limestones is found at a depth of more than 1,000 m. This fact can be explained only by the subsidence of their original foundations (since reef-forming corals live only at depths no greater than 30–50 m). Submerged (Chagos, Robbie, Tuscarora) or raised (Nauru, Ocean, Mare) atolls are formed as a result of changes in the velocity and direction, respectively, of the vertical movements of the earth’s crust.
As biological communities, coral reefs are distinguished by a very large biomass and by the productivity of the inhabiting organisms; the limestone of which the reefs are composed is usually porous and vesicular. During fossilization the former condition is favorable for the formation of oil and gas. Porosity and vesicularity make limestone an excellent reservoir. There-fore, fossil coral reefs frequently contain rich deposits of oil and gas. The study of environmental conditions of modern coral structures and of their association with various tectonic zones is of great importance for petroleum exploration geology.
REFERENCESDarwin, C. ”Stroenie i raspredelenie korallovykh rifov.” Soch., vol. 2. Moscow-Leningrad, 1936.
Ravikovich, A. I. Sovremennye i iskopaemye rify. Moscow, 1954.
Shepard, F. Morskaia geologiia, 2nd ed. Leningrad, 1969. (Translated from English.)
Leont’ev, O. K. Dno okeana. Moscow, 1968.
Wiens, H. Atoll Environment and Ecology. New Haven-London, 1962.
O. K. LEONT’EV