CORAL REEF

ridge or hummock formed in shallow ocean areas by the calcareous skeletons of certain coelenterates, of which coral polyps are the most important. A coral reef may grow into a permanent coral island. Coral polyps resemble sea anemones, to which they are closely related, but, unlike most anemones, most reef corals are colonial. Initial polyps divide themselves into daughter polyps, and they divide in turn, so growing into colonies that can be up to several metres in diameter, all held together in one continuous rigid calcareous skeleton. They remain attached to the seafloor and become so large and heavy that only storms disturb them. Under the right conditions, generally clear and well-circulating water that is not too rough, the corals grow profusely side by side, even on and over each other. The corals in effect build limestone because their skeletons are made of calcium carbonate. Calcareous algae (stony seaweeds), mollusks, echinoderms, and protozoans also contribute to the reef. Different organisms have different reef-building roles. Some, especially the corals, provide the main structural framework of the growing reef, although in parts of the world such as the central Pacific, where the surf is very strong, calcareous algae may be more important in the roughest places. Almost all shelly and calcareous organisms and those with spicules, such as sponges and sea cucumbers, provide fragments that wash or fall into the gaps between corals. Other organisms, especially algae and protozoans, bind and cement everything together with sheetlike growth. The whole structure is attacked by waves and by organisms seeking shelter and food. Fragments of every size from blocks to fine mud result and accumulate within the reef and are shifted by waves and currents away from the main growing structure into calmer areas on top of the reef or around and behind it. A coral reef is actually a complex of features, only part of which is a living coral or algal framework, although the other associated features result from this live segment. The accumulations of carbonate sand and mud provide a habitat for two important groups of marine flowering plants, sea grasses and mangroves, and for almost inconspicuous blue-green algal mats. These plants trap and stabilize sediment, and their accumulations are also accreted to the whole reef complex. Storms and surf heap up fragmental material into beaches and shoals, and the shoals may develop into low sandy or rubbly islets or cays on top of the reef. Coral reefs take four principal forms. Fringing reefs consist of a flat reef area directly skirting a nonreef island, often volcanic, or a mainland mass. Barrier reefs are also close to a nonreef landmass but lie several kilometres offshore, separated from the landmass by a lagoon or channel often about 50 m (160 feet) deep. Some barrier reefs are more or less circular, surrounding an island, but larger barrier reefs, such as those along the Red Sea coast and the Great Barrier Reef of Australia, are complex linear features consisting of chains of reef patches, some of them elongated into ribbon reefs. The third category of reefs consists of atolls, which are like circular barrier reefs but without their central landmass. Finally, there are patch reefs, which have irregular tablelike or pinnacle features. Smaller patches occur inside atoll lagoons. Larger patches occur as isolated parts of larger developments of any of the other three reef categories. They sometimes occur completely separate from other kinds of reefs. A typical coral reef generally faces the open sea, seaward of which is the fore reef, descending into deeper water and floored deeper down by fragmental material derived from the reef. Behind the growing fore-reef edge, which rises to about mean high-water level, is a shallow platform formed partly by a now dead area of reef framework and partly of fragmental material and often colonized by sea grasses, algal mats, or mangroves. Patches of living framework occur in places. Cays may occur along the platform, or the reef may terminate here against the shore of a proper landmass. If there is no immediately adjacent landmass, the reef descends again into deeper water, generally more gently, on this, its leeward side. There are usually live reef frameworks on this slope too, but these are often irregular and patchy. The reef becomes true rock by an almost imperceptible dissolution, redeposition, recrystallization, and chemical transformation of reef material. The shape of coral reefs, while due at least in part to the tendency of reef builders to grow upward and outward toward the prevailing winds and currents, is also the result of changes of sea level during the last 2,000,000 years or more. Coral reefs that developed during this time were left above the sea during the last glaciation, when they were eroded and subjected to solution weathering. As sea level rose again during the last 10,000 years, new reef growth mantled this older, drowned landscape but has still not masked it completely. Coral islands consist of low land perhaps only a few metres above sea level, generally with coconut palms and surrounded by white coral sand beaches. They may extend dozens of kilometres and include almost any tropical limestone island whose structure is integrally part of a living or relatively recent coral reef. Reef building takes place mostly below high-tide level, and a typical coral island or cay usually surmounts the relatively flat top of the whole reef system. Geologically, the island is just one small part of the whole coral reef. Coral reef islands occur in association with all types of reef but especially on reefs whose flat tops are well developed, perhaps a kilometre or more in width. Reef islands may occur in isolation or in a chain along the length of the reef. Sometimes they take the form of long strips of land occupying most of the length of the central area of a reef top. There are two completely different mechanisms of origin of reef islands: uplift and accretion. In the first, part or all of a reef system may become land as a result of crustal movements raising it above sea level (e.g., the Aldabra Islands in the western Indian Ocean). The previously submarine reef top becomes a low plateaulike feature, and such islands are typically rocky, with cliffs and with land surfaces pitted and sculpted by solution weathering. They are often still recognizable as atolls with a lagoon, now much shallower or even completely dry, as an interior basin. If present sea levels were to fall again, as they did in the recent geologic past owing to an increase in polar ice, most of the world's coral reefs would, in effect, become raised features. It is only because present sea levels are the highest for many thousands of years that there are no raised reef islands of this kind now. Coral islands created by accretion have developed from rubbly reef rock broken off from the reef by storms and waves and mixed with finer reef detritus. The exceptional conditions of cyclonic storms are sometimes sufficient to create reef-top shoals in a single event. Other material accumulates by more regular methods such as normal currents and wave action. Beaches develop around the shoal, and wind may heap up the lighter, finer material into dunes. Rainwater can now reach all this material, which, being almost entirely of calcium carbonate, is readily dissolved by it, and the dissolved lime is redeposited around the loose material, cementing it together. The newly formed land is soon colonized by plants and animals, which also contribute their own remains to the island, helping soil to develop. Many of the reef islands in the central and southern Pacific and of the Maldive Islands in the Indian Ocean originated in this way. Reef islands, especially those close to sea level, are not very stable. The cyclones that help to create them may also damage and destroy them. Waves may attack one side and redeposit the material on the other. Precarious though reef islands are, they have nevertheless long been the homes of peoples like the Polynesians and Micronesians in the Pacific and the Maldivians in the Indian Ocean.