Coral reefs are islands-like structures found either underwater or above the surface in the ocean. They are built by corals and many other lime secreting marine organisms under tropical and subtropical climatic conditions. They are an underwater ecosystem built as a result of reef-building corals. They occur mainly in the warm water of the Pacific and Indian Oceans. The conditions, which favour the development of a coral reef are as follow:
- Reef-building corals grow best in water with an average annual temperature of about 24° C. Coral reefs, therefore, develop only between 28° N and 28° S.
- Corals can not grow at a depth where sunlight can not penetrate. This limits the depth of active coral reef growth to about 45 meters.
- Clearwater is necessary for the growth of corals. They do not survive in muddy water.
Types of Coral Reefs:
Coral reefs are of the following types:
- Fringing Reefs: A fringing reef is formed on the margin of an island in continuity with shore. It encircles the island almost completely.
- Barrier Reefs: Barrier reefs are built away from the island and therefore, they enclose a lagoon between the central landmass and themselves. The great barrier reef along the northeast coast of Australia is the famous example of this type.
- Atolls: An attol is a circular reef the central portion of which is occupied by a relatively shallow lagoon.
- Platform reefs: Platform reefs, variously called the bank or table reefs, can form on the continental shelf, as well as in the open ocean, in fact anywhere where the seabed rises close enough to the surface of the ocean to enable the growth of zooxanthemic, reef-forming corals. Platform reefs are found in the southern Great Barrier Reef, the Swain, and Capricorn Group on the continental shelf, about 100–200 km from the coast. Some platform reefs of the northern Mascarenes are several thousand kilometers from the mainland. Unlike fringing and barrier reefs which extend only seaward, platform reefs grow in all directions. They are variable in size, ranging from a few hundred metres to many kilometers across. Their usual shape is oval to elongated. Parts of these reefs can reach the surface and form sandbanks and small islands around which may form fringing reefs. A lagoon may form In the middle of a platform reef.Platform reefs can be found within atolls. There they are called patch reefs and may reach only a few dozen meters in diameter. Where platform reefs form on an elongated structure, e. g. an old, eroded barrier reef, they can form a linear arrangement. This is the case, for example, on the east coast of the Red Sea near Jeddah. In old platform reefs, the inner part can be so heavily eroded that it forms a pseudo-atoll. These can be distinguished from real atolls only by detailed investigation, possibly including core drilling. Some platform reefs of the Laccadives are U-shaped, due to wind and water flow.
- Other reef types or variants:
- Apron reef: Short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore. The initial stage of a fringing reef.
- Bank reef: Isolated, flat-topped reef larger than a patch reef and usually on mid-shelf regions and linear or semi-circular in shape; a type of platform reef.
- Patch reef: Common, isolated, comparatively small reef outcrop, usually within a lagoon or embayment, often circular and surrounded by sand or seagrass. Can be considered as a type of platform reef or as features of fringing reefs, atolls, and barrier reefs. The patches may be surrounded by a ring of reduced seagrass cover referred to as a grazing halo.
- Ribbon reef: Long, narrow, possibly winding reef, usually associated with an atoll lagoon. Also called a shelf-edge reef or sill reef.
- Habili: Reef specific to the Red Sea; does not reach near enough to the surface to cause visible surf; may be a hazard to ships (from the Arabic for “unborn”)
- Microatoll: Community of species of corals; vertical growth limited by average tidal height; growth morphologies offer a low-resolution record of patterns of sea level change; fossilized remains can be dated using radioactive carbon dating and have been used to reconstruct Holocene sea levels.
- Cays: Small, low-elevation, sandy islands formed on the surface of coral reefs from eroded material that piles up, forming an area above sea level; can be stabilized by plants to become habitable; occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including the Caribbean and on the Great Barrier Reef and Belize Barrier Reef), where they provide habitable and agricultural land
- Seamount or guyot: Formed when a coral reef on a volcanic island subsides; tops of seamounts are rounded and guyots are flat; flat tops of guyots, or tablemounts, are due to erosion by waves, winds, and atmospheric processes
Origin of Coral Reefs:
The formation of coral reefs was explained by Charles Darwin in 1836. He said that coral reefs form on the flanks of sinking volcanic islands. Darwin’s theory is known as the ”Subsidence Theory”. This theory is considered to be most probable explanation. It may be summarized as follow;
- The process starts with the formation of a volcanic island on the seafloor. The corals grow around the edges of the volcanic islands and build a ”fringing reef”.
- Then the island slowly sinks beneath the sea as a result of some tectonic movement. The actively growing corals may keep pace with this sinking and continue to build the reef upwards and outwards. In this way, a barrier reef is formed, which is separated from the island by a lagoon.
- With further subsidence, the central island disappears below sea level and an atoll is formed.
The confirmation of Darwin’s theory was found only after World War II, when the United States selected two atolls for testing atomic bombs. During deep drilling of these atolls, volcanic rocks (rocks formed by extrusion of magma as a result of a volcanic activity) were found beneath the thick coral reef structure. Recently the theory of plate tectonics has offered a reasonable explanation for the gradual subsidence of volcanic islands on oceanic crust. This subsidence takes place when crustal plates slowly move away from a higher oceanic ridge to a lower oceanic ridge.