David Britt

4/23/97

Geology I

Wilson

The Great Barrier Reef

Overview:

Off of the coast of Queensland in Northeastern Australia, lies the Great Barrier Reef, one of the great natural wonders of the world. Stretching some 1250 miles from the northern tip to the southern tip, the Great Barrier Reef is home to nearly 500 different types of reef building corals.

The Concise Oxford Dictionary defines a reef as a ridge of rock, shingle, or sand at or just above or below the surface of the water. This definition includes the two major features of coral reef, the first is that they are composed of rock in the form of limestone, and the second feature is that they never extend very far above sea level. The tops of coral reefs are exposed to air but only in very short intervals which occur during times of low tides ("Coral").

Coral reefs, as the name implies, have corals living on their surface, but these are not the only living organisms growing on coral reefs. The major part of a coral reef is nonliving limestone. The living plants and animals associated with coral reefs constitute only a thin layer on the surface. Coral reefs vary greatly in shape and size. Some stretch only a few meters across, in lagoon reefs, to over ten kilometers long in some wall or platform reefs.

Coral reefs consist of consolidated limestone debris, and, once established, tend to grow both outwards and upwards when the plants and animals living on their surface are able to flourish. The life and death processes of these organisms result in a steady contribution of their skeletal remains to the structure of the reef This is especially significant in the tropic seas where plants and animals tend to grow rapidly (Buddemeier, 1976). Also, tropic seas contain large amounts of limestone which is used by reef-forming organisms to construct their shells and skeletons which upon death accumulate to form reefs.

Corals constitute the major building blocks in reefs. It is the coral-zooxanthellae association which enables corals to produce limestone in a very efficient manner. However, it is important to realize that corals are not always necessarily going to be the dominant organism on reefs. Upon close investigation of reef rock, it is found that coral skeletons are not the only component. The corals, along with larger mollusks, provide the bulk of a structure that includes sand-sized fragments of calcareous plants, Foraminifera, and other organisms. Coral reefs may also possess large areas that are completely void of living coral, which is usually the result of unfavorable living conditions (Patton, 1976). Nevertheless, on most reefs of the Great Barrier Reef, the living corals are the most abundant animal, with the genus Acropora being the most dominant.

Coral skeletons make up varying percentages of reef rock. These skeletons range from the fast growing Acropora whose branches average I centimeter to massive Porites, where a single colony may be several meters across. All Scleractinia and the limestone secreting species of hydrozoans contribute to the formation of reefs. The skeletons of dead corals are transported and accumulated by water movement to be cemented into the structure of the reef by calcareous plants.

Algae are marine plants whose role in the formation and maintenance of coral reefs is almost as important as the corals themselves. In some places the role of plants may even be greater than that of the corals. There are many types of algae found on coral reefs, but the most important in reef formation are the calcareous algae. Calcareous secreting algae have the ability to produce limestone in the form of an encrustation over the reef, which acts as a cement, binding together loose fragments of debris to form a hard tough surface. This surface is especially characteristic of the windward areas of a reef where wave action is most violent; here the algae give the reef surface a smooth pavement-like texture of a pink, purple, or light brown color. If coral skeletons could be considered the bricks that make up a reef, the calcareous algae could be looked at as the cement that holds these bricks together.

Another plant that is very important in the formation of coral reefs is the Halimeda. This plant too produces great quantities of calcareous fragments. This genus contains numerous species which produce limestone as a thin outer layer over their fleshy leaf-like segments ("Coral Reefs"). This limestone becomes incorporated into the structure of the reef upon the death of the plant, and in some areas, sand made up primarily of Halimeda fragments is common.

Origin of the Great Barrier :

A reef begins to form when a shallow area of the sea floor within the tropics can provide suitable conditions to encourage and support colonization by benthic, bottom dwelling, plants and animals. Such conditions include the presence of a solid surface that is free from excessive mud and sand. This must be in relatively clear, warm, sea water having sufficient movement to circulate oxygen and plankton amongst the colonizing organisms. As these plants and animals grow ever upwards and outwards they eventually die and their skeletons and shells accumulate and are cemented together by the calcareous algae ("Coral Reefs"). A limestone mound is formed which represents the beginnings of a reef Corals, mollusks, and algae flourish on the surface of the newly formed reef and, when they die, add to its bulk. This process, over thousands of years, raises the reef towards the surface of the water, and it could be described as a 'growing' reef When this reef finally, reaches the surface, upward growth is checked, but lateral (sideways) growth continues. Different types of reefs form in different ways; for example a fringing will grow outwards from a continental island, while a platform reef may form over a solid irregularity anywhere on a continental shelf (Achituv, 1990).

The reefs of Australia's Great Barrier Reef probably originated in this way.

However, special conditions favored their development and continued growth. The first of these conditions was the occurrence of a broad continental shelf off the coast of Queensland which provided a suitable base for coral colonization in shallow water. A second factor that initiated reef development in the past, but was not peculiar to this region alone, was the fluctuation in sea level that occurred in the Pleistocene era many thousands of years ago (Achituv, 1990). During this time the sea level was on several occasions much lower than its present level and parts of the continental shelf were exposed to air.

When the sea began to rise, taking thousands of years, benthic organisms colonizing suitable areas were able to keep pace with the rising sea level until the present reefs were formed (Buddemeier, 1976). Warm water and favorable oceanic and tidally induced currents maintain flourishing coral growths throughout the Great Barrier Reef at the present time.

Types of Coral Reefs:

There are many different types of reefs found within the Great Barrier Reef province and it is useful to be able to identify them so work and observations on one can be compared to another. Coral reefs have been divided into two main categories. The first comprise the oceanic reefs which are found in the open ocean, have a non-limestone reef base and occur in deep water (over I 00 fathoms). The second group includes the shelf reefs which occur in relatively shallow water on continental shelves. Fringing reef are common to both groups although they are best developed in the oceanic province ("Coral Reefs").

The separation of reefs into different types is based on shape, central structure (e.g. development of a lagoon), general zonation and their location on the continental shelf Generally speaking, the reefs of the Great Barrier Reef can be separated into three major categories: wall reefs or linear reefs (commonly referred to as outer barrier reefs) occurring on the northern part of the province near the seaward margin of the shelf, platform reefs, which comprise the majority of reefs, have an oval shape and lie between the seaward margin of the continental shelf and the mainland; and fringing reefs that are found growing out from the shores of continental islands and the mainland.

The Great Barrier Reef today-:

Human intervention along with natural occurrences have created some problems which have proved to be detrimental to the Great Barrier Reef and reefs in general. By the 1990's coral reefs off the coasts of more than 20 countries were being ravaged by coral bleaching, pollution, freighter traffic, and the venomous, coral eating starfish known as the crown of thorns. Bleaching is thought to be caused by rising water temperatures, which the coral cannot withstand. Oil spills and chemical pollution are a major threat to coral. Long-term damage occurs when stands of coral are knocked down by freighters or boat anchors. Over harvesting of fish has resulted in the loss of coral-protecting sea-urchins, leaving the coral open to the crown of thorns (Rowe, 1984). The Australian government, along with other environmental agencies, have been working diligently to preserve and limit destruction of the coral.

WORKS CITED

Achituv, Y. (1990) "Evolution and zoogeography of coral reefs." Ecosystems of the World, 25,

27-49

Buddemeier, R.W. (I 976) "Coral growth." Oceanography and marine Biology Annual

Review, 14, 183-185

Patton, W.K. (I 976) "Animal associates of living reef corals" Biology and Geology of Coral

Reefs, 3, 23-26

Rowe, F.W.E. (1984) "Crown of Thomas" Search, 11, 211-213

"Coral," Microsoft Encarta 96 Encyclopedia

"Coral Reefs," Microsoft Encarta 96 Encyclopedia