Great Barrier Reef

I

Introduction

Great Barrier Reef, world’s largest coral reef system, located off the northeastern coast of Australia bordering the Coral Sea (part of the Pacific Ocean). The reef extends about 2,010 km (about 1,250 mi) roughly parallel to the coast of the state of Queensland, from the Torres Strait in the north to near the town of Bundaberg in the south.

Simple marine animals called corals constructed the Great Barrier Reef over thousands of years. Nearly 3,000 individual coral reefs and some 300 small coral islands form the reef, which ranks as the world’s largest structure made by living organisms. This intricate network of coral formations has an area of about 37,000 sq km (about 14,300 sq mi), encompassing about 13 percent of the world’s total coral reef.

In 1975 the Great Barrier Reef Marine Park was established as the largest marine protected area in the world (though it was overtaken in size by the Northwestern Hawaiian Islands Marine National Monument in 2006). The park covers some 344,800 sq km (133,100 sq mi), including waters surrounding the reefs. In 1981 the park was designated a World Heritage Area by the World Heritage Committee of the United Nations Educational, Scientific and Cultural Organization (UNESCO). The Great Barrier Reef is considered one of the great natural wonders of the world.

II

Formation and Physical Features

A barrier reef lies parallel to a coastline and is always separated from the shore by a wide lagoon. The Great Barrier Reef provides a natural breakwater for the waves of the Coral Sea. The reef’s outer edge ranges from as close as 30 km (18 mi) to the Australian mainland, to as far as 250 km (155 mi). The depth of the lagoon between the Great Barrier Reef and the mainland varies, but it is generally no more than about 35 m (about 115 ft).

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Most barrier reefs lie on the edge of a continental shelf (the submerged edge of a continent). A shelf slopes downward gradually to its outermost edge and then descends abruptly to the ocean bottom. The Great Barrier Reef lies on the edge of the Australian continental shelf, and the waters off the outer margins of the reef drop to a depth of more than 1,000 m (3,280 ft).

A

Formation of the Reef

At least 350 species of coral are responsible for the construction of the Great Barrier Reef. Corals are biologically simple animals that belong to the same animal group (phylum) as jellyfish and sea anemones (see Cnidarians). Living corals have a soft tubular body, called a polyp, which is attached to a surface at its base and is topped by a mouth and ring of tentacles that snare food. Although many types of coral may live in a coral reef, not all are reef builders.

Some corals live in large colonies and lay down a communal skeleton of calcium carbonate (limestone). Where large numbers of these “stony corals” live together, their combined skeletons begin to build a coral reef. Over time, sand and rubble fill gaps between the skeletons of various colonies, cementing them together and forming a solid limestone foundation. The coral reef begins to grow. Other animals, and even plants, also contribute to the formation of the reef. Giant clams and other mollusks, for example, add their own calcium-carbonate shells to the reef structure. Coral reefs grow layer by layer over thousands of years. Scientists can determine the age of coral reefs by drilling thin cores through them to measure the thickness of the underlying limestone.

The stony corals that build coral reefs are slow-growing but long-lived corals. Many types of stony corals grow in the Great Barrier Reef, including branching corals, staghorn corals, massive (or boulder) corals, brain corals, plate corals, and mushroom corals. Branching corals may grow their skeletons vertically by about 10 to 15 cm (about 4 to 6 in) per year, while massive corals may increase in diameter by only 1 to 2 cm (0.4 to 0.8 in) per year. Some individual massive corals in the Great Barrier Reef reach 7 m (23 ft) in diameter and are more than 700 years old. Coral reefs can grow upward (toward the water’s surface) about 1 cm (about 0.4 in) a year. However, corals are marine creatures and cannot grow above the water’s surface; exposure to air and rain, or other fresh water, can kill them.

Coral reefs grow no deeper than 150 m (500 ft) because stony corals require sunlight to survive. Within the tissues of the polyps live single-celled algae called zooxanthellae, which produce life-sustaining nutrients for the corals through photosynthesis, a process that requires sunlight.

Some parts of the Great Barrier Reef began to grow more than 2 million years ago, but most of the reef began to develop about 500,000 years ago. Australia had drifted slowly northward toward the equator by that time (see Plate Tectonics), and the warmer ocean waters surrounding the continent created a coral-friendly habitat. The growth of the Great Barrier Reef was subsequently interrupted by global changes in sea level and temperature during periods of glaciation, when more of the planet’s water was frozen in glaciers due to global cooling (see Ice Ages). The reef’s current period of active growth began about 8,000 years ago, toward the end of the most recent glaciation. At that time melting glaciers caused sea levels to rise, flooding the continental shelves and creating the shallow waters necessary for coral reef growth.

The approximately 300 small coral islands, or cays, in the Great Barrier Reef are accumulations of sand and coral rock. They formed as a result of underwater currents and wave action during storms, rather than from coral growth. Some of the islands are covered with tropical forests or other vegetation, providing an important habitat for sea birds. The coral islands include Heron Island and Green Island. In addition, there are 540 offshore continental islands that are not part of the Great Barrier Reef or the marine park, although many are national parks. The continental islands include the Whitsunday Islands, Palm Islands, Lizard Island, and Magnetic Island.

B

Topography of the Reef

The appearance of the Great Barrier Reef changes gradually from north to south. Patches of reef are found in the northern Torres Strait, which separates Australia’s Cape York Peninsula from New Guinea. Long, narrow ribbon reefs form the outer edge of the Great Barrier Reef for some 570 km (350 mi), from the Torres Strait to near Cooktown. These ribbon reefs are up to 25 km (16 mi) long but less than 500 m (1,640 ft) wide. Inshore from these reefs are many other reef formations, such as broad, flat platform reefs, as well as wide, shallow banks and shoals that are covered by marine algae.

The distance between reef and mainland becomes much greater south of the Whitsunday Islands, which are located about 650 km (about 400 mi) south of Cooktown. This area of the reef includes the Pompey Complex, which comprises some huge reef areas separated by narrow meandering channels, and the smaller Swain Reefs. The Swain and Pompey reefs are rarely visited because they are far from land and hazardous to navigate.

The southernmost reefs, known as the Capricorn-Bunker Group, lie closer to the mainland. They are located at the Tropic of Capricorn (the southern limit of the tropics), where the waters are relatively cool.

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