The Great Barrier Reef is the world's largest coral reef system, composed of roughly 3,000 individual reefs and 900 islands stretching for 2,600 kilometres (1,616 mi) over an area of approximately 344,400 square kilometres (132,974 sq mi). The reef is located in the Coral Sea, off the coast of Queensland in northeast Australia.

The Great Barrier Reef can be seen from outer space and is the world's biggest single structure made by living organisms. This reef structure is composed of and built by billions of tiny organisms, known as coral polyps. The Great Barrier Reef supports a wide diversity of life, and was selected as a World Heritage Site in 1981. CNN has labelled it one of the seven natural wonders of the world. The Queensland National Trust has named it a state icon of Queensland.

A large part of the reef is protected by the Great Barrier Reef Marine Park, which helps to limit the impact of human use, such as overfishing and tourism. Other environmental pressures to the reef and its ecosystem include water quality from runoff, climate change accompanied by mass coral bleaching, and cyclic outbreaks of the crown-of-thorns starfish.

Physiography

The Great Barrier Reef is a distinct physiographic province of the larger East Australian Cordillera division. It encompasses the smaller Murray Islands physiographic section.

Geology and geography

Australia moved northwards at a rate of 7 cm per year, starting during the Cainozoic. Eastern Australia experienced a period of tectonic uplift, leading to the drainage divide in Queensland moving 400 km inland. Also during this time, Queensland experienced volcanic eruptions leading to central and shield volcanoes and basalt flows. Some of these granitic outcrops have become high islands. After the Coral Sea Basin was formed, coral reefs began to grow in the Basin, but until about 25 million years ago, northern Queensland was still in temperate waters south of the tropics - too cool to support coral growth. The history of the development of the Great Barrier Reef is complex; after Queensland drifted into tropical waters, the history is largely influenced by how reefs fluctuate (grow and recede) as the sea level changes. They can increase in diameter from 1 to 2 centimetres per year, and grow vertically anywhere from 1 to 15 centimetres (0.4–12 in) per year; however, they are limited to growing above a depth of 150 metres (492 ft) due to their need for sunlight, and cannot grow above sea level. The land that formed the substrate of the current Great Barrier Reef was a coastal plain formed from the eroded sediments of the Great Dividing Range with some larger hills (some of which were themselves remnants of older reefs or volcanoes). When Queensland moved into tropical waters 24 million years ago, some coral grew, but a sedimentation regime quickly developed with erosion of the Great Dividing Range; creating river deltas, oozes and turbidites, which would have been unsuitable conditions for coral growth. 10 million years ago, the sea level significantly lowered, which further enabled the sedimentation.

The Reef Research Centre, a Cooperative Research Centre, has found coral 'skeleton' deposits that date back half a million years. The GBRMPA considers the earliest evidence to suggest complete reef structures to have been 600,000 years ago.

According to the Great Barrier Reef Marine Park Authority, the current, living reef structure is believed to have begun growing on the older platform about 20,000 years ago. The Australian Institute of Marine Science agrees, which places the beginning of the growth of the current reef at the time of the Last Glacial Maximum. At around that time, the sea level was 120 metres (394 ft) lower than it is today.

From 20,000 years ago until 6,000 years ago, the sea level rose steadily. As it rose, the corals could then grow higher on the hills of the coastal plain. By around 13,000 years ago the sea level was 60 metres (197 ft) lower than the present day, and corals began to grow around the hills of the coastal plain, which were, by then, continental islands. As the sea level rose further still, most of the continental islands were submerged. The corals could then overgrow the hills, to form the present cays and reefs. Sea level on the Great Barrier Reef has not risen significantly in the last 6,000 years. The CRC Reef Research Centre estimates the age of the present, living reef structure at 6,000 to 8,000 years old.

The remains of an ancient barrier reef similar to the Great Barrier Reef can be found in The Kimberley, a northern region of Western Australia.

The Great Barrier Reef World Heritage Area has been divided into 70 bioregions,of which 30 are reef bioregions, and 40 are non-reef bioregions. In the northern part of the Great Barrier Reef, ribbon reefs and deltaic reefs have formed; these structures are not found in the rest of the Great Barrier Reef system. There are no atolls in the system, and reefs attached to the mainland are rare.

Fringing reefs are distributed widely, but are most common towards the southern part of the Great Barrier Reef, attached to high islands, for example, the Whitsunday Islands. Lagoonal reefs are also found in the southern Great Barrier Reef, but there are some of these found further north, off the coast of Princess Charlotte Bay. Cresentic reefs are the most common shape of reef in the middle of the Great Barrier Reef system, for example the reefs surrounding Lizard Island. Cresentic reefs are also found in the far north of the Great Barrier Reef Marine Park, and in the Swain Reefs (20-22 degrees South). Planar reefs are found in the northern and southern parts of the Great Barrier Reef, near Cape York, Princess Charlotte Bay, and Cairns. Most of the islands on the reef are found on planar reefs.