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.
The Reef Research Centre, a Cooperative Research Centre, has found coral 'skeleton' deposits that date back half a million years.The Great Barrier Reef Marine Park Authority says that corals have been growing in the region for as long as 25 million years, but have not always formed coral reef structures. The GBRMPA considers the earliest evidence to suggest complete reef structures to have been 600,000 years ago.
Dating inconsistencies stem from 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.
According to the Great Barrier Reef Marine Park Authority, the current, living reef structure is believed to have begun growing on an 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. The land that formed the substrate of the Great Barrier Reef was a coastal plain with some larger hills (some of which were themselves remnants of older reefs).
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 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.