What is the Great Barrier Reef?

Changing sea level affects the history of the Great Barrier Reef Image: GBRMPA

A coral cay may form on platform reefs Photo: GBRMPA

The Great Barrier Reef is about 500,000 years old, but it has not been present in its current form for all of that time. Reefs have grown and receded on Australia's continental shelf, depending on sea level changes. The present reef structure is only about 6,000 to 8,000 years old, and is growing on top of the underlying structure of old reefs formed during periods of higher sea levels.

The Great Barrier Reef is not a single continuous reef along Australia's continental shelf. Rather, it is composed of around 2,900 individual reefs of many shapes and sizes, and includes some continental (or rocky) islands surrounded by reefs. The Great Barrier Reef comes quite close to the mainland, within a few kilometres, in the northern Great Barrier Reef, and is hundreds of kilometres offshore in the southern parts including the Swains Reefs.

A fringing reef can form adjacent to the mainland Photo: GBRMPA

Most reefs of the Great Barrier Reef have formed on the continental shelf rather than in the deep ocean like atolls, and are generally known as shelf reefs. The reefs on the Great Barrier Reef include fringing reefs, ribbon reefs, deltaic reefs and platform reefs. Fringing reefs form adjacent to the mainland or a high island. Platform reefs are oval in shape, 3km to 10 km long, and often have a lagoon in the middle. Some platform reefs accumulate sand in one section to form a coral cay, and this may be stable enough to support vegetation, and hence populations of birds other small animals. Ribbon reefs occur only in the northern section of the Great Barrier Reef, close to the edge of the continental shelf. They are long and thin and lack a lagoon. Deltaic reefs occur only in the northern Great Barrier Reef, and resemble a river delta.