What is the Great Barrier Reef?
|Changing sea level affects the history of the Great
|A coral cay may form on platform reefs
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
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.