How the Great Barrier Reef was formed - Twenty thousand years ago

Ningaloo Reef in Western Australia is the longest fringing reef in the world

20,000 years ago, the world was starting to emerge from the most recent ice age. The sea level around the Australian coast was then about 120 meters lower than it is today.

The coastal mountain ranges we see today in north-eastern Australia were further inland. A coastal plain, dotted with hills, separated these mountains from the sea.

Ningaloo Reef is a coral reef that fringes the coast in north-western Western Australia.

The north Queensland coast probably looked something like this 20,000 years ago.

Aboriginals then living in the region where the coastal city of Townsville is today would have had to travel more than 70 kilometres to reach the nearest beach.

The beaches 20,000 years ago would not have looked any different from the beaches of today.

Sea-level was approximately 120 meters lower than present time.

Why did reefs form on the edge of the continental shelf?

The corals that build reefs are not single animals. They are colonies made up of many individual animals called polyps. A coral polyp is rather like a sea anemone. It is a sack-like animal with a mouth surrounded by tentacles. The polyps are joined together to form a colony.

Figure 1 : Cross section of coastal north Queensland 20,000 years ago.

The corals that build reefs have an association with a single-celled alga. Photosynthesis by algae in coral tissues allows the animal to make skeleton about 3 times faster in the light than in the dark. Reefs are formed because of this association between an animal and a plant. The association makes it possible for corals to deposit skeleton faster than destructive agencies can remove it. Coral skeletons are made of a form of calcium carbonate called aragonite.

Coral skeletons are the bricks from which coral reefs are built. The bricks are cemented together by calcium carbonate deposited by encrusting algae. Like the corals, these calcareous algae also use light to help them form calcium carbonate.

Reefs are only formed in shallow water where there is adequate light for the organisms that build them. Reefs do not form at depths greater than about 30 meters. They also do not form where the water temperature drops below about 18 degrees Celsius. This is why coral reefs mostly form in clean, clear, shallow tropical seas.