Figure 1 The reefs are generally flat–topped underwater hills around 50–100 metres high sitting on the plain of the continental shelf. In this image the reefs to the right are the Ribbon Reefs offshore from Cooktown. (Courtesy R. Beaman, deepreef.org)
The GBR is an underwater chain of 3 000 flat–topped hills sitting on a wide underwater plain called the continental shelf (Figure 2.5). The continental shelf surrounding the reefs is between 50 and 100 metres deep and the sides of each reef are very steep. The shape of the reefs gives a clue to how they form. They are just a pile of dead coral with a surface layer of live coral and other organisms. The reef literally grows on the skeletons of dead ancestors, rising slowly until it reaches sea level
The continental shelf upon which the reefs sit has a very steep edge on the eastern side that drops down into the deep water of the Coral Sea and Queensland Trough (Figure 2.6) where the water depth exceeds 1 000 meters. Many reefs such as the Ribbon Reefs are on the edge of this cliff.
Figure 2 The Great Barrier Reef shelf north of Cairns. The reefs are the red patches. The seafloor drops away from around 100 metres to 1 000 metres just offshore from the edge of the barrier reefs. During periods of low sea level (18 000 years or more ago) all the red and yellow areas were dry land. (Bathymetry image: R Beaman, deepreef.org)
For more spectacular images of the bathymetry of the GBR go to deepreef.org