10. Great Barrier Reef, East Coast of Australia
The Great Barrier Reef is the largest structure ever built by living organisms. At least 350 different species of coral are found in the reef, which is 2000 kilometers (1250 miles) long and forms a natural breakwater for the east coast of Australia. Underlying sediments, twice as old as the reef itself, indicate that the region was once above sea level. Geological evidence shows that the reef began growing more than 25 million years ago. As the photograph shows, the “reef” is in fact composed of many individual detached reefs, separated by deep water channels. The calcareous remains of tiny creatures called coral polyps and hydrocorals provide the basic building material for the reefs, while the remains of coraline algae and organisms called polyzoas provide the cement that holds the structure together. When fossilized, such reefs and the debris eroded from them form thick limestone units.
The Great Barrier Reef is the largest reef on Earth at the present day. The reasons for its size and longevity are concerned with the very stable geological setting of the Australian platform, and the favorable oceanic circulation. Coral cannot exist at temperatures below 21° Centigrade. The warm waters of the Australian continental shelf vary little with depth because of the stirring action of the southeast trade winds. These winds pound the outer edge of the reef for 9 months of the year, and this also keeps the reef supplied with seawater rich in the organic material needed by the growing coral.
STS-8, August-September 1983. Picture #8-46-0973.