What Made The Great Barrier Reef?

DID YOU KNOW?

That the Great Barrier Reef is the only living thing on earth that can be seen from outer space!

All reefs are similar yet all very different, most reefs fall into three categories:

fringing reefs, which occur around the edges of the continental islands that were once a part of the mainland.
ribbon or outer, barrier reefs which grow on the edge of the continental shelf.
platform or patch reefs, which support a cay, or island formed by sedimentary debris swept onto the reef.

One of the major reasons the reef holds such a fascination for scientists and tourist alike is its infinite variety. Some reefs are a little more than a few thousand metres in area while others could be spread over 120 square kilometres. Shapes vary from round, oblong, crescent and indescribable. Some of the very outer reefs are up to 25 kilometres long. At low tide some of the reefs expose themselves while others remain fully submerged. An aerial fly over is a must to appreciate the full extent of the reef.

About 2800 species of fish are known to live in the reef region. Of the 500 or so species of reef building corals found throughout the Indian and Pacific Oceans, about 350 are known to be on the Barrier Reef. It could be decades before scientists have a complete list of all the plants and animals found on any one reef. Many species are still to be identified and named. This clearly shows a staggering diversity of life.

But how were they formed?

Coral reefs are underwater wave-resistant mounds that are virtual ecosystems all by themselves. They are made up of corals, algae, mollusks, bryozoans, brachiopods, echinoderms, and sponges. Coral sands and solid limestone also play a large part in the buildup of a coral reef. Coral reefs grow upward by a rock like accumulation of calcium containing (calcareous) exoskeletons of past generations of coral animals.

In their ideal environments, coral reefs will grow from 1 to 100 centimeters per year. It has been found that reefs are very sensitive to changes in their environment. High rainfall and mainland runoff can kill reefs by increasing the amount of sediment in the water as well as decreasing the salinity of the water. Another danger to reefs is the absence of waves. In this environment, silt will accumulate and suffocate the coral. Following is a list of ideal conditions for coral growth:

At or slightly below sea level
Shallow water
Salinity 30-40 parts per thousand
Areas of strong wave action
Temperatures of 73-80 degrees F

Tropical waters along western coasts of ocean basins (eastern coasts of continents), and around oceanic islands are the best places to find coral reefs. They are usually only found within 30 degrees north or south of the equator.

Theories about how they are created

There are several theories of formation but the most correct ones are from Charles Darwin and Reginald Daly. Darwin's theory is the Subsidence Theory. According to this theory fringing reefs form along the edges of an island and then, as the island subsides, a barrier reef is created. Actual subsidence is caused usually by one of two primary reasons: sea-floor spreading or the great weight of the island itself. Continued subsidence causes the central island to become totally submerged. This leaves a circular reef, an atoll, surrounding an open body of water. This theory was proven by boring into Eniwetok Island, (an atoll in the Marshall Islands of the West Pacific). During the deep drillings, a basaltic summit of an old volcano was encountered at depths of 1200 meters.

A second theory was suggested by Daly. This theory gives another view of a beginning fringing reef. He suggested glaciation as a reason for reef formation. According to his theory, he believes that the string of events leading to the buildup of fringing reefs on a flat surface began with a small fringing reef that encountered an ice age. The cold waters prevented new growth and sea level lowered due to alot of water being used to form a glacier. The part of the reef left out of water was eroded and cut down by waves. After the end of the ice age and the melting of the glaciers, sea level rose and coral reefs began growing once more. This time they grew on flat, planed surfaces. This theory has also been proven.

A third theory suggests that the formation of reefs and atolls is due entirely to sea level rise. All of the theories describe how some reefs form and all are correct to an extent. However, none of these are entirely correct, nor do they explain how all reefs form.