Coral reefs are the world's only major L2 501 / geological feature constructed by plants and animals] and the Great Barrier Reef is by far the largest of them all. On an [G 11 / evolutionary] scale, corals have been very successful. The first forms, called Rugosa, appeared more than 450 million years ago and may have formed reefs similar to those today. The Rugosa became extinct about 200 million years ago and were replaced by the Scleractinia, which probably flourished in the warm waters of the ancient Tethys Sea.
Until about 53 million years ago, Australia was located in the colder latitudes attached to Antarctica. As a continuation of the earth processes that broke up the supercontinent of Pangea, Australia 'drifted' into [L2 502 / a position whose environmental conditions were well suited to coral growth] - surface water temperature from 70-100 degrees Fahrenheit, an average [G 26 / salinity] of 35 parts per 1,000, a high oxygen content with 90 percent saturation and crystal clear water with visibility up to 200 feet.
The Great Barrier Reef is, therefore, geologically young. Parts began to form no more than 18 million years ago in the north, with others hardly beginning more than one million years ago in the south. The major factor that determined reef growth was the changes in sea level. The two main causes of changes in sea level are environmental fluctuations that cause [L2 503 / ice ages] and [L2 504 / plate tectonics] that have changed the geology of the world's ocean basins. These have had a significant impact on the formation of the Barrier Reef and are, to a large degree, responsible for what we see today.
