Australia: The Land Where Time Began

A biography of the Australian continent 

Great Barrier Reef, Queensland

The Great barrier Reef is the world's largest system of coral reefs. It stretches for 2300 km down the coast of Queensland, from 9o 15' S, near the most northerly point on the Australian continent, the tip of Cape York Peninsula, to Lady Elliot Island, to the southeast of Gladstone, 24o 7' S, with a total area of about 230,000 km2. It is comprised of more than 2,600 reefs that range in size from small pinnacles to more than 100 km2. There are places along the reef where the coralline rock overlies old mangrove forests.

The reefs occur from the low tide mark to beyond the 200 m bathymetric contour on the continental shelf. The shelf width increases from north to south, as does the width of the reef, increasing from 23 km wide in the north to 260 km in the south. The shelf also deepens from north to south. It is believed that at time of high sea level in the Pleistocene the northern part of the shelf would probably have been exposed for a longer time than the southern part, the southern extent of the shelf would probably have been below the sea at times of higher sea levels. The northern part of the shelf has a generally steeper continental slope than in the south, and also extends deeper than in the south.

The climate has a strong influence on the development of the reef, and because the reef is spread across such as wide range of latitudes the reef development is varied. At Cairns the average temperature in summer is 30o C, and at Gladstone, further to the south, it is 26o C. The average winter temperatures range from 26o C to 15o C respectively. There is a strong climate seasonality because of the rainfall maximum in summer, that is highest in the central region, that is wetter, and towards the southern limits of the reef, cooler winters. In summer, tropical cyclones also have an influence on the reef morphology.

Reef development is also affected by run-off of fresh water and sediment load from major rivers such as the the Burdekin. In the northern part of the reef it is more susceptible to influences from the mainland than in the south, where it is further away from the coast. These influences in the north, such as the amount of rainfall and the consequent amount of freshwater discharged by rivers into the sea, is increased by human activity in the region, especially by the increased sediment load that results from denudation associated with logging and mining. There are also chemical pollutants from human habitation and the chemicals used in agriculture.

There is a wide range of tidal heights in the area of the reef, from broad Sound, to the south of Mackay, at 10 m, to less than 3 m on the southern part of the reef and to the north of Cairns, and the central region, which has a significant effect on the flushing and water mixing. There are generally moderate wave-energy conditions on the reef, though they can be high on the margins of the reef. The elongation of many reefs, in a northwest-southeast direction, is believed to result from the consistency of the prevailing southeast trade winds.

The morphological complexity of forms present on the Great Barrier Reef is a result of the widely varying climatic conditions, and the foundations the reef has formed on, over the great extent of the reef, as well as the fluctuating sea level of the region over time as global sea levels rose and fell. According to the authors, the simple reef classification proposed by Charles Darwin is still valid, though with many varieties in each of the categories. The barrier reefs, that make up the bulk of the Great Barrier Reef, display a great variety of forms, depending on such factors as the depth and foundation morphology, the exact nature of Holocene sealevel rise, and the calcium carbonate production rate.

A classification of shelf reefs of the Great Barrier Reef has been constructed by D. Hopley, based on the extent of mimicking or masking of antecedent morphology by reef growth in the Holocene. His assumption being that Initial colonisation occurred on the antecedent foundation, and subsequent growth upward of the reef has a tendency to lag behind the rising sea level. Based on this model, highs in the antecedent surface, especially around reef margins, are the sites where reef flats develop. The inherited relief is eventually masked as lagoons that are infilled. The final stage is the lateral movement of sediment from the windward to the leeward side.

The authors suggest that the basic form of the genetic classification proposed by Darwin is probably more complex than Darwin envisioned, as evidence is lacking that the primary form is a fringing reef that is converted to a barrier reef, becoming atolls when submerged. The most southerly extent of the Great Barrier Reef is in the form of fringing reefs, either attached to the mainland or surrounding continental islands. These extend as far south as the Solitary Islands off Coffs harbour in New South Wales (29o 52' S to 30o 14' S), as well as around Lord Howe Island, 31o 35' S. Cape Kimberley, 16o 15' S, is the most southerly extent of well formed fringing reefs. Most fringing reefs are 250 m in diameter or lass, the main constituent organisms being algae rather then coral.

Yule Point, to the north of Cairns, was originally a patch reef that has been linked by sediment to the coast. North of the Whitsunday Passage, it is common for fringing reefs to surround continental islands, though proximity to the mainland and run-off from the larger islands has a tendency to limit their development. An exception is Double Island, 20 km to the north of Cairns and 1 km from the mainland, that has a fringing reef that is 1200 m wide. On the windward side, facing the southeast, the reefs are narrower with steeper margins, while on the leeward side the wider reefs have more gently sloping margins. It has been suggested, based on evidence from Hayman Island, that fringing reefs on the Great Barrier Reef originated at depths of about 20 m on foundations that were attached to the shoreline about 9,500 years ago in the Holocene. The migration of the shoreline in a landward direction resulting from rapid rise of the sea level. Growing upward at an average rate of about 2.9 mm/year, addition of carbonate detritus allowing the lateral spread of the reef. Sediments from the land have contributed no more than 10 % of the material of the reef, 90 % being carbonate throughout the Holocene.

One reef form that is not present in the region of the Great Barrier Reef is the true atoll, that are annular ribbon reefs that enclose a lagoon, that rise from the deep ocean. Shelf atolls, variants of patch reefs, were described by Hopley as being present among platform reefs.

Sources & Further reading

Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd

Author: M. H. Monroe
Last updated 20/04/2011  



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