1301.0 - Year Book Australia, 2009–10
ARCHIVED ISSUE Released at 11:30 AM (CANBERRA TIME) 04/06/2010
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GEOGRAPHY OF AUSTRALIA
In a jurisdictional and economic sense, Australia extends well beyond the mainland continent and Tasmania, including about 12,000 islands. There are many near-coastal islands which are parts of states or the Northern Territory, the largest being Melville Island (Northern Territory) at 5,786 sq km. Other major near-coastal islands include Kangaroo Island (South Australia), King and Flinders Islands (Tasmania), Bathurst Island and Groote Eylandt (Northern Territory) and the Torres Strait Islands (Queensland).
Australia also has jurisdiction over a large number of islands remote from the coast. Some of these, such as Macquarie Island (Tasmania) and Lord Howe Island (New South Wales) are legally parts of states, but many are included in separate territories such as the Cocos Islands, Heard and McDonald Islands, Norfolk Island, Christmas Island, the Coral Sea Islands and Ashmore and Cartier Islands. Australia also administers a portion of Antarctica, the Australian Antarctic Territory. While most of these islands are small, the United Nations Convention on the Law of the Sea allows Australia jurisdiction over large tracts of the ocean and seafloor that surround them (see the Forestry and Fishing chapter).
Australia has an Exclusive Economic Zone (EEZ) that is 200 nautical miles (370.4 km) wide, and also incorporates areas of the continental shelf outside the 200-mile boundary. This is measured from the lowest astronomical tide, defined as the lowest level that sea level can be predicted to fall to under normal meteorological conditions. Where the boundary overlaps with potential boundaries of other countries (such as Papua New Guinea, Indonesia, East Timor and some French island territories), a boundary has to be negotiated. The EEZ gives Australia jurisdiction over a marine area of some ten million sq km.
The land area of Australia is almost as great as that of the continental United States of America (excluding Alaska and Hawaii), about twice the size of the European Union, and 32 times greater than that of the United Kingdom. Tables 1.2 and 1.3 show the area of Australia relative to that of other continents and selected countries.
Australia is the lowest, flattest and, apart from Antarctica, the driest of the continents. Unlike Europe and North America, where some landscapes date back to only around 10-20,000 years ago, when great ice sheets retreated, the age of landforms in Australia is generally measured in many millions of years. This gives Australia a very distinctive physical geography.
Map 1.4 shows the elevation of the Australian continent. Most of the continent is at a relatively low elevation, with less than 1% of the country above 1,000 metres elevation. Elevations exceeding 2,000 metres are found only in the Snowy Mountains of New South Wales, with the highest peak being Mt. Kosciuszko (2,228 metres). Higher peaks are found in some external territories, with Mawson Peak on Heard Island reaching 2,745 metres, and much of the Antarctic plateau is above 3,000 metres.
The mainland continent can be divided into three large areas:
The areas have no defined boundaries, however, an indication of the location and size of each of the regions can be obtained from the following description of each of the areas with reference to map 1.4.
The Central Lowlands stretch from the Gulf of Carpentaria through the Great Artesian Basin to the Murray-Darling Plains. Most of this area is flat and low-lying. The main exception occurs in South Australia, where relatively recent faulting has occurred, and the area takes the form of a number of blocks which have been moved up to form a series of ranges (e.g. the Flinders Ranges and Adelaide Hills), with the down-faulted blocks in between forming plains, some of them submerged (e.g. Spencer Gulf). Much of the Central Lowlands is occupied by the Great Artesian Basin, which consists of sedimentary rocks which hold water that enters in the wetter Eastern Highlands.
The Eastern Highlands, stretching along most of the length of the east coast, are characterised over much of their length by a steep escarpment on the coastal side, a series of high plateaus, and then more gentle sloping towards the inland. While the highest elevations (over 1,800 metres) are found in the Snowy Mountains and Victorian Alps, many of the plateaus further north in New South Wales exceed 1,000 metres elevation. In Queensland, however, 1,000 metres is only reached in a few locations and the highlands are generally less pronounced.
The coastal escarpment is particularly marked along much of the New South Wales and southern Queensland coast, as well as more isolated ranges further north, such as those around Cairns. Australia’s highest waterfalls (Wollombi on the Macleay, Wallaman Falls on a tributary of the Herbert, Barron Falls near Cairns, and Wentworth Falls in the Blue Mountains) occur where rivers flow over this escarpment. In the Victorian part of the highlands, the old plateau has been eroded into separate ranges and high plains, and is relatively steep on both the coastal and inland sides. Between the escarpment and the coast lies a coastal strip, sometimes flat but quite hilly in many places, and rarely more than 100 km wide.
As a result of the plateau-like nature of much of the Eastern Highlands, the Great Dividing Range, which separates rivers flowing to central Australia or the Murray-Darling Basin from those flowing to the Pacific Ocean or Bass Strait, is not very pronounced in most locations. In some places, such as the northern Snowy Mountains and Brindabella Ranges, the highest ranges do not coincide with the Great Dividing Range (which in that area is east of Canberra).
The article Landforms and their history in Year Book Australia 1988 provides a more detailed description of Australia's landforms.
The history of Australia's landforms
As noted earlier, much of the Australian landscape is many millions of years old. The Western Plateau is especially old, and includes some of the oldest rocks on earth, more than 3,500 million years old. Most of this region has existed as a landmass for over 500 million years.
The present topography results from a long landscape history which can be considered as starting about 290 million years ago, the last time Australia was subjected to large-scale glaciation. Once the ice melted, parts of the continent subsided and were covered with sediment to form sedimentary basins such as the Great Artesian Basin. By early-Cretaceous times, about 140 million years ago, Australia was already so flat and low that a major rise in sea level divided it into three landmasses as the shallow Cretaceous sea spread over the land. The main separation of Australia from Antarctica took place between 100 and 80 million years ago.
In the following Tertiary times, Australia can be regarded as a landscape of broad swells varied by a number of sedimentary basins (Murray, Gippsland, Eucla, Carpentaria, Lake Eyre and others). These slowly filled up and some are now sources of coal or oil. Most of the Eastern Highlands were uplifted at about this time, although a few parts were still experiencing uplift as recently as one million years ago. The central Australian region was also uplifted, and then eroded, leaving remnant mountains and individual peaks such as Uluru (Ayers Rock), which was exposed about 65 million years ago. Another feature of this era is the Nullarbor Plain, an uplifted limestone sea floor dating to about 25 million years ago.
Throughout the Tertiary, volcanoes erupted in eastern Australia. Some individual volcanoes were the size of modern Vesuvius, and huge lava plains covered large areas. Volcanic activity continued up until a few thousand years ago in Victoria, south-east South Australia and Queensland, and a resumption at some time in the next few thousand years cannot be ruled out. Australia’s youngest volcano is Mt. Gambier in South Australia, about 4,600 years old.
Between 55 and 10 million years ago, Australia drifted across the surface of the Earth as a plate, moving north from a position once adjacent to Antarctica. During much of this period the Earth was much warmer and wetter than it is today, with little or no ice cover even at the poles, and hence Australia retained a warm, relatively moist climate through most of this period despite its latitudinal shift. It was probably under this climate that the deep weathered, iron-rich profiles that characterise much of Australia were formed. Aridity only seems to have set in after Australia reached near its present latitude range about five million years ago, with no known landforms (such as dunes or salt lakes) associated with aridity that are more than one million years old, and the northern part was probably never arid. Australia's deserts in Year Book Australia 2006). Large parts of the arid zone are covered with sand dunes, which are typically aligned longitudinally according to prevailing wind directions (south-east to east in the north, north-west to west in the south). These dunes were formerly mobile but are now mostly fixed. Plains covered with small stones (stony deserts or gibber plains) are found in areas without a sand cover. Salt lakes are found in many low positions, in places following lines of ancient drainage. They are often associated with lunettes (dunes formed on the downwind side of lakes), which have been the location of many important finds of Aboriginal prehistory. In addition to the present arid zone, some of these landforms are found in areas which were formerly arid but have become wetter, such as parts of western Victoria and south-eastern South Australia.
On a global scale, the last few million years were notable for the Quaternary ice age. There were many glacial and interglacial periods (over 20) during this time, with the last ending about 12,000 years ago. As in the rest of the world, Australia’s climate during this time was much cooler (and probably generally drier) than it is today, but only small parts of the continent were glaciated - the Central Plateau of Tasmania and an area of about 25 sq km around the summit of Mount Kosciuszko, above 1,800 metres elevation. These ice sheets disappeared about 20,000 years ago. A more significant impact of glacial periods on Australian landforms was through its impact on sea level; during peak glacial periods the sea level was more than 100 metres lower than it is now, Tasmania and New Guinea were joined to the Australian continent, and in some areas, such as the east coast of Queensland, the coastline was several hundred kilometres away from its present location.
River erosion has been important in carving the detail of much of the Australian landscape. Those rivers which flow directly to the sea have dissected a broad near-coast region into plateaus, hills and valleys. Other rivers drain inland, and while they may be eroding the valleys near their highland sources, their lower courses are filling up with alluvium. Most rivers of the Murray-Darling Basin reach the sea, but many elsewhere either end in salt lakes which are dry for most of the time (such as Lake Eyre), or terminate on the plains of the Central Lowlands (such as the Paroo). Many of the features of the drainage patterns of Australia have a very long history, and some individual valleys have maintained their position for hundreds of millions of years. The salt lakes of the Yilgarn Plateau in Western Australia are the remnants of a drainage pattern that was active before continental drift separated Australia from Antarctica.
During glacial periods of low sea level, coastal rivers tended to cut down to that level, especially towards the sea. When sea levels rose again, some of these valleys were drowned (such as Sydney Harbour), while others filled with alluvium as the sea rose, creating flat lowland valleys.
Coastal geomorphology is also largely the result of the accumulation of sediment in drowned coasts. In some areas, such as Ninety Mile Beach (Victoria) or the Coorong (South Australia), there are long beaches made simply from this accumulation. Further north along the east coast, many parts of the coastline consist of alternating long beaches and rocky headlands, with the beaches backed by plains filled with river and marine sediments.
The offshore shape of Australia, revealed in isobath contours, results mainly from the pattern of break-up of the super-continent of which Australia was once a part. The continental shelf around Australia varies greatly in width; in some areas it is several hundred kilometres wide, while in other areas, such as off far south-eastern New South Wales and much of Tasmania, it is less than 40 km in width. In South Australia, the continental shelf is cut by submarine canyons up to 4,600 metres deep offshore from the mouth of the Murray River. The Queensland coast is bounded by a broad plateau which has been exposed during the various glacial periods. Coral reefs have grown on this plateau at various times during the last 700,000 years when it has been submerged, although the present Great Barrier Reef, which did not start to form until after the last glaciation, is only a few thousand years old.
The Australian landforms of today are thus seen to result from long continued processes in a unique setting, giving rise to typical Australian landscapes, which in turn provide the physical basis for the distribution and nature of biological and human activity in Australia.
Rivers and lakes
As described earlier, the rivers of Australia may be divided into two major classes; those of the coastal margins with moderate rates of fall, and those of the central plains with very slight fall. Australia’s longest river system, the Murray-Darling, drains part of Queensland, most of New South Wales and northern Victoria, and a section of South Australia, finally flowing into the arm of the sea known as Lake Alexandrina, on the South Australian coast. The length of the Murray is about 2,520 km, while the longest branch of the combined Murray-Darling system, with its headwaters in the Culgoa catchment, is about 3,370 km long.
Most of the east coastal rivers are short, the exceptions being those rivers which penetrate the coastal escarpment, such as the Burdekin and Fitzroy in Queensland, and the Hunter in New South Wales. The south-west of Western Australia also has a number of short coastal rivers.
A number of river systems reach the tropical or sub-tropical coast. Many of these are of considerable length, such as the Mitchell, Gregory and Leichhardt in northern Queensland, the Daly and Victoria in the Northern Territory, and the Ord, Fitzroy, Ashburton, Fortescue and Gascoyne in Western Australia. All of these rivers have extremely large variations in flow between wet and dry seasons, arising from the great seasonal rainfall variations typical of this region, and some only flow intermittently. The Mitchell, whose annual discharge of about 12 cubic kilometres rivals the Murray-Darling as Australia’s largest river system in terms of volume, has discharges in February and March about 100 times those of July.
Australian river discharges are very small compared with those of many rivers elsewhere, reflecting the very low runoff from the Australian continent. By way of comparison, the annual discharge from the Amazon basin in South America is approximately 7,000 cubic kilometres.
There are many lake types in Australia. The largest are salt lakes which are, or were, drainage sumps from internal rivers. For most of the time these lakes are beds of salt and dry mud. Lake Eyre, which has only filled three times in the last century, is the largest of these (9,500 sq km), while other large salt lakes include Lake Torrens (5,745 sq km) and Lake Gairdner (4,351 sq km).
Other natural lake types include coastal lakes formed by damming of valleys by marine sediments, fault angle lakes (such as Lake George near Canberra), volcanic lakes (mostly in Victoria, south-eastern South Australia and Queensland), and glacial lakes (most common in Tasmania, but also found in the Snowy Mountains). Many of these lakes are permanent, but some, such as Lake George, dry out during drought periods, and all are small compared with the inland salt lakes - Australia has no natural, unmodified, permanent freshwater lake larger than 100 sq km. Many artificial lakes, or lakes expanded by artificial means, also exist in all states and territories. The combined Lakes Gordon and Pedder in south-western Tasmania are the largest of these, both in surface area (513 sq km) and volume (11,320 megalitres (ML)), while other very large artificial lakes include Lake Argyle on the Ord in northern Western Australia (5,720 ML) and Lake Eucumbene in the Snowy Mountains Scheme (4,870 ML).