World of Earth Science | Australia

Of the seven continents, Australia is the flattest, smallest, and except for Antarctica, the most arid. Including the southeastern island of Tasmania, the island continent is roughly equal in area to the United States, excluding Alaska and Hawaii. Millions of years of geographic isolation from other land-masses accounts for Australia's unique animal species, notably

marsupial mammals like the kangaroo, egg laying mammals like the platypus, and the flightless emu bird. Excluding folded structures (areas warped by geologic forces) along Australia's east coast, patches of the northern coastline and the relatively lush island of Tasmania, the continent is mostly dry and inhospitable.

Australia has been less affected by seismic and orogenic (mountain building) forces than other continents during the past 400 million years. Although seismic (earthquake) activity persists in the eastern and western highlands, Australia is the most stable of all continents. In the recent geological past, it has experienced none of the massive upheavals responsible for uplifting the Andes in South America, the Himalayas in south Asia, or the European Alps. Instead, Australia's topography is the result of gradual changes over millions of years.

Australia is not the oldest continent, a common mis-conception arising from the continent's flat, seemingly unchanged expanse. Geologically, it is the same age as the Americas, Asia, Africa, Europe, and Antarctica. Australia's crust, however, has escaped strong Earth forces in recent geological history, accounting for its relatively uniform appearance. As a result, the continent serves as a window to early geological ages.

About 95 million years ago, tectonic forces (movements and pressures of Earth's crust) split Australia from Antarctica and the southern supercontinent of Gondwanaland. Geologists estimate that the continent is drifting northward at a rate of approximately 18 in (28 cm) per year. They theorize that south Australia was joined to Antarctica at the Antarctic regions of Wilkes Land, including Commonwealth Bay. Over a period of 65 million years, beginning 160 million years ago, Australia's crust was stretched hundreds of miles by tectonics before it finally cleaved from Antarctica.

Testimony to the continental stretching and splitting includes Kangaroo Island off South Australia, made up of volcanic basalts, as well as thick layers of sediment along the coast of Victoria. Other signs are the similar geology of the Antarctic Commonwealth Bay and the Eyre Peninsula of South Australia, especially equivalent rocks, particularly gneisses (metamorphic rocks changed by heat and pressure) of identical age. The thin crust along Australia's southern flank in the Great Australian Bight also points to continental stretch.

As it drifts north, the Australian plate is colliding with the Pacific and Eurasian plates, forming a subduction zone (an area where one continental plate descends beneath another). This zone, the convergence of the Australian continental plate with Papua New Guinea and the southern Indonesian islands, is studded with volcanos and prone to earthquakes. Yet, Australia is unique in that it is not riven by subduction zones like other continents. There are no upwelling sections of the earth's mantle below Australia (the layer below the crust), nor are there intracontinental rift zones like the East African Rift System which threatens to eventually split Africa apart.

Furthermore, Australia and Antarctica are dissimilar to other landmasses; their shapes are not rough triangles with apexes pointing southward like South America, Africa, and the Indian subcontinent, Gondwanaland's other constituent parts. However, like its sister continents, Australia is composed of three structural units. These include in Western Australia a stable and ancient block of basement rock or craton as geologists call it, an ancient fold mountain belt (the Great Dividing Range along the east coast), and a flat platform-like area in-between composed of crystalline or folded rocks overlaid by flat-lying or only gently deformed sediments.

Millions of years of erosion have scoured Australia's surface features. One notable exception to Australia's flat topography is the Great Dividing Range stretching 1,200 mi (1,931 km) along Australia's east coast. The Great Dividing Range was thrust up by geological folding like the Appalachian Mountains in the eastern United States. The mountains are superimposed on larger geological structures including the Tasman and Newcastle geosynclines, troughs of older rocks upon which thick layers of sediment have been deposited. Those sediments in turn have been transformed by folding as well as magmatic and volcanic forces.

Twice, during a 125 million year period beginning 400 million years ago, the geosynclines were compressed, forming mountains and initiating volcanoes. Volcanic activity recurred along the Great Dividing Range 20–25 million years ago during the Miocene Epoch when early apes evolved as well as seals, dolphins, sunflowers, and bears. However, over millions of years the volcanic cones from this epoch have been stripped down by erosion. Still, volcanic activity persisted in South Australia until less than a million years ago. In Queensland, near Brisbane in the south and Cairns in the north of the state, the Great Dividing Range hugs the coast, creating beautiful Riviera-like vistas.

East of the Great Dividing Range, along Australia's narrow eastern coastal basin are its two largest cities, Sydney and Melbourne, as well as the capital, Canberra. The Dividing Range tends to trap moisture from easterly weather fronts originating in the Pacific Ocean. Rivers and streams also course the Range. West of the Range the landscape becomes increasingly forbidding and the weather hot and dry.

Although unrelated to geological forces, the world's largest coral formation, the Great Barrier Reef stretches for 1,245 mi (2,003 km) along Australia's northeast coast. Most of Australia is referred to as outback—desert and semi-desert flatness, broken only by scrub, salt lakes which are actually dry lakebeds most of the year, and a few spectacular sandstone proturburances like Uluru (also known as Ayers Rock) and the Olgas (Kata Tjuta).

In 1991, geologists discovered a subterranean electrical current in Australia, the longest in the world, which passes through more than 3,700 mi (6,000 km) across the Australian outback. The current is conducted by sedimentary rocks in a long horseshoe arc that skirts a huge mass of older igneous and metamorphic rock comprising most of the Northern Territory. It begins at Broome in Western Australia near the Timor Sea and then dips south across South Australia before curling northward through western Queensland where it terminates in the Gulf of Carpenteria.

A side branch runs from Birdsville in South Australia near the Flinders Ranges into Spencer Gulf near Adelaide. Geologists say the current is induced by the Earth's everchanging magnetic field and that it runs along fracture zones in sedimentary basins that were formed as the Earth's ancient plates collided. Although the fracture zones contain alkaline fluids that are good conductors of electricity, the current is weak and cannot even light a lamp. Geologists say the current might provide clues to deposits of oil and gas and help explain the geological origins of the Australian continent.

Australian topography is also punctuated by starkly beautiful mountain ranges in the middle of the continent like the McDonnell and Musgrave Ranges, north and south respectively of Uluru (Ayers Rock). Uluru, the most sacred site in the country for Australia's aborigines, is a sandstone monolith of which two-thirds is believed to be below the surface. Ayers Rock is about 2.2 mi (3.5 km) long and 1,131 ft (339 m) high. Also in the center of the country, near Alice Springs, are the Henbury Meteorite craters, one of the largest clusters of meteorite craters in the world. The largest of these depressions, formed by the impact of an extraterrestrial rock, is about 591 ft (177 m) long and 49 ft (15 m) deep.

The continent's oldest rocks are in the Western Australian shield in southwest Australia. The basement (underlying) rocks in this area have not been folded since the Archean eon over three billion years ago, when the planet was still very young. The nucleus of this shield (called the Yilgarn craton) comprising 230,000 sq mi (59,570,000 ha), consists mostly of granite with belts of metamorphic rock like green-stones, rich in economic mineral deposits as well as intrusions of formerly molten rock.

The Yilgarn craton does not quite extend to the coast of Western Australia. It is bounded on the west by the Darling Fault near Perth. To the south and east the Frazer Fault sets off the craton from somewhat younger rocks that were metamorphosed between 2.5 billion and two billion years ago. Both fault lines are 600 mi (960 km) long and are considered major structures on the continent.

Along the north coast of Western Australia near Port Hedland is another nucleus of ancient rocks, the Pilbara Craton. The Pilbara craton is composed of granites over three billion years old as well as volcanic, greenstone, and sedimentary rocks. The Hammersley Range just south of the Pilbara Craton is estimated to contain billions of tons of iron ore reserves.

Other ancient rock masses in Australia are the Arunta Complex north of Alice Springs in the center of Australia which dates to 2.25 billion years ago. The MacArthur Basin, southwest of the Gulf of Carpenteria in the Northern Territory is a belt of sedimentary rocks that are between 1.8 billion and 1.5 billion years old.

The Musgrave block near the continent's center, a component of the Adelaidian geosyncline, was formed by the repeated intrusion of molten rocks between 1.4 billion to one billion years ago during the Proterozoic Era when algae, jellyfish, and worms first arose. At the same time, the rocks that underlay the Adelaidian geosyncline were downwarped by geological pressures, with sediments building up through mid-Cambrian times (about 535 million years ago) when the area was inundated 400 mi (640 km) by the sea inland of the present coastline.

The rocks of the Adelaidian geosyncline are as thick as 10 mi (16 km) with sediments that have been extensively folded and subjected to faulting during late Precambrian and early Paleozoic times (about 600 million to 500 million years ago). Some of the rocks of the Adelaidian geosyncline, however, are unaltered. These strata show evidence of a major glacial period around 740 million years ago and contain some of the continent's richest, most diverse fossil records of soft-bodied animals.

This glaciation was one manifestation of global cooling that caused glacial episodes on other continents. Geologists say this Precambrian glacial episode was probably one of coldest, most extensive cooling periods in Earth history. They also consider the Adelaide geosyncline to be the precursor of another downwarp related to the most extensive folded belts on the continent, namely the Tasman geosyncline along Australia's east flank.

Victoria is also characterized by a belt of old rocks upon which sediments have been deposited called the Lachlan geosyncline. Marine rocks were deposited in quiet water to great thicknesses in Victoria, forming black shales. Some of the sediment was built up by mud-laden currents from higher areas on the sea floor. These current-borne sediments have produced muddy sandstones called graywackes.

At the end of the Ordovician and early Silurian Periods (about 425 million years ago) there was widespread folding of the Lachlan geosyncline called the Benambran orogeny. The folding was accompanied by granite intrusions and is thought to be responsible for the composition and texture of the rocks of the Snowy Mountains in Victoria, including Mt. Kosciusko, Australia's tallest peak at 7,310 ft (2,193 m).

In eastern Australia, Paleozoic Era volcanic activity built up much of the rock strata. Mountain glaciation during the late Carboniferous period when insects, amphibians, and early reptiles first evolved also transformed the landscape. Mountain building in eastern Australia culminated during the middle and later Permian Period (about 250 million years ago) when a huge mass of magma (underground molten rock) was emplaced in older rocks in the New England area of northeastern New South Wales. This huge mass, or batholith, caused extensive folding to the west and ended the sedimentation phase of the Tasman geosyncline. It was also the last major episode of orogeny (mountain building) on the continent.

In parts of Western Australia, particularly the Carnarvon Basin at the mouth of the Gascoyne River, glacial sediments are as thick as 3 mi (5 km). Western Australia, particularly along the coast, has been inundated repeatedly by the sea and has been described by geologists as a mobile shelf area. This is reflected in the alternating strata of deposited marine and non-marine layers.

In the center of Australia is a large sedimentary basin or depression spanning 450 mi (720 km) from east to west and 160 mi (256 km) north to south at its widest point. Sedimentary rocks of all varieties can be found in the basin rocks which erosion shaped into spectacular scenery including Ayres Rock and Mt. Olga. These deposits are mostly of Precambrian age (over 570 million years old), while sediment along the present-day coastline including those in the Eucla Basin off the Great Australian Bight are less than 70 million years old. North of the Eucla Basin is the Nullarbor (meaning treeless) Plain which contains many unexplored limestone caves.

Dominating interior southern Queensland is the Great Artesian Basin, which features non-marine sands built up during the Jurassic Period (190 million to 130 million years ago), sands which contain much of the basin's artesian water. Thousands of holes have been bored in the Great Artesian Basin to extract the water resources underneath but the salt content of water from the basin is relatively high and the water supplies have been used for livestock only.

The Sydney basin formed over the folded rocks of the Tasman geosyncline and is also considered to be an extension of the Great Artesian Basins. Composed of sediments from the Permian and Triassic Periods (290 million to 190 million years old) it extends south and eastward along the continental shelf. The sandstone cliffs around Sydney Harbor, often exploited for building stones, date from Triassic sediments.

Minerals in Australia have had a tremendous impact on the country's human history and patterns of settlement. Alluvial gold (gold sediments deposited by rivers and streams) spurred several gold fevers and set the stage for Australia's present demographic patterns. During the post-World War II period there has been almost a continuous run of mineral discoveries, including gold, bauxite, iron, and manganese reserves as well as opals, sapphires, and other precious stones.

It is estimated that Australia has 24 billion tons (22 billion tones) of coal reserves, over one-quarter of which (7 billion tons/6 billion tones) is anthracite or black coal deposited in Permian sediments in the Sydney Basin of New South Wales and in Queensland. Brown coal suitable for electricity production in found in Victoria. Australia meets its domestic coal consumption needs with its own reserves and exports the surplus.

Australia supplies much of its oil consumption needs domestically. The first Australian oil discoveries were in southern Queensland near Moonie. Australian oil production now amounts to about 25 million barrels per year and includes pumping from oil fields off northwestern Australia near Barrow Island, Mereenie in the southern Northern Territory, and fields in the Bass Strait. The Barrow Islands, Mereenie, and Bass Strait fields are also sites of natural gas production.

Australia has rich deposits of uranium ore, which is refined for use for fuel for the nuclear power industry. Western Queensland, near Mount Isa and Cloncurry contains three billion tons (2.7 billion tones) of uranium ore reserves. There are also uranium deposits in Arnhem Land in far northern Australia, as well as in Queensland and Victoria.

Australia is also extremely rich in zinc reserves, the principal sources for which are Mt. Isa and Mt. Morgan in Queensland. The Northern Territory also has lead and zinc mines as well as vast reserves of bauxite (aluminum ore), namely at Weipa on the Gulf of Carpenteria and at Gove in Arnhem Land.

Gold production in Australia has declined from a peak production of four million fine ounces in 1904 to several hundred thousand fine ounces. Most gold is extracted from the Kalgoorlie-Norseman area of Western Australia. The continent is also well known for its precious stones, particularly white and black opals from South Australia and western New South Wales. There are sapphires and topaz in Queensland and in the New England District of northeastern New South Wales.

Because of its aridity, Australia suffers from leached, sandy, and salty soils. The continent's largely arid land and marginal water resources represent challenges for conservation and prudent environmental management. One challenge is to maximize the use of these resources for human beings while preserving ecosystems for animal and plant life.

See also Beach and shoreline dynamics; Continental drift theory; Desert and desertification; Earth (planet); Industrial minerals; Plate tectonics; Weathering and weathering series