Australia: The Land Where Time Began
The Venerable Elder Continent of the Earth Australian geology, the Geological History of Australia
Although the building blocks of Australia are the oldest, those of other continents are not much younger. Australia is "older" because much of it is little changed from the early days of the Earth. In places, later sediments were deposited only to be eroded away again, once again exposing the ancient land surfaces, which are again subjected to erosion. In effect it has continued to "age" while the other continents have been rejuvenated a number of times since their formation, by volcanic and seismic activity. More than half of the surface rocks of Australia formed in the Precambrian, more than 600 million years ago. It is widely believed by scientists that rocks of a similar age underlie the younger rocks over much of the remainder of the continent.
Between 2.3 and 1.9 billion years ago the original Pilbara Block and Yilgarn block were joined by a number of other blocks of granite that rose above sea level. During that time period the foundations of the Australian continent are believed to have drifted from the North Pole to the South Pole and to be on their way north yet again. Between 1.8 and 1.4 billion years ago the aggregation of granite blocks that comprised the Australian land mass, the western 2/3 of the present continent, were welded together by crustal movement and massive intrusion of granite between the original blocks to form 3 large blocks that now comprised a rigid base for the remainder of the continent to be built from. These blocks were separated by mobile belts of thin crust, that fused them into a single continental landmass, acting like shock absorbers as blocks were moved around and they absorbed some of the shock of the collision with Antarctica The rise of the Petermann Ranges resulted from collision of land masses on the far side of Antarctica with that continent, the crust of the mobile belts being thrust up as the Himalayas are being thrust up as India collides with Asia.
The 3 shields that formed the basis of the future continent are gently contoured. In the Officer Basin and Canning Basin they formed depressed areas like huge bowls. These depressed areas accumulated sediment over millennia and are now the sites of the Great Victoria Desert and the Great Sandy Desert. In the very centre of the continent, the Red Centre, is the Amadeus Basin, where Uluru (Ayers Rock) and Kata Tjuta (The Olgas) are situated. Both these formations were believed to have been formed from the sediment washed into the Amadeus Basin from the eroding Petermann Ranges. Later research has found that the sediments that formed Kata Tjuta were actually from a different source, from the south and southwest, as opposed to from the north in the case if Uluru.
Its geological and climatic history over the last 4 billion years are what makes it different. The surface of the Earth is very dynamic, chunks of crust being continuously shunted all over the globe. When they collide, mountain ranges are thrust up, accompanied by earthquakes and volcanoes, as with the Himalayas as India ploughs into Asia. The last time Australia collided with anything larger than a few islands was more than a billion years ago when it rammed Antarctica to become part of Gondwana. And it is the only continent that has no active, though possibly dormant volcanoes, and very few and mostly minor earthquakes. In the past it has had cataclysmic volcanic eruptions on the scale of the Deccan Traps of India during the Cambrian Period about 520 million years ago. But not for a long time, the latest large-scale geological event was the uplift of the eastern part of the continent 2-3 million years ago, and a few million years earlier, the uplift of the Kimberley region.
The most extensive area of recent volcanic activity stretches in a band down the full length of the eastern coast. This is the "new" part of Australia, that formed much later than the "old", original, 2/3 of the continent. It formed as material scraped off the Pacific Plate accumulated along the subduction zone that was situated along the east coast at the time. Prior to this time there were volcanic island arcs off the east coast. During this process of the growth of the eastern part of the continent, these island arcs were swept up and incorporated into the growing margin of Australia. At one point in this process, the continental crust at the subduction zone locked on the oceanic crust of the plate that was diving under the continental crust. This caused the continental crust to buckle down. This allowed the ocean to invade the lower lying parts along the east coast, and some parts of central Australia. Eventually the locked crust slipped and the crust was then free to return to its original height, draining the seas that had formed on the continental crust.
This process is familiar to scientists studying earthquakes and tsunamis, it occurs at subduction zones around the world from time to time. It occurred in 2004, when two plates near Java locked, then released catastrophically, causing the devastating tsunami. The release of the plate in Indonesia allowed the crust to suddenly bounce back up by 15 metres. When the plates locked along the Australian subduction zone the crust was bent down about 350 metres. The rebound probably wasn't as sudden as with the Indonesian tsunami.
No Australian volcano has erupted for 5000 years and some are considered to be dormant, though possibly extinct. When volcanic activity did occur in Australia in the distant past it was confined to relatively localised events. The subduction zone now lies along the east coast of New Zealand. As a result New Zealand, situated on the eastern edge of Indo-Australian tectonic plate, has the highest concentration of youthful volcanoes in the world and is one of the most seismically active areas of the world. The stability of the Australian continent, with limited volcanic activity for many millions of years, and the relatively small amount of seismic activity, is the result of Australia being situated in the centre of its tectonic plate, well away from the active regions along its margins, particularly in New Guinea and New Zealand. The most concentrated area of volcanic activity in Australia occurred along the east coast, near the margin of the continent, as it moved north over a 'hot-spot'. The result is a chain of volcanoes becoming progressively younger from north Queensland to near Melbourne in Victoria. The oldest dated in the Great Dividing Range in Queensland is 40 million years old. The youngest in Victoria is about 5,000 years old. The chain of hot spot volcanoes spread large amounts of basalt around them which is now a rare example (for Australia) of rich, renewed soils.
The uplift of the Kimberley region did little or nothing to improve the impoverished soils of the continent, no extra material was added that could bring a fresh supply of nutrients to the leached soils, it just increased the rate of erosion of the uplifted areas which provided more material for the formation of more soil, but in areas of intense seasonal rainfall such as the Kimberleys much of this soil is soon washed out to sea. The soils over much of Australia have been through a number of cycles of erosion, deposition, consolidation, and further erosion. A further insult to the soils is that vast areas have spent millions of years under shallow seas that receded and evaporated, only to return some millions of years later, only to dry out again, leaving behind a vast amount of salt that inhibits plant growth when it is brought to the surface by the clearing of deep-rooted native vegetation to grow shallow-rooted crops, allowing the water table to rise. Irrigation adds water that causes the water table to rise still further bringing salt to the root zone of plants.
The result of all this is that Australia is different and even unique in a number of ways, both in its geology and its flora and fauna. The animals and plants have been evolving in isolation from the life of the rest of the world for 45 million years, with only a few minor exchanges with islands as its drift north brought it towards the Indonesian Archipelago, and birds, and to a lesser extent, bats. The evolution of life in Australia was also influenced to a large extent by the erratic climate and the soils that are impoverished like nowhere else.
As the other continents moved apart and came together again the life they carried had to contend with massive invasions as continents came together after being separated for millions of years. An example of this is when North and South America joined allowing the exchange of animals. Many species had to adapt rapidly in new directions to cope with the new competitors or predators. Those who couldn't died out.
In Australia, the moving force for adaptation was mostly climate change as Australia drifted north from the Antarctic Circle towards the Equator. As well as temperature change over much of the continent they had to cope with a much more arid environment as the climate became much drier. About 15 million years ago the deserts that cover so much of present-day Australia began to form, and the drying intensified about a million years ago, then about 20,000 years ago things got really bad. The last of the lush forests of the present arid zone disappeared.
Although the Australian landmass of today is much different from the original one that emerged from the sea, parts of it have remained unchanged, or mostly so, from that first dry land, possibly the first dry land in the world. Parts of Australia emerged from the sea at least 3000 million years ago. In the southern part of Western Australia is an ancient block of rock called Yilgarnia by geologists. This block has not been covered by the sea since it first rose from the water. The granite rock of Yilgarnia crystallised at least 2,700 million years ago.
Glaciers during the Permian were a major factor shaping the topography of the Great Western Plateau, the current landscapes of the area resulting from erosion since the Permian. The basins in the region were subsequently shaped by the retreat of the seas that covered them in the Cretaceous. The eastern highlands are of uncertain age, but they arose prior to the late Mesozoic. All major landscapes were formed more than 90 Ma, making them more ancient than most places in other parts of the world. The continent has been subjected to very little tectonic activity since the epicontinental seas of of the Cretaceous retreated, with only miner episodes of uplift or subsidence.
Over the time since then, climatic changes have had a major effect on the landscapes and their regoliths. The climate was mostly humid between the Cretaceous and the Pliocene, resulting in deep weathering and leached acid soils. About 2.5 Ma the onset of aridity changed the weathering regimes to mostly alkaline resulting in the soils becoming salt- and carbonate-rich. Much of the coastal and interior regions underwent a period of extensive aeolian landscape construction as the winds intensified in this period.
The vegetation growing in these drastically changed soils that developed under the windy, arid conditions underwent equally drastic changes of the vegetation types, evolving from the vegetation of the humid forests that flourished over much of the Tertiary to the arid-adapted flora of the present. Taylor has suggested in Hill (1994) that changes in earlier vegetation can be viewed in the same way, though there is little data on earlier landscape evolution and soil development.
Another of the many oddities of the Australian continent is the age of land surfaces, with ancient rocks underlying much of the surface. Large areas of the Australian continent have been found to be older than the Cainozoic, more than 60 Ma. Among these ancient surfaces are Uluru, Kata Tjuta, high plains in many parts of the Eastern Uplands, the Flinders Ranges, the Mt. Lofty Ranges, the Hamersley Range, etc. The ages of these surfaces are at odds with the convention, and widely accepted, models of landscape evolution, that no landscape or landform should survive for more than 30 My. The Oligocene is the oldest surface age that is suggested by the model to exist on Earth, unless it has been exhumed. Yet again Australia is different.
Though some of these surfaces have been exhumed, many are of epigene and etch type. Research has continued on the reasons for the survival of such ancient surfaces in Australia. One important factor in long term survival of forms and surfaces is structure. The presence of quartzite, an inherently tough material, or sandstone in remnants of palaeoforms in many of the fold mountain ranges and plateaux, has contributed to their survival. Survival is also favoured by compressive fields. A very important feature is the frequency of open fractures, even inherently weak rocks, such as siltstone and shale, can be resistant if they are in compression, and water cannot enter because all partings are closed tightly. In the Flinders Ranges, the summit surfaces are preserved, though formed of a weak rock, siltstone, located in a core of an anticline that is deeply eroded, is suggested by the authors to probably be in compression.
Origin of plate tectonics
In an article in Nature, vol 456, p 493, scientists report evidence in zircon deposits, that have been dated to about 4 billion years ago, from Western Australia that plate tectonics was occurring in the first 500 million years after the formation of the Earth. They concluded that the zircon crystals had formed under conditions of lower temperature and higher pressure than was expected to have prevailed at that time. They concluded that the crystals probably formed in a subduction zone. This is the first direct evidence from this time that tectonics had already begun. See link 5 below.
The Oldest or First
The Video Across Australia visits many of the places of geological interest on this site.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|