Australia's geological history

…In brief.

In very general terms, Australia is an ancient continent which has grown from west to east. It contains some of the most significant evidence of past tectonic processes and of the development of life on Earth. It is imperative that any further analysis be done with regard to the fact that the Australian landmass has existed independent of any other since around 55 million years ago (mya); ergo, most of Australia’s history has been in the context of a comparatively miniscule landmass linked to many others (especially in the south and west). The eastern border was usually situated adjacent to shallow seas or island arcs¹.

Some of the most profound finds have been made in Western Australia, where the oldest rocks are located. These are the Pilbara, Kimberley and Yilgarn cratons² and they are one of the primary sources which suggest (via radiocarbon dating) that the Earth could be around 4.5 billion years old. More recent history is much better documented, as rocks older than the Permian (240-290 mya) have largely been subducted into the mantle. Even thereafter, though, there must be a wealth of evidence that has been lost. Still, let’s not mope overmuch.

The first stage and second stages were largely concerned with the formation of cornerstone blocks (the aforementioned cratons) and their subsequent fusion. This occurred sometime around 2,500 mya, when three such blocks were established in Western Australia. From this time until 900 mya they were separated by active, linear mountain chains. These contained metamorphic rocks and granite; they were highly deformed and folded. By 900 mya they had been cratonised and formed the western two thirds of the modern continent. Australia, however, was still a piece of Gondwanaland.

The third stage involved subduction and accretion in the east; from 500 mya to 250 mya, the Tasman Fold Belt was formed and eastern rocks exhibit evidence of the presence of former island arcs and ocean trenches. Sediment would have accumulated between the past continental edge and island arc, eventually filling the seaway. From 320 mya to 280 mya, eastern and central Australia experienced major mountain building (including the Lachlan Fold Belt, New England Fold Belt and others, each of which supplied sediment for basins which developed along Australia’s eastern flank). The mobile belt then moved eastward⁴, producing the Lord Howe Rise. By 200 mya, the eastern third was cratonised.

The fourth stage saw a significant degree of separation and disintegration. Firstly, around 160 mya, an area named Argoland moved away to the north-west. Around the same time, rift valleys formed in Western Australia and the area between it and the Indian continent - here we observe the beginning of the break-up of Gondwana. Sea levels rose, flooding the Great Artesian Basin. A narrow seaway separated Argoland by 132 mya, while to the south and west spreading began (which marked out the continental shapes). By 118 mya, an ocean up to 600 km wide separated India, Antarctica and Australia. Various divisions and sea floor spreading occurred from 96 mya onwards; the Tasman Sea stopped around 49 mya, although the Southern Ocean continued from 45 mya until the present day. Downwarping of the continent allowed shallow seas to cover the Murray Basin.

The fifth stage was an age of intra-continental earthquakes and hot spot volcanoes⁵. We would recognise Australia at this point, but there were still some formative processes to be undergone; as the continent drifted north, it passed over several hot spots. This created a series of parallel lines of volcanoes, the largest of which are Mount Warning (on the border of New South Wales and Queensland) and Mount Canobolas (in the central west of NSW). These volcanoes are not active today, but the most recent eruption occurred at Mount Gambier, 4000 years ago. Pressure caused by interaction with northern plate boundaries caused some old intra-plate faults to shift and the Great Dividing Range was uplifted by this process.

The sixth -and arguably final - stage brings us, of course, to the present. We continue our northward passage; tension from the interaction of the Australian and Pacific plates has created the current New Guinea mobile belt.

Footnotes:
¹Island arc: a curved chain of volcanic islands (such as the Aleutian Islands, or the islands of Japan).
²Craton: an area which has been through a full cycle of continental crust-building processes (including folding, igneous emplacement, crustal thickening and eventual stability after continuous erosion and isostatic uplift); they are usually about 35 Kilometres thick. An exposed craton is also referred to as a ‘shield’, although they are usually covered in parts by a ‘platform’ layer.
³These are also known as mobile belts or mountain belts.
⁴This mobile belt currently resides along the Tonga-Kermadec-New Zealand Line in the Pacific Ocean.
⁵These are the result of tensions within the plate itself; not all seismic activity or vulcanism is the product of interactions between tectonic plates.

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