ARTICLES
What’s Australia made of? (continued)
New exploratory drilling by state and federal geological surveys
also suggests that an ocean once separated Western Australia
and South Australia until at least 1,400 million years ago. This
ocean was named after the Mirning people who are indigenous
to the region.
Surprisingly, the cratons that came from the three continents are
broadly defined by our present state and territory boundaries.
The South Australian Craton (SAC) was connected to a big chunk
of modern day Antarctica. The North Australian Craton (NAC) –
matching roughly to the Northern Territory, the Kimberley and
northern Queensland – was likely linked to parts of what is now
northern China. The West Australian Craton (WAC) was made up
of both the Yilgarn and Pilbara cratons that had collided together
earlier.
Volcanoes along the border
Closing the Mirning ocean bought Australia together for the
first time, around 1,350 million years ago. Oceans close by
subduction: this is the process when an oceanic plate moves
beneath an overriding continent, and is similar to that occurring
in the Andes, Japan, or New Zealand today.
As the ancestral plates moved around, these three continents
smashed into each other and formed ancient mountain belts.
These mountains are now eroded to low stumps – such as the
Musgrave Ranges in central Australia – or buried beneath much
later sediments, such as those that make up the vast western
deserts.
This process is thought to have created volcanic mountains
along the region now straddled by the South Australia-Northern
Territory-Western Australia borders. Our latest research paper
proposes that as this mountain range formed, then eroded over
time, the sediment produced was moved by rivers and deposited
in a large inland sea that covered a lot of the Northern Territory
under a blanket of sand and mud. This now forms the bedrock
under much of the area between Tennant Creek and Katherine.
What makes this particularly exciting is that this increased
volcanic erosion bought nutrients into the sea. As a result, we
suggest that huge bacteria growths injected our atmosphere with
oxygen.
Illustration of the amalgamation of 'ancient' Australia
When the bacteria died, they were buried, underwent decay and
resulted in vast gas reserves encased in rocks of the region.
Illustration of the amalgamation of ‘ancient’ Australia. The first
event is the loss of the western Mirning Ocean followed by
the collision of the Western Australian Craton (WAC) with the
combined South Australian Craton (SAC) and North Australian
Craton (NAC). Volcanoes and mountains that formed during
this period eroded to form sands that buried much of northern
Australia. Marcella Cheng for The Conversation, modified from
Yang et al, CC BY-ND
The eastern states are young
So these cratons, and the roots of the mountain belts that mark
where they collided, make up Western Australia, South Australia,
northern Queensland and the Northern Territory, but what about
the eastern states?
Well for quite a long period of time, these areas of Australia simply
did not exist. These relative newcomers consist mainly of rocks
formed on the edge of old Australia as the Pacific Ocean evolved
over the last 500 million years (in the last ninth of Earth history).
An ocean between states
The boundary between the West Australian Craton and the other
cratons lies buried beneath the Nullabor Plain in the south, and
beneath the Great Sandy Desert further north.
Tasmania is older and may well have been a part of North America
until within the last billion years.
Until very recently, geologists thought this collision, which
brought together much of the continent, occurred 1800 million
years ago. This was based on the ages of some rocks poking out
of the Great Sandy Desert in remote Western Australia. But apart from that, mountain ranges have grown, faults have
moved, but the bulk of Australia has been together for the last
billion years or so, only to have broken out finally as our present
landmass 55 million years ago.
But intriguing recent research using new dating techniques
suggests that the collision represented by these rocks is actually
500 million years younger than anyone thought. This article was first published in ‘The Conversation’ on 21st
November, 2017. Once again SEN is grateful to ‘The Conversation’
for its generous policy of encouraging republishing of articles,
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SCIENCE EDUCATIONAL NEWS VOL 67 NO 1