Extract from ABC News
ABC Science
By Dr Tom Raimondo
Posted about an hour ago
Australia is often thought of as an ancient and quiescent
continent — the sleeping giant in a world where landscapes
dramatically change in front of our very eyes.
Earthquakes have shattered New Zealand and Italy, tsunamis have inundated Japan and Indonesia, and volcanic eruptions have blasted Iceland and Russia, to name a few.
In Australia, meanwhile, our activity seems to be limited to a steady northward drift at a rate of centimetres per year.
But Australia's past is far from sleepy.
Here are five places where evidence of our extreme history is written in stone.
But hidden beneath the red earth are some of the biggest subsurface anomalies you've never seen.
Just north of the outback town of Marla, the layer underneath the Earth's crust called the mantle rises up 30 kilometres closer to the surface along an extensive fault system known as the Woodroffe Thrust-Mann fault.
This subterranean mountain is so large that it actually warps the Earth's gravity field.
The mantle is much denser than typical continental crust. So the Woodroffe Thrust-Mann Fault is visible in gravity maps as a linear streak running east-west.
Gravitational acceleration is faster in the red areas at the core
of the fault zone compared to the blue areas either side.
The difference in gravity along the fault zone is so dramatic that it has been recognised as the largest continental gravity gradient anywhere in the world.
So if you were to drop a rock on your foot on the journey from Adelaide to Darwin, better to do it at a fuel stop in Marla than while sightseeing at the border of South Australia and the Northern Territory — the lower gravity at Marla means the rock will be slightly lighter so it might hurt a little less.
This lake is actually the eroded remains of a giant meteorite
impact crater that is 580 million years old and estimated to be up
to 90 kilometres wide — one of the largest craters ever
identified.
No surprises then that the impact energy was equivalent to more than 5 million megatons of TNT; enough to produce a global catastrophe that included seismic shocks, tsunamis and super-hurricanes.
Material ejected from the crater has been identified more than 300 kilometres east in rocks of the Flinders Ranges.
Here, features such as shatter cones, shocked quartz and pulverised rock fragments are seen within a distinctive sedimentary layer known as an ejecta horizon.
Today the crater is much flatter than when it first formed, with up to 5 kilometres of material stripped off its surface over time — yet it still remains as the unmistakeable imprint of an extraterrestrial visitor that entered with a giant bang!
This region contains a lava field that stretches for 500 kilometres in diameter, with individual eruptions up to 300 metres thick and a total lava volume as high as 500,000 cubic kilometres.
That's enough to fill Sydney Harbour a million times over.
What's even more remarkable is that the piping-hot lava reached
temperatures above 1,000 degrees Celsius and erupted from the
volcano almost instantaneously — most likely from large fissures
in the crust that burst open nearly 1600 million years ago.
But perhaps the most extreme consequence of this geological phenomenon was that it also produced the world's largest hydrothermal deposit, a spectacular ore system filled with huge reserves of copper, uranium, silver and gold.
Olympic Dam contains approximately 9 billion tonnes of ore, a remarkable by-product of the hugely enriched ore-forming fluids associated with the Gawler Ranges volcanic system.
Geologists have discovered that this bizarre idea was a reality some 650 million years ago, using evidence from glacial rocks of the Flinders Ranges that were deposited at sea level and close to the equator.
In fact, this evidence has been used to argue that the whole
planet was once covered in ice sheets up to several kilometres thick
— a scenario called Snowball Earth.
Amazingly, global glaciation was thawed by a sudden reversal to warmer temperatures about 635 million years ago.
In the Flinders Ranges, this event is marked by the sharp transition from glacial sediments to dolomite, a distinctive sedimentary rock that formed in a warm and shallow sea.
Nearby quartzite rocks contain Ediacaran fossils, the oldest soft-bodied organisms ever discovered.
The Ediacaran Period is named after the hills that contain these fossils, and is indicated by a bronze plaque known as a golden spike — the only one in the Southern Hemisphere.
The mighty Murray River is an ancient watercourse that can seem
to only live up to its name during periods of flooding, with a small
and shallow mouth that is often choked with sand.
But Australia's longest river has also produced some of the deepest submarine canyons ever found.
These incredible features sit far beneath sea level near Kangaroo Island, reaching lengths of 80 kilometres and depths of over 5,000 metres — greater than twice the height of Mt Kosciusko and tall enough to easily swallow Colorado's Grand Canyon whole.
One of the largest is named Sprigg Canyon after the geologist Reg Sprigg, who discovered them with the help of the Royal Australian Navy in 1947.
The canyons were produced when the Murray River wound its course far beyond the present coastline, when sea levels were much lower during the most recent ice age and the continental shelf was dry land.
So while the true erosive power of the Murray is rarely seen, the scars of its forgotten passage to the deep ocean can still be found in a spectacular underwater world.
Dr Tom Raimondo is a geoscientist and program director for
environmental and geospatial science at the University of South
Australia. He is also one of RN's Top
5 under 40 scientists. Hear more about Australia's
ancient past with Dr Raimondo on RN's Off Track.
Earthquakes have shattered New Zealand and Italy, tsunamis have inundated Japan and Indonesia, and volcanic eruptions have blasted Iceland and Russia, to name a few.
In Australia, meanwhile, our activity seems to be limited to a steady northward drift at a rate of centimetres per year.
But Australia's past is far from sleepy.
Here are five places where evidence of our extreme history is written in stone.
Central Australia: World's biggest gravity warp
Driving along the Stuart Highway from Adelaide to Darwin can seem like an uneventful journey, with miles and miles of flat country only occasionally interrupted by low hills and dry creeks.But hidden beneath the red earth are some of the biggest subsurface anomalies you've never seen.
Just north of the outback town of Marla, the layer underneath the Earth's crust called the mantle rises up 30 kilometres closer to the surface along an extensive fault system known as the Woodroffe Thrust-Mann fault.
This subterranean mountain is so large that it actually warps the Earth's gravity field.
The mantle is much denser than typical continental crust. So the Woodroffe Thrust-Mann Fault is visible in gravity maps as a linear streak running east-west.
(Supplied: Geoscience Australia)
The difference in gravity along the fault zone is so dramatic that it has been recognised as the largest continental gravity gradient anywhere in the world.
So if you were to drop a rock on your foot on the journey from Adelaide to Darwin, better to do it at a fuel stop in Marla than while sightseeing at the border of South Australia and the Northern Territory — the lower gravity at Marla means the rock will be slightly lighter so it might hurt a little less.
Lake Acraman: A crater of cataclysmic proportions
Looking down from space, Lake Acraman seems innocuous enough on the gentle slopes of the northern Eyre Peninsula in South Australia. But its circular shape hints at a truly cataclysmic origin.
GIF:
Lake Acraman is the site of one of
the largest meteorites to strike our planet. Source: Google Earth
No surprises then that the impact energy was equivalent to more than 5 million megatons of TNT; enough to produce a global catastrophe that included seismic shocks, tsunamis and super-hurricanes.
Material ejected from the crater has been identified more than 300 kilometres east in rocks of the Flinders Ranges.
Here, features such as shatter cones, shocked quartz and pulverised rock fragments are seen within a distinctive sedimentary layer known as an ejecta horizon.
Today the crater is much flatter than when it first formed, with up to 5 kilometres of material stripped off its surface over time — yet it still remains as the unmistakeable imprint of an extraterrestrial visitor that entered with a giant bang!
Olympic Dam: Earth's super supervolcano
The Yellowstone Caldera is the largest active supervolcano on the planet. But even it is dwarfed by the remnants of a much older supervolcano found in the Gawler Ranges of South Australia.This region contains a lava field that stretches for 500 kilometres in diameter, with individual eruptions up to 300 metres thick and a total lava volume as high as 500,000 cubic kilometres.
That's enough to fill Sydney Harbour a million times over.
(Supplied: Stacey McAvaney)
But perhaps the most extreme consequence of this geological phenomenon was that it also produced the world's largest hydrothermal deposit, a spectacular ore system filled with huge reserves of copper, uranium, silver and gold.
Olympic Dam contains approximately 9 billion tonnes of ore, a remarkable by-product of the hugely enriched ore-forming fluids associated with the Gawler Ranges volcanic system.
Flinders Ranges: A snowball in the tropics
Can you image the hot and humid tropics instead being blanketed by freezing ice?Geologists have discovered that this bizarre idea was a reality some 650 million years ago, using evidence from glacial rocks of the Flinders Ranges that were deposited at sea level and close to the equator.
(Supplied: Tom Raimondo)
Amazingly, global glaciation was thawed by a sudden reversal to warmer temperatures about 635 million years ago.
In the Flinders Ranges, this event is marked by the sharp transition from glacial sediments to dolomite, a distinctive sedimentary rock that formed in a warm and shallow sea.
Nearby quartzite rocks contain Ediacaran fossils, the oldest soft-bodied organisms ever discovered.
The Ediacaran Period is named after the hills that contain these fossils, and is indicated by a bronze plaque known as a golden spike — the only one in the Southern Hemisphere.
Murray River: Spectacular submarine canyons
(Getty Images: The Washington Post)
But Australia's longest river has also produced some of the deepest submarine canyons ever found.
These incredible features sit far beneath sea level near Kangaroo Island, reaching lengths of 80 kilometres and depths of over 5,000 metres — greater than twice the height of Mt Kosciusko and tall enough to easily swallow Colorado's Grand Canyon whole.
One of the largest is named Sprigg Canyon after the geologist Reg Sprigg, who discovered them with the help of the Royal Australian Navy in 1947.
The canyons were produced when the Murray River wound its course far beyond the present coastline, when sea levels were much lower during the most recent ice age and the continental shelf was dry land.
So while the true erosive power of the Murray is rarely seen, the scars of its forgotten passage to the deep ocean can still be found in a spectacular underwater world.
(Supplied: Peter J Hill and Patrick
De Dekker)
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