If you take a trip this summer, you may pass several ancient sentinels of Australia’s volcanic past.
Many of them, like the Glass House Mountains, are hard to miss.
These ancient volcanic plugs are all that remains of eruptions that occurred around 25 million years ago.
They are among hundreds of ancient volcanic remnants that stretch 3,500 kilometers from Tasmania to northern Queensland.
While the Glass House Mountains stand out against the landscape, other remnants are just weathered bumps hidden in the bush or a series of lava-carved caves.
But why there have been so many eruptions over the past 100 million years in eastern Australia – some dating back a few thousand years – remains a mystery.
How are Australian volcanoes different from other volcanoes around the world?
Massive volcanoes, such as those in the Pacific Ocean’s “Ring of Fire,” typically occur near the edges of tectonic plates when one plate slides under another.
But Australia is right in the middle of a tectonic plate.
“Australian volcanoes aren’t really tied to plate boundaries…and most of them aren’t part of a larger island chain,” said University of Sydney geologist Ben Mather.
Chains of smaller volcanoes can also arise from the edges of tectonic plates if the plate slides over a hot spot.
And in fact, Australia is home to three ancient chains of volcanoes, created when the continent moved northeast over the Pacific Plate after separating from Antarctica.
The Cosgrove Track, which stretches over 2,000 kilometers from Cape Hillsborough in Queensland to Cosgrove in Victoria, is the longest chain of ancient volcanoes in the world.
There are two other ranges off the east coast in the Tasman and Coral Seas.
But most volcanoes in Australia weren’t created that way, Dr Mather said.
“One would normally expect volcanoes [created by hotspots] would be quite old in the north of the continent and get younger towards the south,” he said.
Again most of the volcanoes in eastern Australia and Zealandia – a piece of continental crust that includes New Zealand and is mostly submerged beneath the Tasman and Coral Seas – have random ages.
“The volcanoes we have studied are much smaller and much more frequent eruptions.”
Previously, scientists have proposed that some volcanic regions such as those around Mount Gambier in South Australia were formed by whirlpools of magma left in the wake of the continent’s edge – much like a boat leaves a whirlpool behind. of water behind it as it glides over a lake – or by multiple plumes of molten rock cracking through the crust.
But neither do these mechanisms explain how all of Australia’s volcanoes formed.
A new theory to explain Australia’s volcanoes
Now, after studying how tectonic plates move over time and analyzing the chemistry of rock samples from several volcanoes, Dr. Mather and his colleagues have developed a new hypothesis.
“[Our mechanism] may apply to volcanoes from the southern tip of Tasmania to northern Queensland,” Dr Mather said.
The answer lies in the nature of the seabed that has been pushed under the continent from the east, they reported in the journal Science Advances.
“Over the last 120 million years, a lot of seabed has been pushed under Australia and Zealandia,” explained Dr Mather.
“But what’s special is that this seabed is permeated with water and carbon.”
Usually, the plate that is pushed underneath sinks towards the center of the Earth, but the material pushed under the Indo-Australian plate has continued to drag into the upper mantle for the past 60 million years.
“So every now and then this volatile cocktail is released and seeps to the surface in the form of volcanoes.”
The team found that peaks in volcanic activity over the past 2 to 20 million years coincided with flow flows along the Tonga-Kermadec Trench in the Pacific Ocean to the east.
“The plate that lies beneath Zealandia and Australia… shook up and released all of these volatiles from that reservoir into the Earth’s mantle.”
These volatiles bubble through the crust, which is much younger and thinner in the east than in other parts of Australia.
“You only get volcanism along the eastern third of Australia,” Dr Mather said.
Some of the volcanoes you might encounter on a road trip that researchers believe were created by this process include:
- Mount Gambier, South Australia
- Organ pipes, Victoria
- Sawn Rocks, near Narrabri New South Wales
- Barrington Shield Volcanoes (at Barrington Tops Park, New South Wales)
- Belmore Volcanic Province, near Baryugil, New South Wales
- Mount Canobolas, New South Wales
- Glass House Mountains, Queensland
- Undara Lava Tubes, Queensland
“Most of the volcanoes you encounter along the Pacific Highway will be these kinds of volcanoes,” Dr. Mather said.
“But we probably haven’t fully identified all the eruption locations yet.”
“No shoe fits all”
Monash University volcanologist Ray Cas said the new model proposed by Dr Mather and colleagues gives us a better understanding of Australian volcanoes on a regional scale and adds a few other factors such as the timing of eruptions and the composition rocks.
“But no one shoe fits all,” Professor Cas said.
“That definitely doesn’t give us the absolute answer as to why we have volcanism and why we have these variations.”
Different volcanoes and different volcanic provinces probably arose from different influences.
For example, there are a range of other factors that come into play in the region around Mount Gambier.
At 5,000 years old, Mount Gambier is the youngest volcano on the Australian continent.
It is just one of approximately 400 volcanoes in an area of Victoria and South Australia known as the Newer Volcanics Province.
“There are other factors such as upwelling CO2 in the mantle and geophysical anomalies below the mantle that suggest to us that this is a good candidate for new eruptive activity in the future,” said Professor Cas.
Editor’s note: An earlier version of this story was based on information that included Cradle Mountain in Tasmania as a location that had been formed by this volcanic process. This has been corrected.