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If you are heading on a road trip this summer, you might drive past 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 is left of eruptions that occurred around 25 million years ago.
They are among hundreds of ancient volcano remnants that extend 3,500 kilometres from Tasmania to northern Queensland.
While the Glass House Mountains stand out in the landscape, other remnants are just weathered nubs hidden in the bush or a series of caves hollowed out by lava.
But just why there have been so many eruptions over the past 100 million years in eastern Australia — some of them as recent as a few thousand years ago — is a mystery.
How are Australia's volcanoes different from others in the world?
Massive volcanoes, such as those in the Pacific Ocean's "Ring of Fire", usually occur near the edges of tectonic plates as one plate slips beneath another.
But Australia is bang in the middle of a tectonic plate.
"Australia's volcanoes are not really tied to any plate boundaries ... and most of them don't form part of a larger island chain," said Ben Mather, a geologist at the University of Sydney.
Chains of smaller volcanoes also can pop up away from the edges of tectonic plates if the plate slides over a hotspot.
And in fact, Australia is home to three ancient volcano chains, created as the continent moved north-east over the top of the Pacific plate after splitting from Antarctica.
The Cosgrove Track, which stretches more than 2,000 kilometres from Cape Hillsborough in Queensland to Cosgrove in Victoria, is the world's longest chain of ancient volcanoes.
There are two other chains off the east coast in the Tasman and Coral Seas.
But most volcanoes in Australia were not created this way, Dr Mather said.
"You would normally expect that volcanoes [created by hotspots] would be quite old in the north of the continent and get younger towards the south," he said.
Yet most volcanoes across eastern Australia and Zealandia — a piece of continental crust that includes New Zealand and is mostly submerged underneath the Tasman and Coral Seas — are of random ages.
"The volcanoes we've studied are much smaller eruptions, and they are far more frequent."
Previously, scientists have proposed that some volcanic regions such as those around Mount Gambier in South Australia were formed by magma eddies left in the wake of the edge of the continent — much in the same way as a boat leaves an eddy of water behind it as it glides over a lake — or by multiple plumes of molten rock cracking through the crust.
But these mechanisms also fail to explain how all Australia's volcanoes formed.
A new theory to explain Australia's volcanoes
Now, after studying how tectonic plates moved over time and analysing the chemistry of rock samples from several volcanoes, Dr Mather and colleagues developed a new hypothesis.
"[Our mechanism] can apply to volcanoes from the southern tip of Tasmania all the way up to northern Queensland," Dr Mather said.
The answer lies in the nature of the seafloor that was pushed under the continent from the east, they reported in the journal Science Advances.
"During the past 120 million years, lots and lots of seafloor has been shoved underneath Australia and Zealandia," Dr Mather explained.
"But what's special is this seafloor is imbued with water and carbon."
Usually, the plate that is pushed underneath sinks towards the centre of the Earth, but the material pushed under the Indo Australian plate has continued to hang around in the upper mantle for the past 60 million years.
"So occasionally that volatile cocktail gets released and that percolates to the surface in the form of volcanoes."
The team found peaks in volcanic activity over the past 2 million to 20 million years coincided with fluxes of movement along the Tonga-Kermadec Trench in the Pacific Ocean to the east.
"The plate that is beneath Zealandia and Australia ... has been shaken and it's liberated all those volatiles from that reservoir in the Earths' mantle."
These volatiles bubble up 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 the researchers believe were created by this process include:
"Most of the volcanoes you encounter along the Pacific Highway are going to be these sorts of volcanoes," Dr Mather said.
"But we probably haven't completely identified all the eruption locations yet."
'No shoe fits all'
Volcanologist Ray Cas of Monash University said the new model put forward by Dr Mather and colleagues gives us a better understanding of Australia's volcanoes at a regional scale and adds a few other factors such as the timing of eruptions and composition of the rocks.
"But no shoe fits all," Professor Cas said.
"It doesn't definitively give us the absolute answer as to why we've got volcanism and why we have these variations."
Different volcanoes and different volcanic provinces probably originated through different influences.
For example, there are a range of other factors that come into play in the area around Mount Gambier.
At 5,000 years old, Mount Gambier is the youngest volcano on the Australian mainland.
It is just one of about 400 volcanoes in an area in Victoria and South Australia known as the Newer Volcanics Province.
"There are other factors such as the upwelling of CO2 in the mantle and geophysical anomalies beneath the mantle which suggest to us that that is a good candidate for some further eruption activity in the future," Professor Cas said.
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.