Geologically speaking, Australia is a relatively quiet place to live. Earthquakes are rare and usually mild, and as for volcanoes, there has not been as much as a magma eruption on the continent for thousands of years.
It wasn’t always like that. In fact, the earth is full of remnants of mysterious eruptions that have shattered the crust with surprising regularity over the past 80 million years.
A new study suggests that some of the more unusual periods of activity were the result of an ancient seabed that made its way under the plate, providing the perfect dough for an extensive course of eruptions right along the continent.
From the rocks at Cradle Mountain in Tasmania to the famous organ pipes in Victoria to the dramatic peaks of the Glass House Mountains in Queensland, an amazing array of monoliths cover the east coast of Australia, like a crooked spine.
Exactly what caused them is not at all obvious.
“We are not in the famous ‘Ring of Fire’ in the Pacific, which produces so many volcanoes and earthquakes,” says geologist Ben Mather of the University of Sydney.
“So I needed another explanation for why there were so many volcanoes on Australia’s east coast.”
In many parts of the world, far from the edges of any tectonic plate, volcanoes are the product of rising heat that burns the shell. As the crust slides over high temperature spots, the weak points can channel the growing magma to the surface.
However, hot spot volcanoes tend to look like ellipses that dot the landscape. Many of Australia’s volcanoes are solitary pillars – they stand alone in the middle of nowhere.
“Rather than huge explosions like Krakatoa or Vesuvius or iconic volcanoes like Mount Fuji, the effect is more like the bubbles that appear when you heat your pancake mixture,” says Mather.
An alternative mechanism could be in the pockets of explosive chemistry.
Volatile mixtures of materials can react to form gases that push up through the weak parts of the continent, generating volcanoes just like many of those we find in Australia.
It is a solid hypothesis, with good evidence to support it, but it leaves open the question of where such mixtures could originate.
Perhaps the swirls of molten stone lifted them as one continent slid beneath another, or perhaps the currents swirling in the mantle rubbed the lower sections of the crust. These events could explain millions of years of volcanism covering a strip of earth right at the edge of the knife of a sinking piece of bark.
To cover 100 million years of volcanic history, we will need a broad, resilient model.
Moreover, it should also explain similar volcanoes that dot the surface of its underwater neighbor, the “lost” continent of Zealand.
In fact, this volcanic “backbone” spreads over a 5,000-kilometer-wide section of the Earth’s crust.
None of the current theories hopes to explain all this.
Fortunately, this vast period of geological activity aligns perfectly with the sinking of a large section of the Pacific plate, far to the east of Australia, starting from the Tonga-Kermadec trench to the east and north of New Zealand.
Thousands of miles of seabed are slowly sloping under Australia’s continental shelf, drawing with them a glaze of volatile minerals deposited by marine life as dinosaurs stepped around.
“What differentiates the East Australia-Zealand region is that the seabed pushed below the continent in the western Pacific is highly concentrated with water-rich materials and carbon-rich rocks,” says Mather.
“It creates a transition zone just below Australia’s east coast, which is enriched with volatile materials.”
The theory explains not only the periods of volcanic activity that date back millions of years, but also the extent of its coverage deep inside the continent.
Finding ample evidence to support the theory is not easy, given that we do not yet have adequate coverage of seismic sensors in Zealand and Eastern Australia. But researchers are confident that the data we have is consistent with this new model.
Isotopes collected from volcanic rock formations on the coast also match the types of signatures expected from the thick layers of goop on the ocean floor that are cooked in the mantle.
“The peaks of volcanic activity correlate nicely with the amount of seabed that is recycled at the Tonga-Kermadec Trench east of New Zealand,” says Mather.
Not only the geological history of Australia requires an explanation; the theory could be used to explain similar areas of activity around the world, including eastern China and the western United States.
It is unclear whether Australia can expect new explosive formations in the near future. With a belly full of volcanic dough, its days of geological activity may be far from over.
This research was published in Scientific advances.