Hotspot volcanoes

Why has the East Coast had hundreds of volcanoes since the age of the dinosaurs?

Australia’s east coast is littered with the remains of hundreds of volcanoes – the most recent of which is only a few thousand years old – and scientists are struggling to explain why so many eruptions have taken place over the past 80 years. last million years.

Now geoscientists from the University of Sydney have discovered why part of a stable continent like Australia is such a hotbed of volcanic activity. And the results suggest there may be more volcanic activity in the future.

Ben Mather

Dr Ben Mather, School of Geosciences, University of Sydney. Credit: University of Sydney

“We are not on the famous Pacific ‘Ring of Fire’ that produces so many volcanoes and earthquakes,” said Dr Ben Mather of the University of Sydney’s School of Geosciences and EarthByte group.

“So we needed another explanation as to why there have been so many volcanoes on the east coast of Australia.”

Many volcanoes that form in Australia are one-time events, he said.

“Rather than huge explosions like Krakatoa or Vesuvius, or iconic volcanoes like Mount Fuji, the effect is more like the bubbles that emerge when you heat your pancake mix,” Dr. Mather said.

Their remains can look like regular hills or standout structures like Cradle Mountain in Tasmania, the Organ Pipes in Victoria, the Undara Lava Tubes in Queensland, and Sawn Rocks near Narrabri, New South Wales. Many have yet to be identified, Dr Mather said.

So what’s going on?

“Beneath our east coast we find a special volatile mixture of molten rock bubbling to the surface through the younger, thinner Australian east coast crust,” he said.

The study is published today (December 16, 2020) in the journal Scientists progress.

Dr Mather and his team looked at how hundreds of eruptions have occurred along the east coast from northern Queensland to Tasmania and across Tasman to the largely submerged continent of Zealandia. They were particularly interested in “recent” peaks of volcanic activity 20 million and 2 million years ago.

“Most of these eruptions are not caused by the Australian tectonic plate moving over hot plumes in the mantle beneath the Earth’s crust. Instead, there is a fairly consistent pattern of activity, with a few noticeable spikes,” said co-author Dr Maria Seton, from the School of Geosciences and the EarthByte group.

Animation of the movement of tectonic plates and the impact of volcanoes over the last 120 million years. Blue shading shows material subducted from the Pacific Plate beneath the Australian Plate. Credit: University of Sydney

What warned them was that these peaks were occurring at the same time as there was an increased volume of seabed material being pushed under the continent from the east by the Pacific Plate.

“Peaks in volcanic activity correlate well with the amount of recycled seabed in the Tonga-Kermadec trench in eastern New Zealand,” Dr Mather said.

Taking this evidence, Dr Mather and his team have built a new model that unifies observations of so many eruptions occurring over millions of years along the east coast of Australia.

“The most recent event was at Mount Gambier in Victoria just a few thousand years ago,” he said.

Although the model explains constant volcanic activity, it cannot predict when the next volcano will emerge.

Geoscience Research Team on RV Investigator

Geoscience team on the CSIRO RV Investigator vessel. The author of the research, Dr Ben Mather of the University of Sydney, is third from the right. Credit: provided

How’s it going

The seafloor of the Pacific plate to the east is pushed under the Australian plate. This process is called subduction. The material is literally pushed under the Australian continental shelf, starting with the Tonga-Kermadec Trough to the east and north of New Zealand.

“From there it is thrown into the transition zone between crust and magma at depths of about 400 to 500 kilometers. This material then reappears as a series of volcanic eruptions along the east coast. of Australia, which is thinner and younger than the central and western parts of the continent,” said Dr Mather.

This subduction process is not unique to the Australian east coast.

“What distinguishes the eastern region from Australia and Zeeland is that the seabed pushed under the continent from the western Pacific is highly concentrated in hydrous materials and carbon-rich rocks. This creates a transition zone just under the east coast of Australia which is enriched with volatile materials.

The new explanation improves on previous models that suggested Victoria’s volcanoes were due to convection eddies in the mantle due to being near the trailing edge of the tectonic plate or models that rested on the passage of the plate above mantle hotspots.

“None of these gave us the full picture,” Dr. Mather said. “But our new approach can explain the volcanic pattern along Australia’s east coast.”

Dr Mather said this pattern could also explain other intraplate volcanic regions in the western United States, eastern China and around Bermuda.

Co-author Professor Dietmar Müller, co-coordinator of the EarthByte group at the School of Geosciences, said: “We now need to apply this research to other corners of the Earth to help us understand how other examples of enigmatic volcanism have occurred.”

Reference: “Intraplate volcanism triggered by slab flow explosions” by Ben R. Mather, R. Dietmar Müller, Maria Seton, Saskia Ruttor, Oliver Nebel and Nick Mortimer, December 16, 2020, Scientists progress.
DOI: 10.1126/sciadv.abd0953

The research, led by geoscientists from the University of Sydney, was undertaken with researchers from Monash University and GNS Science in Dunedin, New Zealand.