Most volcanoes either sit atop boundaries where tectonic plates meet, or above plumes where magma rises from the mantle with such heat it forces its way through the crust. However, some anomalies don't fit neatly into either category, most notably many former eruption sites across Eastern Australia. These may finally have been explained, and the same forces may be responsible for non-typical volcanoes elsewhere.
It's at least 3,000 years since the Australian mainland experienced an eruption (although the memory of some of the later ones has survived). However, from Queensland to South Australia rocks reveal more than 100 million years of eruptions, some 5,000 kilometers (3,000 miles) from the plate boundary. “These volcanoes have shaped the landscapes [Australians] live on,” Dr Ben Mather of the University of Sydney told IFLScience. “They have left a lasting legacy in the rich volcanic soils that provide for much of Australia's agriculture.”
Yet their existence is puzzling. “We aren't on the famous Pacific 'Ring of Fire' that produces so many volcanoes and earthquakes," Mather noted in a statement. Moreover, rather than rare enormous explosions like Krakatoa, the region's geology indicates “a fairly consistent pattern of activity,” co-author Dr Maria Seton noted.
Mather and Seton found the timing and location of the volcanoes line up with the volume of material subducted as the Pacific Plate was pushed beneath the Australian Plate over the last 100 million years, albeit far from the boundary.
In Science Advances, Mather and Seton conclude the western side of the Pacific plate is unusually high in water and carbon-rich rocks. “This creates a transition zone right under the east coast of Australia that is enriched with volatile materials,” Mather said.
Moreover, Mather explained to IFLScience, subducted plates usually head quickly down into the mantle, but the Pacific slab has stayed quite buoyant, sliding a long way under the Australian plate.
Animation of tectonic plate movement. Mather et al, 2020, Science Advances
The presence of all this material, slowly enriching the mantle at that point with gasses has caused magma upwellings. Combined with the relative thinness of the eastern part of the Australian plate, this has allowed hot material to frequently pierce the plate, leading to eruptions in both Australia and Zealandia, the largely submerged continent that includes New Zealand.
Mather told IFLScience, “In Eastern China there are very similar fingerprints in the eruptions, telling us they must be sourced from a mantle reservoir with a subduction signature.” The authors are yet to put together a similarly detailed model of how this might have occurred. However, Mather considers it likely ancient volcanoes far from plate boundaries in China, Western North America, and Bermuda are all the result of similar processes to those they have described for Australia.
The work appears unlikely to allow volcanologists to predict eruptions on human timescales. Nevertheless, Mather thinks it might provide broader predictions about eruption frequency, including that Australia can expect more at some point, either in Tasmania or off the southern coast.