The world's longest chain of continental volcanoes has been discovered stretching for more than 2,000 kilometres along eastern Australia.

The ancient volcanic chain, reported in the journal Nature, runs from Cape Hillsborough on the central Queensland coast, south-west through central New South Wales to Cosgrove in Victoria.

"This volcanic chain was created over the past 33 million years, as Australia moved north-northeast over a mantle plume hotspot which we believe is now located in Bass Strait," said the study's lead author, Dr Rhodri Davies of the Australian National University.

"This track, which we've named the Cosgrove hotspot track [after an extinct Victorian volcano in the chain], is nearly three times as long as the famous Yellowstone hotspot tracks on the North American continent."

This kind of volcanic activity is surprising because it occurs away from tectonic plate boundaries where most volcanoes are found.

These hotspots are thought to form above mantle plumes, narrow upwellings of hot rock that originate at Earth's core-mantle boundary almost 3,000 kilometres below the surface. A volcano chain is created as the tectonic plate moves over the hotspot.

The newly identified volcanic chain is the most westerly of three major volcanic chains running along eastern Australia.

Cosgrove volcano hotspot track

The Cosgrove volcano chain extends 2,000 kilometres from Cape Hillsborough in Queensland to Cosgrove in Victoria. ( Drew Whitehouse/NCI National Facility )

The authors examined 15 extinct volcanoes in eastern Australia that had been known about for quite some time and appeared to follow a generally similar track.

"The volcanoes in central Queensland showed an age progression, so they got younger towards the south, and so too did those in New South Wales and Victoria," Dr Davies said.

The researchers looked at the movement of the Australian tectonic plate.

"Australia is actually the fastest moving continent on Earth, moving towards Indonesia at around seven centimetres per year," Dr Davies said.

The researchers found the chain of now-extinct volcanoes in Queensland, New South Wales and Victoria had all passed over the same fixed mantle plume hotspot as the Australian continental plate tracked north-northeast.

"We showed that these volcanoes are surface manifestations of the same mantle plume," Dr Davies said.

"However, the two groups of volcanoes were geochemically very distinct from each other and were separated by a gap of 700 kilometres, so no-one ever put these two volcanic chains together."

Chain helps give better understanding of volcanism

Dr Davies and colleagues used seismology to map the thickness of Earth's crust and mantle — known as the lithosphere — that lies under eastern Australia.

They found volcanoes in central Queensland erupted through lithospheres about 80 kilometres thick while those in New South Wales and Victoria had melted through lithospheres about 100 kilometres thick.

But the gap between the Queensland volcanoes and those in New South Wales and Victoria occurred because the lithosphere in this region was at least 150 kilometres thick.

"So the mantle plume can't melt through in those regions, so there's no volcanoes on the surface," Dr Davies said.

The thickness of the lithosphere also explained differences in the chemical composition of the volcanic rocks at different locations.

"If you take a mantle plume of a specific temperature and raise that to a depth of say 130 kilometres below the surface, specific minerals from the surrounding rock will enter that melt," Dr Davies said.

"And if the plume reaches shallower depths of say 100 kilometres, additional elements will enter the melt, changing the chemical composition.

"By looking at this chain of volcanoes in Australia and understanding the composition of the volcanic rock and how they evolved with time, we will understand volcanism on other continents and through earlier periods in Earth's history which is still poorly understood."

Finding the spot

Based on the speed at which the Australian plate is moving, Dr Davies and colleagues believe the mantle plume which generated these volcanoes is currently located between King Island and Tasmania.

The researchers are now trying to determine if the lack of a volcano at the plume's current location is caused by the thickness of the lithosphere there.

"There is some seismicity in this region, there's been some earthquakes around that location recently which does hint that something is going on there, but we haven't been able to find any seamounts or volcanic regions at present," Dr Davies said.