The world's longest continental volcanic chain has been found running the length of Australia, hidden deep beneath the Earth's crust. It runs for over 2,000 kilometres (1,243 miles), from the Whitsunday Islands in the north of Queensland all the way down near Melbourne in the country's southern coast.
The chain, said Rhodri Davies, lead author of a new journal by Australian National University's School of Earth Sciences, was created by the drift of the Australian continent over the last 33 million years. As the land mass drifted northward, it passed over the top of a hotspot, an active volcanic region in the Earth's mantle. This hotspot, still thought to be in existence under the Tasman sea, a little to the northwest of Tasmania, created the long volcanic chain.
"We realised that the same hotspot had caused volcanoes in the Whitsundays and the central Victoria region, and also some rare features in New South Wales, roughly halfway between them," Davies said.
"The track is nearly three times the length of the famous Yellowstone hotspot track on the North American continent."
Its length isn't the only unusual feature of what the research team has named the Cosgrove hotspot track. It's also located far away from the edge of a tectonic plate; most volcanic activity occurs between tectonic plates, where the instability of the Earth's crust allows magma escape to the surface.
Australia sits in the middle of a tectonic plate that stretches from the Indian Ocean to the west and New Zealand to the right. The Cosgrove hotspot track is thousands of kilometres from a parallel edge.
The chain isn't one long line of volcano, but a string of individual hotspots, which are thought to form over mantle plumes. These are regions of unusually hot rock that spring from the boundary between the Earth's core and the mantle, some 3,000 kilometres (1,864 miles) below the Earth's surface.
Thankfully, because the Australian land mass is so thick, the plumes cannot penetrate close enough to the surface to melt into magma. However, where the Earth's outer layer, the lithosphere, is less than 130 kilometres (80 miles) thick, the plumes can create volcanic activity.
This discovery will allow scientists to understand the movements of the continents in the past, as well as the telltale signs of volcanic activity. For instance, the team found deposits of a rare igneous rock called leucitite in low-volume magmas within northern New South Wales. Its presence is an indication that the lithosphere in that region is thin enough to melt into magma.
"Now that we know there is a direct relationship between the volume and chemical composition of magma and the thickness of the continent, we can go back and interpret the geological record better," said co-author Ian Campbell from the ANU School of Earth Sciences.
The team's paper can be found online, published this week in the journal Nature.