The Australian mainland lacks active volcanoes, but its geologic history looks very different with the identification of a 2,000-kilometer-long (1,240-mile-long) volcanic track, the longest known on any continent. The first volcanoes on the track are 33 million years old, but the forces responsible may still be having an effect on the sea floor off Tasmania.
Most of the world’s volcanoes lie where tectonic plates meet, particularly around the Ring of Fire, or at mid-ocean ridges. However, exceptions such as volcanoes in Hawaii can be found within plates. The most popular, although still disputed, explanation is that these sit above mantle plumes, where hot material rises from the boundary between the mantle and core, forcing its way through the crust to be released at the surface.
Image Credit: Hannes Grobe/AWI / Wikimedia Commons
Plumes are thought to be fixed relative to the Earth’s core. As the tectonic plates move, new locations are exposed, with active volcanoes above the plume and a track of extinct peaks left behind. The Hawaiian track, including older mountains now eroded beneath the sea, is far longer, but Dr. Rhodri Davies of the Australian National University says Yellowstone Snake River Plain was thought to be the most extended example within a continental plate.
However, Davies reports in Nature that Australia hosts a track three times as long as Yellowstone. Dubbed “Cosgrove track,” it begins with ancient volcanoes like Pinnacle Rock in north Queensland. These have been identified as the likely remnants of a mantle plume for decades.
However, other parts of the track are less obvious. Central New South Wales and Victoria host thin layers of the volcanic mineral leucitite. In between are stretches of up to 700 kilometers (435 miles) without any sign of volcanic activity at all, leading geologists to reject an association between the volcanic provinces. However, Davies says the ages are perfect to be formed by the continent passing above a single plume.
Image: The path of the volcanic track across eastern Australia. Credit: Drew Whitehouse/NCI NationalFacilityVizLab.
Davies attributes the surface differences to “dramatic variations in the thickness of the lithosphere” in the region. “In north Queensland, the lithosphere is only 80km [50 miles] thick,” he told IFLScience, allowing magma to make its way to the surface. However, evidence from the speed at which earthquakes propagate through the Australian crust indicates that for other parts of the track, the lithosphere is more than 130 kilometers (81 miles) thick, preventing volcanic activity. Intermediate thicknesses produce the potassium-rich leucitite.
Things get complicated in the south, as the plume’s patch crosses a chain of extinct volcanoes that Davies told IFLScience are caused by quite a different phenomenon, known as edge-driven convection. where volcanic activity is a result of a sharp boundary between thick and thin sections of the lithosphere.“This is the first documented case of the interaction of a mantle plume and edge-driven convection,” Davies said.
“The chain is so long because Australia is the fastest moving continent, traveling north at about 7cm a year.” Davies told IFLScience. This movement has carried the Australian mainland beyond the plume, now located off the coast of Tasmania. “There are no known volcanic eruptions. Nothing obvious on the sea floor, but there have been earthquakes in the region and it needs to be investigated further.”
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