An international team of researchers has discovered the world’s longest known continental volcanic hotspot track — a 1,245 mile (2,000 km) long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago.
The Cosgrove hotspot track. Image credit: Drew Whitehouse / NCI National Facility VizLab.
The volcanic chain, dubbed the Cosgrove hotspot track, was found to be nearly 3 times the length of the famous Yellowstone hotspot track in the United States. It was created over the past 33 million years, as Australia moved northwards over a hotspot in the Earth’s mantle.
The scientists were studying hotspot tracks in northern and southern Australia when they discovered that the tracks form part of the same 1,245 mile long track stretching across the continent from near Townsville in the north to near Melbourne in the south. Both tracks were previously thought to be unconnected.
“We realized 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,” said Dr Rhodri Davies of the Australian National University, lead author of a paper published in the journal Nature.
“This kind of volcanic activity is surprising because it occurs away from tectonic plate boundaries, where most volcanoes are found.”
“The Cosgrove hotspot track essentially consists of two parts in the north and south, with a center section where the lithosphere is too thick to allow melt to form,” said co-author Dr Nick Rawlinson of the University of Aberdeen, UK.
These hotspots are thought to form above mantle plumes, narrow upwellings of hot rock that originate at Earth’s core-mantle boundary almost 1,870 miles below the surface.
The team found that sections of the track have no volcanic activity because the Australian continent is too thick to allow the hot rock in mantle plumes to rise close enough to the Earth’s surface for it to melt and form magma.
They found that the plume created volcanic activity only where Earth’s lithosphere is thinner than 80 miles.
“Ultimately this new understanding may help us to reconstruct the past movements of continents from other hotspots,” Dr Rawlinson said.
The giveaway that the continent is just thin enough for melting to begin, such as in northern New South Wales, is the formation of an unusual mineral called leucitite. This mineral is found in low-volume magmas that are rich in elements such as potassium, uranium and thorium.
“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 Prof Ian Campbell of the Australian National University.
“The mantle plume that formed the Australian volcanoes is probably still in existence, under the sea a little to the northwest of Tasmania,” Dr Davies said.
“There are observations of higher mantle temperatures and increased seismicity in this region.”
_____
D.R. Davies et al. Lithospheric controls on magma composition along Earth’s longest continental hotspot track. Nature, published online September 14, 2015; doi: 10.1038/nature14903
