Massive magma chamber hiding beneath volcanic system of Yellowstone National Park
© Mark Ralston, AFP/GETTY IMAGES
The 'Grand Prismatic' hot spring in the Yellowstone National Park, home of a massive underground supervolcano.
We knew of a smaller magma chamber closer to surface, holding some 10,000 cubic kilometres of magma and feeding heat upwards. The newly discovered reservoir sits under it and has a volume of 46,000 cubic kilometres. Together, the two form the largest known magma reservoir in the world.
"We can't say definitively that this is the biggest magma reservoir in the world, but we currently don't know of any other that has been imaged that is as large as the two we see beneath Yellowstone," says Fan-Chi Lin of the University of Utah in Salt Lake City.
The discovery of the much larger reservoir at a depth of 20 to 50 kilometres helps to solve a long-running puzzle relating to the carbon dioxide spewing out from the huge steaming caldera volcano at Yellowstone, creating ripples of tiny earthquakes it does so. The problem is that the upper magma chamber is much too small to account for the 45 kilotonnes of carbon dioxide discharged daily.
"If you combine the upper and lower crustal magma reservoirs, it better accounts for the amount of carbon dioxide coming out of the ground at Yellowstone," says Lin.
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Mantle plume
It also solves another mystery: how molten material from much deeper down makes it to the surface. The main source of magma feeding Yellowstone is a mantle plume found around 60 kilometres deep, fanning out like a huge pancake.
"This transports heat and magma-containing fluids to the crust above," says Lin.
Geologists had been puzzling over how this molten material could travel all the way between the plume and the upper magma reservoir, which is just 5 to 16 kilometres down.
The newly discovered reservoir is the missing connection between the two, interlinking with them via narrow vertical and horizontal channels respectively called dykes and sills.
"Now we have a much better understanding of how the entire magmatic system beneath Yellowstone works, from the mantle to the surface," says Lin.
The other good news is that despite the massive magma find, the risk of an supervolcano eruption remains unchanged. The last huge eruption there happened 640,000 years ago, forming the current Yellowstone caldera, 60 kilometres long and 40 kilometres wide.
The findings can be applied to other volcanic systems around the world to help identify imminent volcanic activity and the risk of eruptions, says Lin.
Under the radar
So how did we not notice this chamber before?
To visualise underground structures, geologists look at the way seismic waves from earthquakes pass through the areas of interest. Some types of rock - particularly molten material - slow down the waves, so when the wave speeds are measured by detectors at the surface, geologists can work out rock profiles below ground. "It's analogous to a medical CT scan," says Lin.
Features at the depths where the new chamber was found have been difficult to study because the speed of waves from below are dictated by rock closer to the surface. Lin's team got round this by combining readings from local, shallower, and distant, deeper tremors, allowing the distorting effects of the rocks nearer the surface to be filtered out.
"This allows us to simultaneously image both shallow and deeper crustal structures," says Lin. Altogether, they analysed data from 4520 local and 329 distant quakes.
Other volcanologists say the results finally prove what was long suspected, that there was another source of heat and magma beneath Yellowstone.
"This newly-discovered magmatic body supplies the heat and volatile gases essential to account for how the Yellowstone system functions," says Luca Caricchi of the University of Geneva in Switzerland.
"These results advance significantly our ability to identify the global distribution of volcanic systems with the potential to produce extremely large volcanic eruptions, which may have very large impacts on society," says Caricchi.
Good places to look for similar systems to Yellowstone include Toba in Indonesia, the Taupo volcanic zone in New Zealand, and the southern Rocky mountains in the US, he says.
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