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Supervolcano plume sized up

April 12, 2011


University of Utah

This image, based on variations in electrical conductivity of underground rock, shows the volcanic plume of partly molten rock that feeds the Yellowstone supervolcano. Yellow and red indicate higher conductivity,green and blue indicate lower conductivity.

By John Roach

The volcanic plume beneath Yellowstone is larger than previously thought, according to a new study that measured the electrical conductivity of the hot and partly molten rock.

The findings say nothing about the chances of another cataclysmic eruption at Yellowstone, but they give scientists another view of the vast and deep reservoir that feeds such eruptions.

“It’s a totally new and different way of imaging and looking at the volcanic roots of Yellowstone,” study co-author Robert Smith, an emeritus professor of geophysics at the University of Utah, said in a press release. Supervolcano history
The supervolcano has erupted three times over the past 2 million years – 2 million, 1.3 million, and 642,000 years ago. While researchers don’t expect another eruption any time soon, it could eventually explode, destroying life for hundreds of miles around it and blanketing North America in ash.

In recent years, scientists have detected an unprecedented rate of rising for the caldera and increases in seismic activity, including a peculiar swarm of earthquakes.

The U.S. Geological Survey has ranked the Yellowstone caldera as a high threat for volcanic eruption, calling it the 21st most dangerous of 169 volcano centers in the U.S.

While the new measurements don’t raise the threat level, scientists are keen to gain a deeper understanding of the plume beneath the national park renowned today for geysers and hot springs.

“We are just getting more and more understanding of what is going on,” Michael Zhdanov, a professor of geophysics at the University of Utah and lead author of the study, told me today.

New measurements
In previous work, published in 2009, researchers used seismic waves from earthquakes to image the hotspot plumbing that feeds the supervolcano. Seismic waves move more quickly through cold rock than hot rock. By clocking seismic waves, researchers made 3D images of the plume.

Those images showed the plume of hot and molten rock dips downward from Yellowstone at an angle of 60 degrees and extends 150 miles west-northwest to a point at least 410 miles under the Montana-Idaho border, which as far as the imaging could “see.”

In the new study, scientists used images of the plume’s electrical conductivity that is generated by molten silicate rocks and hot briny water that is naturally present and mixed in partly molten rock. This shows the conductive part of the plume dipping more gently — an angle of about 40 degrees to the west and extending about 400 miles from east to west. The geoelectric image can only see 200 miles deep.

“It looks a little bigger,” Zhdanov said. “It looks like our image put an envelope around this seismic image.”

Geoelectrical data
The geoelectrical data was collected by Earthscope, a National Science Foundation-funded effort to collect seismic, magnetotelluric, and ground deformation data to study the structure and evolution of North America.

Magnetotelluric measurements record very low frequencies of electromagnetic radiation — about 0.0001 to 0.0664 Hertz, which is far below the frequencies of radio and TV signals and electric power lines. The low frequency, long wavelength field penetrates about 200 miles into the Earth.

Data for the study was collected by 115 stations in Wyoming, Montana, and Idaho — the three states straddled by Yellowstone National Park. It was crunched by a supercomputer, which produced the geoelectric plume picture.

The study has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

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