From: Andy Soos, ENN
Published April 13, 2011 08:30 AM

Yellowstone Supervolcano Size

University of Utah geophysicists made the first large-scale picture of the electrical conductivity of the gigantic underground plume of hot and partly molten rock that feeds the Yellowstone supervolcano. The image suggests the plume is even bigger than it appears in earlier images made with earthquake waves. The Yellowstone Caldera is the volcanic caldera located in Yellowstone National Park in the United States, sometimes referred to as the Yellowstone Supervolcano. The caldera is located in the northwest corner of Wyoming, in which the vast majority of the park is contained. The major features of the caldera measure about 34 miles by 45 miles.

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"It's like comparing ultrasound and MRI in the human body; they are different imaging technologies," says geophysics Professor Michael Zhdanov, principal author of the new study and an expert on measuring magnetic and electrical fields on Earth's surface to find oil, gas, minerals and geologic structures underground.

"It's a totally new and different way of imaging and looking at the volcanic roots of Yellowstone," says study co-author Robert B. Smith, professor emeritus and research professor of geophysics and a coordinating scientist of the Yellowstone Volcano Observatory.

The new University of Utah study has been accepted for publication in Geophysical Research Letters, which plans to publish it within the next few weeks.

In a December 2009 study, Smith used seismic waves from earthquakes to make the most detailed seismic images yet of the "hotspot" plumbing that feeds the Yellowstone volcano. Seismic waves move faster through cold rock and slower through hot rock. Measurements of seismic-wave speeds were used to make a three-dimensional picture, quite like X-rays are combined to make a medical CT scan.

The source of the Yellowstone hotspot is controversial. Some geoscientists hypothesize that the Yellowstone hotspot is the effect of an interaction between local conditions in the lithosphere and upper mantle convection. Others prefer a deep mantle origin (mantle plume). Part of the controversy is due to the relatively sudden appearance of the hotspot in the geologic record.

The 2009 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 - as far as seismic imaging could "see."

In the new study, images of the Yellowstone plume's electrical conductivity - generated by molten silicate rocks and hot briny water mixed in partly molten rock - shows the conductive part of the plume dipping more gently, at an angle of perhaps 40 degrees to the west, and extending perhaps 400 miles from east to west. The geoelectric image can see only 200 miles deep. 

Smith says the geoelectric and seismic images of the Yellowstone plume look somewhat different because "we are imaging slightly different things." Seismic images highlight materials such as molten or partly molten rock that slow seismic waves, while the geoelectric image is sensitive to briny fluids that conduct electricity.

The lesser tilt of the geoelectric plume image raises the possibility that the seismically imaged plume, shaped somewhat like a tilted tornado, may be enveloped by a broader, underground sheath of partly molten rock and liquids, Zhdanov and Smith say.

Despite differences, he says, "this body that conducts electricity is in about the same location with similar geometry as the seismically imaged Yellowstone plume."

The new study says nothing about the chances of another cataclysmic caldera (giant crater) eruption at Yellowstone, which has produced three such catastrophes in the past 2 million years.

The hotspot finally reached Yellowstone about 2 million years ago, yielding three huge caldera eruptions about 2 million, 1.3 million and 642,000 years ago. Two of the eruptions blanketed half of North America with volcanic ash, producing 2,500 times and 1,000 times more ash, respectively, than the 1980 eruption of Mount St. Helens in Washington state. Smaller eruptions occurred at Yellowstone in between the big blasts and as recently as 70,000 years ago.

Seismic and ground-deformation studies previously showed the top of the rising volcanic plume flattens out like a 300-mile-wide pancake 50 miles beneath Yellowstone. There, giant blobs of hot and partly molten rock break off the top of the plume and slowly rise to feed the magma chamber - a spongy, banana-shaped body of molten and partly molten rock located about 4 miles to 10 miles beneath the ground at Yellowstone.

For further information: http://unews.utah.edu/p/?r=032411-5

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