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The upper contours of a hot blob of molten rock plunging at least 400 miles below Yellowstone National Park are coming into sharper relief, thanks to new data generated by measurements of electrical conductivity.
The funnel-shaped chamber could be larger than previously pictured, according to University of Utah geophysicists poised to publish a large-scale image of the magma plume believed to be responsible for Yellowstone's famous thermal features and a series of cataclysmic eruptions.
Robert Smith, an emeritus professor of geophysics, used seismic waves to paint a detailed picture of the plume in 2009, indicating it rises at an angle of 60 degrees from the northwest, covering about 150 miles of horizontal distance.
The latest study exploits the plume's electrical conductivity to chart a picture in which the plume rises at a shallower angle, about 40 degrees, and is larger, extending as far as 400 miles east to west, versus the 150 miles pictured by the seismic data.
The U. scientists cautioned that the studies don't contradict each other, but simply measure different things. Seismic waves are sensitive to density so they slow as they travel through molten rock, while electricity passes more readily through the materials associated with the plume.
"It's like comparing ultrasound and MRI in the human body," said geophysics professor Michael Zhdanov, Smith's co-author on the latest study. "They are completely different methods. They provide complementary data about the same object."
The study has been accepted by Geophysical Research Letters, which plans to publish it within the next few weeks.
The plume is conductive because it's rich in silicate rocks and is perhaps surrounded by hot water, according to Smith.
"Conductivity is affected by different properties of the rock," said Zhdanov, an expert on measuring magnetic and electrical fields in search of oil, gas, minerals and other underground geological structures. "It's probably the same rocks but under different physical condition. Solid rocks are less conductive."
The lesser tilt of the geoelectric image suggests that the seismically imaged plume may be enveloped by a broad sheath of partly molten rock and liquids, the geophysicists report.
The new geoelectrical data, which can "see" 200 miles below the Earth's surface, was generated by EarthScope, a massive National Science Foundation-funded project to characterize the structure of the geology under North America. Smith's seismic image exceeds 400 miles in depth.
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