Robert Smith, University of Utah geophysicist, has spent much of his career studying the subsurface conditions and modeling the size of the plume that underlies Yellowstone. In 2009, Smith reported on a seismic study that showed that the plume of hot, possibly partially melted rock, dips downward from Yellowstone at an angle of 60 degrees. This plume extends 150 miles west-northwest and reaches at least 410 miles depth under the Montana-Idaho border. This is as far as the seismic imaging permits resolution of the structure.
In a work to be published in Geophysical Research Letters this month**, summarized in this Science Daily article, Smith and colleagues now report on a study of the electrical conductivity beneath Yellowstone, the first of its kind. Electrical conductivity yields an image of melted rocks plus hot salty water--the geothermal system that surrounds the magmatic plume. This plume appears larger than that revealed through seismic imaging. The plume dips at an angle of ~40 degrees to the west, extending ~400 miles east-west, and goes at least 200 miles deep. The overall picture is of a tilted molten core that looks something like a tilted tornado surrounded by a sheath of hot water. Seismic and ground-deformation studies show that the top of the plume flattens out like a pancake about 50 miles beneath Yellowstone--a 300 mile diameter pancake! Blobs of hot partially molten rock break off of the top of this reservoir and rise to feed the shallow magma chamber that is 4-10 miles below the surface in Yellowstone.
**Zhdanov, M.S., Smith, R.B., Gribenko, A., Cuma, M., and Green, M., Three-dimensional inversion of large-scale EarthScope magnetotelluric data based on the Integral Equation Method: Geoelectricla imaging of the Yellowstone conductive mantle plume, Geophys. Res. Lett., doi:10.1029/2011GL047346, in press. 2011.