Deep in the Earth below Yellowstone National Park is a plume of molten rock that provides the heat and energy to drive Old Faithful, the colorful Norris Geyser Basin, Mud Volcano, and all the other geothermal features in the park.
And now scientists tell us it's larger than they once thought.
Past surveys of the deep-seated plume often used seismic waves, which move faster through cold rock than through hot rock, to gauge the size of the plume. The new approach taken by University of Utah researchers involves mapping the electrical conductivity of the hot spot that is at the root of Yellowstone's supervolcano.
“It’s like comparing ultrasound and MRI in the human body; they are different imaging technologies,” explains geophysics Professor Michael Zhdanov, the 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.
The new calculation shows the hot spot to be somewhat larger, east to west, than previously thought, according to a university news release.
In a December 2009 study, Robert Smith, professor emeritus and research professor of geophysics at the University of Utah and a coordinating scientist of the Yellowstone Volcano Observatory, used seismic waves from earthquakes to make the most detailed seismic images yet of the “hotspot” plumbing that feeds the Yellowstone volcano. Those measurements, which allowed him to make a three-dimensional image of the hot spot, 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," the release notes.
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 release adds. (The geoelectric image can “see” only 200 miles deep.)
“It’s a totally new and different way of imaging and looking at the volcanic roots of Yellowstone,” says Dr. Smith, who co-authored the new study.
Geophysical Research Letters has accepted the study for publication and plans to publish it within the next few weeks, according to university officials.
Two Approaches To Measuring Yellowstone National Park's Hot Spot
Dr. Smith says the resulting 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.
“It [the plume] is very conductive compared with the rock around it,” Professor Zhdanov says. “It’s close to seawater in conductivity.”
The lesser tilt of the geoelectric plume image raises the possibility that the seismically imaged plume, shaped somewhat like a tilted tornado, might be enveloped by a broader, underground sheath of partly molten rock and liquids, the two geophysicists say.
Despite any differences, Dr. Smith says, “this body that conducts electricity is in about the same location with similar geometry as the seismically imaged Yellowstone plume.”
The study was conducted by Professor Zhdanov, Dr. Smith, two members of Professor Zhdanov’s lab – research geophysicist Alexander Gribenko and geophysics Ph.D. student Marie Green – and computer scientist Martin Cuma of the University of Utah’s Center for High Performance Computing. Funding came from the National Science Foundation and the Consortium for Electromagnetic Modeling and Inversion, which Zhdanov heads.
The Yellowstone Hotspot at a Glance
This new approach to measuring the hotspot says nothing about the chances of another cataclysmic caldera eruption at Yellowstone, which has produced three such catastrophes in the past 2 million years, the university release said.
Almost 17 million years ago, the plume of hot and partly molten rock known as the "Yellowstone hotspot" first erupted near what is now the Oregon-Idaho-Nevada border. As North America drifted slowly southwest over the hotspot, there were more than 140 gargantuan caldera eruptions – the largest kind of eruption known on Earth – along a northeast-trending path that is now Idaho’s Snake River Plain.
The hotspot 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, university researchers say. 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," university researcher say. "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."
Computing a Geoelectrical Image of Yellowstone’s Hotspot Plume
Producing a geoelectric image of the hotspot is not easily done. Professor Zhdanov and his colleagues used data collected by EarthScope, an National Science Foundation-funded effort to collect seismic, magnetotelluric and geodetic (ground deformation) data to study the structure and evolution of North America. Using the data to measure the Yellowstone plume was a computing challenge because so much data was involved, according to the university researchers.
"Inversion" is a formal mathematical method used to “extract information about the deep geological structures of the Earth from the magnetic and electrical fields recorded on the ground surface,” Professor Zhdanov says. Inversion also is used to convert measurements of seismic waves at the surface into underground images.
Magnetotelluric measurements record very low frequencies of electromagnetic radiation – about 0.0001 to 0.0664 Hertz – far below the frequencies of radio or TV signals or even electric power lines. This low-frequency, long-wavelength electromagnetic field penetrates a couple hundred miles into the Earth. By comparison, TV and radio waves penetrate only a fraction of an inch.
The EarthScope data were collected by 115 stations in Wyoming, Montana and Idaho – the three states straddled by Yellowstone. The stations, which include electric and magnetic field sensors, are operated by Oregon State University for the Incorporated Research Institutions for Seismology, a consortium of universities.
In a supercomputer, the researchers explain, a simulation predicts expected electric and magnetic measurements at the surface based on known underground structures. That allows the real surface measurements to be “inverted” to make an image of underground structure.
Professor Zhdanov says it took about 18 hours of supercomputer time to do all the calculations needed to produce the geoelectric plume picture.