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.
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.
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.
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.
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.”
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.
Two views of the Yellowstone volcanic plume
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.
“It’s a bigger size” in the geoelectric picture, says Smith. “We can infer there are more fluids” than shown by seismic images.
differences, he says, “this body that conducts electricity is in about
the same location with similar geometry as the seismically imaged
says that last year, other researchers presented preliminary findings
at a meeting comparing electrical and seismic features under the
Yellowstone area, but only to shallow depths and over a smaller area.
study was conducted by Zhdanov, Smith, two members of 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 (NSF) and the Consortium for Electromagnetic
Modeling and Inversion, which Zhdanov heads.
The Yellowstone hotspot at a glance
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.
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.
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.
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.
Computing a geoelectrical image of Yellowstone’s hotspot plume
and colleagues used data collected by EarthScope, an NSF-funded effort
to collect seismic, magnetotelluric and geodetic (ground deformation)
data to study the structure and evolution of North America. Using the
data to image the Yellowstone plume was a computing challenge because so
much data was involved.
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,” Zhdanov says. Inversion also is
used to convert measurements of seismic waves at the surface into
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.
EarthScope data were collected by 115 stations in Wyoming, Montana and
Idaho – the three states straddled by Yellowstone National Park. 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.
a supercomputer, 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.
says it took about 18 hours of supercomputer time to do all the
calculations needed to produce the geoelectric plume picture. The
supercomputer was the Ember cluster at the University of Utah’s Center
for High Performance Computing, says Cuma, the computer scientist.
has 260 nodes, each with 12 CPU (central processing unit) cores,
compared with two to four cores commonly found on personal computer,
Cuma says. Of the 260 nodes, 64 were used for the Yellowstone study,
which he adds is “roughly equivalent to 200 common PCs.”
To create the geoelectric image of Yellowstone’s plume required 2 million pixels, or picture elements.
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.
“It [the plume] is very conductive compared with the rock around it,” Zhdanov says. “It’s close to seawater in conductivity.”