By Amanda Enbysk
A geological mystery lies beneath the majestic beauty of Yellowstone National Park. Once thought solved, the enigma continues to unfold through the lens of a young science known as magnetotellurics.
As accepted theory goes, over the past 16 million years a rising plume of magma in the Earth’s mantle produced massive amounts of lava and ash in a path stretching from the Snake River Plain to its current caldera — a volcanic crater in Wyoming, the Yellowstone “supervolcano.” It is widely believed that the Yellowstone caldera currently sits on top of that hotspot, a vertical “blowtorch” in the mantle beneath the Earth’s crust. The North American tectonic plate slowly creeps over the plume of magma, no faster than the rate at which fingernails grow. The plume sometimes oozes and other times violently erupts lava across an area the size of Rhode Island. Adam Schultz, a geophysics professor in Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences, describes this mantle hotspot idea as “almost a cartoon view that Earth scientists have of why you get features like Yellowstone.”
Magnetotellurics (MT), the study of the Earth’s electric and magnetic fields, may turn this cartoon view on its head. The use of magnetotelluric surveys has exploded in the last decade thanks to progress in computing technology and geophysical instrumentation. Schultz’s colleagues at Oregon State — Anna Kelbert and Gary Egbert — have used magnetotellurics to reveal that large volumes of partially molten rock and potentially superheated water (hydrothermal systems) snake west underneath the crust and into the uppermost mantle west of Yellowstone. This molten trail continues westward along much of the Snake River Plain in Idaho and into Oregon. These findings complicate the expectation that a nearly vertical magma plume lies directly under the present day Yellowstone supervolcano, which was what is anticipated from a hotspot. Magnetotellurics has opened doors to stunning breakthroughs and fascinating discoveries, providing new perspectives that were once invisible to science.
From Magnetics to Melted Rock
With magnetotellurics, scientists measure variations in the direction and intensity of the planet’s natural magnetic and electric fields over time. They use these measurements to understand the properties of the rock, one of the most important being electrical conductivity. Generally, greater electrical conductivity can suggest the presence of extensively interconnected bodies of fluid within the rock. West of Yellowstone, magnetotellurics reveal a relatively shallow, hot, highly conductive region under the Snake River Plain.
Schultz compares magnetotelluric surveys to MRIs commonly used in medical diagnostics. In fact the underlying principles are similar. “If you go to a radiology department and they do a CT scan of your head, for example, they see some weird thing, and they’re not quite sure what it is. You have an MRI and go, ‘ah! that’s a brain tumor,’” says Schultz.
In the same way, MT can be thought of as a very large MRI. And just as doctors put together multiple types of scans to see inside our bodies, geophysicists combine seismology, magnetotellurics and measurements of the on-going deformation of the Earth’s surface through GPS and satellite radar data to see what’s underground. Schultz’s focus on the Yellowstone caldera is part of a larger project, the magnetotelluric component (also known as EMScope) of the National Science Foundation’s EarthScope Program.
Schultz, a former program director for the NSF, heads EMScope. In the quest to understand more about the history of the North American continent, EarthScope makes seismic, GPS and MT surveys of the United States and part of Canada. EMScope provides the geomagnetic facet of the survey, producing 3-D images of Earth’s electrical conductivity variations beneath the continent.
Sweeping west to east, scientists are deploying portable arrays of magnetometers and electric field sensors in plastic boxes buried a foot or two in the ground. These small devices silently collect data over a period of one to three weeks, depending on the level of solar storm activity, which provides the source of their signal. Remarkably, the stream of charged particles emitted from the Sun’s atmosphere, the “Solar Wind,” is what makes this all happen. Some of those particles are captured by the Earth’s magnetic field and form gigantic electric currents that flow above the atmosphere, the most famous of which are the aurora (the Northern and Southern Lights). These currents cause other electric currents to flow inside the Earth’s crust and mantle, generating a signal that is detectable by MT devices.
Ancient Rift Revealed
Schultz first encountered geophysics at Brown University in 1979 when MT systems and computers were the size of travel trailers. Instruments today are small, rugged and more mobile. Teams of scientists are currently creating 3D images of the electrical conductivity beneath the comparatively flat landscape of the Midwest. Early results already reveal a billion-year-old ancient rift down the center of the continent, a feature hidden by vast seas of crops and flattened by millions of years of erosion. Magnetotellurics provides a view that goes below the region’s apparent horizon-to-horizon uniformity.
In Oregon, Schultz also leads a magnetotelluric study contributing to the potential geothermal development of Newberry Volcano just south of Bend. Nearly 20 times larger than Mount St. Helens, Newberry is Oregon’s largest volcano. Its flanks slope so gently that it’s hardly visible from any roadside viewpoint. In fact, the city of Bend sits close to the northern flank. The volcano isn’t dead, however. Massive amounts of heat lie just beneath the surface, a potentially large source of alternative energy waiting to be utilized.
Adam Schultz directs Oregon State’s National Geoelectromagnetic Facility.
The U.S. Department of Energy’s National Energy Technology Lab (NETL) has contracted with Oregon State to monitor and assist in the development of a geothermal system on the caldera’s western rim. AltaRock, a geothermal energy company, aims to demonstrate that sufficient heat can be harnessed from deep beneath the surface. It might be possible to generate electricity at commercially competitive levels. To do so, technicians begin by injecting cold fluids at high pressure into the cracks and crevices in the blistering but otherwise dry basalt underground. Ultimately, those heated fluids could then be extracted to create steam and drive electric turbines to generate power.
Unfortunately, water changes the rock to clay, creating a slimy obstacle that would block the cracks and shut off the water flow back to the surface. However, the fluids also change conductivity, and this property allows geophysicists like Schultz to make 3-D surveys that help identify clogs in the plumbing and keep the water flowing and creating steam.
There’s even a future for magnetotellurics in ocean-wave energy. Turbine buoys used in wave-energy projects generate electromagnetic fields. Since some marine species may be sensitive to electric and magnetic fields, the turbines could potentially disrupt marine ecosystems. To ensure the safety of these fragile areas, Schultz and his team are developing new sensors to gather electromagnetic, seismic and other data. The latest sensor, affectionately called Beaver 1 by the National Geoelectromagnetic Facility, Schultz’s lab, is destined for the ocean floor beneath wave turbines off the Oregon coast.
Back at Yellowstone, data from MT surveys offer evidence of a more complex explanation for the heat beneath the world’s first national park. While the EMScope sensors have moved on to other areas, early results show the melted remains beneath and to the west of the giant volcano. They whisper of a subducted past. Over 200 million years ago, the Farallon plate, the ancient piece of crust between the North American and Pacific tectonic plates, began to dive beneath young North America. Geologists have known for some time that rather than angling steeply toward the mantle, the Farallon hugged the base of the continent all the way to the current Rocky Mountains. About 16 million years ago, interactions between the diving plate and a mantle plume began forming the volcanic features of the Snake River Plain and Yellowstone before eventually descending to be recycled. All that’s left of the Farallon, mere slivers of its past size, grinds today beneath the coast of North and Central America. Off the Pacific Northwest coast, those remains are called the Juan de Fuca plate.
Geoscientists are still debating what the MT data mean for the evolution of the continent and for specific areas such as Yellowstone. Kelbert, Egbert and Schultz plan to refine their understanding with more magnetotelluric studies of the crust in higher resolution. EMScope is only a first step in 3-D geomagnetic surveys, and the discovery beneath Yellowstone is only a chapter of a complex history. This young science will undoubtedly illuminate more untold stories that lie beneath our feet. Geophysicists will have their hands full for years to come.
Amanda Enbysk is a senior in the College of Earth, Ocean, and Atmospheric Sciences.
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