A stretch of once-lost crust lies just beneath the surface, tracing the eastern flank of the Appalachian Mountains. This 200-kilometer long section of the Earth’s shell, running from Maine down to Georgia, likely originated from volcanic eruptions during the breakup of the supercontinent Pangaea approximately 200 million years ago, subsequently buried under mountain erosion sediment.
Known as the Piedmont Resistor, this piece of Pangaea represents one of the key findings from the magnetotelluric (MT) Array—a network of 1,800 temporary stations deployed across the continental United States to measure the electrical conductivity of deep rock layers. Now, two decades after its inception, the MT Array has released its final map and model in a paper published in Reviews of Geophysics. This final product illustrates how the continent’s formation left behind hidden structures, like the Piedmont Resistor, and preserved mineral wealth. “You can see all the seams,” notes co-author Paul Bedrosian, a geophysicist with the U.S. Geological Survey (USGS). “And those boundaries, even those that are a billion years old, are where people look for mineral resources.”
“The magnetotelluric Array has set a global benchmark,” says geophysicist Hao Dong of the China University of Geosciences, inspiring similar endeavors in China, Australia, and Brazil. In addition to illuminating the continent’s geological foundations, it revealed how subsurface geology can amplify hazards from solar storms for power grids. All of this was achieved with a lean budget of only $20 million when adjusted for inflation, according to Adam Schultz, a geophysicist at Oregon State University who previously managed the project. “The amount of data quality you received is an incredible return on investment.”
The Magnetotelluric (MT) system began as a secondary effort alongside the US Array project, which used seismic waves to probe the continental crust and mantle. The original plan involved deploying seismic and MT stations concurrently, but those goals soon diverged. Seismologists favored the quiet, dense rocks typical of elevated, high-and-dry terrain, whereas MT researchers required moist, electrically conductive ground for their sensors. “We spend a lot of time in swamps,” Bedrosian mentions.
Their objective was to detect electric currents induced in the bedrock by the constantly fluctuating magnetic fields generated by solar wind disturbances and the upper atmosphere. Pairs of electrodes, set 50 meters apart in a cross formation, capture these currents; to measure the strength of the magnetic fields driving them, each station also incorporates a magnetometer. The ratio between the currents and the magnetism measurements reveals the conductivity of the rocks beneath the surface, with lower electrical frequencies probing conductivity at greater depths. While seismic systems map variations in rock density, the MT array is especially adept at sensing deep compositional changes, including water, frequently found beneath continents within subducting tectonic plates. It can also identify graphite and sulphides created during magmatic pulses when continental plates collide or rift apart.
Stations were spaced 70 kilometers apart and operated for several weeks with minimal public interaction, although one team did have a run-in with a local sheriff after being misidentified as potential terrorists. When U.S. National Science Foundation funding for the project ended in 2018, a large portion of the southern U.S. remained unmapped. The USGS took stewardship of the remaining effort, seeing it through to the decommissioning of the final station in Louisiana in 2024.
Three-dimensional subsurface images derived from the MT data allowed researchers to observe, for instance, how water is drawn into the Cascadia subduction zone where oceanic crust dives beneath Oregon and Washington. The system visualized the crustal extension responsible for the Basin and Range province of Nevada and helped illustrate how the Wasatch Range in Utah acts as a restraint controlling that stretching. It also helped delineate the subtle boundaries of ancient crustal blocks forming the continent’s core and reconstruct buried mountain belts that generate the complex conductive veins beneath the Great Plains. The lack of crustal boundaries further east, meanwhile, suggests the eastern half of the country formed more haphazardly from continental fragments and volcanic islands, says Bedrosian.
One of these fragments is the Piedmont Resistor, which seismic surveys in the region had overlooked. Bedrosian and his colleagues interpret it as crust formed from massive eruptions that contributed to the separation of the Americas from the rest of Pangaea 200 million years ago. “It’s very plausible that it exists,” says geophysicist Alexander Grayver of the University of Cologne.
This isn’t merely a tectonic leftover. When a severe solar storm agitates Earth’s magnetosphere, causing powerful currents to surge through the crust, the Resistor magnifies the threat to power grids, as depicted in space weather hazard maps generated using the MT Array data. As its name implies, its cooler magmatic rocks resist the induced current, constraining and concentrating it in shallower layers closer to populated areas.
Geophysicist Anna Kelbert of the Harvard-Smithsonian Center for Astrophysics states that MT data shows how geological features can increase the solar storm risk tenfold or even a thousandfold. And while the U.S. government is doing a good job updating its hazard maps, American electric utility companies are not incorporating the latest data—and no one is compelling them to. “Utilities are lagging behind,” Kelbert remarks.
Mining companies interested in discovering new deposits of critical minerals might pay closer attention now. Although the maps are too coarse to pinpoint individual ore bodies, deposits are often found near the magmatic systems that the MT array can trace, notes geophysicist Graham Heinson of the University of Adelaide. “Mapping the edges [of the crustal blocks] gives us a target-rich idea of where the next big system might be.”
Microwave technology has always been a relatively niche field in the U.S., and now Chinese scientists are the primary users of the data gathered by this system. Concluding the creation of the array is a moment filled with mixed emotions, Bedrosian adds. “There’s a sense that we’re celebrating a triumph, but simultaneously, we’re turning off the stadium lights.”
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