A version of this story appeared in Science, Vol 380, Issue 6648.Download PDF

From the air, Maine is a uniform sea of green: Forests cover 90% of the state. But beneath the foliage and the dirt lies an array of geological terrains that is far more diverse, built from the relics of volcanic islands that collided with North America hundreds of millions of years ago.

Two years ago, sensor-laden aircraft began to survey these geochemically rich terrains for precious minerals. Researchers spotted an anomalous signal streaming out of Pennington Mountain, 50 kilometers from the Canadian border. State geologists bushwhacked through the paper mill–bound pine forests, taking rock samples. They eventually uncovered deposits containing billions of dollars’ worth of zirconium, niobium, and other elements that are critical in electronics, defense, and renewable energy technologies. “It was a perfect discovery,” says John Slack, an emeritus scientist at the U.S. Geological Survey (USGS) who worked on the Maine find. He expects more like it. “We think there’s potential throughout the Appalachians.”

Few topics draw more bipartisan support in Washington, D.C., than the need for the United States to find reliable sources of “critical minerals,” a collection of 50 mined substances that now come mostly from other countries, including some that are unfriendly or unstable. The list, created by USGS at the direction of Congress, contains not only the 17 rare earth elements produced mostly in China, but also less exotic materials such as zinc, used to produce steel, and cobalt, used in electric car batteries. “These commodities are necessary for everything,” says Sarah Ryker, USGS’s associate director for energy and minerals. “They’re also a flashpoint for conflict.”

But last decade, when lawmakers began to ask USGS about U.S. supplies, the response was unsettling: The agency didn’t even know where to look. For decades, companies had been moving mining operations abroad, in part to avoid relatively stringent U.S. environmental regulations. The basic exploration needed to identify mineral resources and spur corporate interest had languished. The last nationwide survey, a quest for uranium, ended in the 1980s. Ryker says the U.S. is “undermapped” compared with most developed countries, including Australia, Canada, and even Ireland. “We’re at an embarrassing point.”

Magnetic anomalies (red) beneath southeastern Missouri reveal iron oxide deposits formed 1.4 billion years ago.U.S. Geological Survey

To start filling in this knowledge void, USGS in 2019 began what it calls the Earth Mapping Resources Initiative, or Earth MRI. With a modest $10 million annual budget, the agency began working with state geological surveys to digitize data and commission fieldwork to map the most promising terrain in fine detail.

Then, in 2021, the Bipartisan Infrastructure Law directed $320 million into the program—nearly one-third of the entire USGS budget—to be spent over 5 years. That spending has already enabled hundreds of survey flights, and it is opening a golden age for economic geology. It is also a boon for basic science—filling in gaps in geologic history, identifying unknown earthquake faults, and revealing geothermal systems. “We’re seeing a renaissance throughout the whole country,” says Virginia McLemore, an economic geologist at the New Mexico Bureau of Geology and Mineral Resources. “I’ve been training all my life to get to this point.”

The discoveries could spur a rash of mining, and environmentalists are wary. If USGS spots promising ore systems, companies will have to show that they can develop them safely and with minimal environmental impact, says Melissa Barbanell, director of U.S.-international engagement at the World Resources Institute, an environmental nonprofit. “It can never be zero harm,” she says. “But how can we minimize the harm and keep it to the mine itself?”

Mining companies, meanwhile, are embracing Earth MRI. Donald Hicks, a geophysicist at global mining giant Rio Tinto, which has dozens of operations worldwide but only a few in the U.S., says he has encouraged fellow miners to collaborate and share data with the program. Rio Tinto even funded some USGS flights in Montana, in return for 1 year’s exclusive access to the data. “Having this high-quality, large-scale data in the public domain will drive new ideas and new discoveries,” Hicks says.

For most of the history of mining, the origin story of a mineral lode was beside the point. Prospectors found it and miners dug it up. But by now, most of the obvious finds are gone, says Anne McCafferty, a USGS geophysicist. “The low-hanging fruit has been picked.”

This scarcity has pushed Earth MRI into adopting a “mineral systems” approach, first pioneered in Australia, that attempts to predict where critical minerals might be found based on the processes that form them. For example, a search for rare earth minerals might begin by looking for an unusual kind of carbon-rich rock called a carbonatite, which often contains pockets of rare earths formed when it crystallized out of lava. Or geologists might seek out clay-rich rocks or sediments that can capture concentrations of the rare earths after water erodes them from a source rock. Prospectors would also look for signs that these ore rocks were preserved across the eons.

To assemble these telltale rock histories, USGS scientists need to integrate a variety of information sources. Some already exist: large-scale geological maps based on decades of fieldwork, and surveys of the deep structure of rock formations based on the reflections of seismic waves from artificial or natural earthquakes.

Earth MRI’s airborne surveys, with flights just 100 meters above the surface, will add much more detail and inform a new generation of sharper geologic maps. One tool affixed to the aircraft is a magnetometer, which detects rocks rich in iron and other magnetic minerals—often a clue that they hold critical minerals. Another is a gamma ray spectrometer, which like a Geiger counter can capture the radiation emitted by thorium, uranium, and potassium. Those elements frequent the same volcanic rocks as rare earth minerals and are often incorporated into their crystal structures. Other aircraft carry laser altimeters that can map surface relief to reveal geologic history. And a pioneering “hyperspectral” instrument developed by NASA can identify minerals exposed on the surface based on the specific wavelengths of light they absorb. In the combined data, “You can see all the geology underneath,” says Anjana Shah, the USGS geophysicist leading the agency’s East Coast airborne surveys. “It’s a very powerful way of understanding the Earth.”

Hunting high and low

Armed with a $320 million boost from Congress, the U.S. Geological Survey is funding airborne and field campaigns to identify rocks likely to hold minerals critical for renewable energy and electronics, like lithium and rare earth elements. The campaign, called the Earth Mapping Resources Initiative (Earth MRI), is the first major assessment of the country’s mineral wealth in nearly half a century. It is deploying different techniques depending on the geology of each region.

Geophysics  Shown in blue.

Low-flying aircraft outfitted with magnetometers can survey iron-bearing rocks hidden in the shallow earth. Gamma ray spectrometers hunt for the radioactive signature of rare earth elements.

Lidar  Shown in purple.

Earth MRI is helping complete a high-resolution topographic airborne laser altimeters, or lidar. These data are essential for geological mapping and can reveal the surface expression of ancient landforms.

Hyperspectral  Shown in green.

In the arid West, where trees don’t block the view, flights using a NASA hyperspectral instrument will hunt for the signature minerals in hundreds of channels of reflected light.

Ground-based  Shown in tan.

The agency is sponsoring field mapping campaigns by state geologists. It is also funding broader geochemical surveys and studies of mineral resources left in old mine waste piles.

(Graphic) D. An-Pham/Science; (Data) Earth Mapping Resources Initiative via USGS

In early forays, Earth MRI aircraft criss-crossed North and South Carolina, tracing the ancient roots of the landscape. Hidden beneath the states’ tobacco farms are fossilized beaches that mark shorelines left during the warm periods between past ice ages, when sea levels were higher than today. Laser altimeter maps capturing subtle relief bloom with those shorelines and the paleorivers that dissected them, says Kathleen Farrell, a geomorphologist at the North Carolina Geological Survey. “There’s a lot more coastal plain than anyone thought.”

The ancient beaches hold deposits of black sands, eroded from mountains and deposited by rivers, that are rich in heavy elements. By combining the new airborne data collected by Shah with field mapping and boreholes drilled to sample the deep sediments, Farrell and her colleagues hope to learn how the Carolina sands originated. They want to know how the coastal plains were assembled over time, why the heavy sands formed only during certain periods, and where upriver those sands came from. The answers should help guide geologists to new heavy metal deposits; similar sites in northern Florida are among the few commercial sources of titanium in the U.S.

The airborne campaigns in South Carolina will have another benefit, Shah adds: They flew over Charleston, collecting magnetic data that, by identifying shifts and offsets in subsurface rocks, reveal the hidden seismic faults that ruptured in 1886 in an earthquake as large as magnitude 7. Such a quake, if it struck again today, would cause billions of dollars in damage.

This year, an Earth MRI survey covering parts of Missouri, Kentucky, Tennessee, Arkansas, Illinois, and Indiana will probe another mysterious seismic zone. Buried under kilometers of sediment lurks the Reelfoot Rift, a gash in the continent’s bedrock likely created some 750 million years ago when the Rodinia supercontinent began to crack apart. In 1811 and 1812, faults tied to this rift caused the New Madrid earthquakes, the largest to ever strike the U.S. east of the Rocky Mountains. But despite the potential hazard, the fault zone remains poorly understood.

The Reelfoot and nearby bedrock deformations not only create hazards; they also create opportunities for minerals to form. The rifts provided conduits for magma to well up much later in geologic time, when Africa collided with North America to form the Appalachian Mountains. This magma is thought to have expelled gases that flowed into limestones, chemically altering them. One result is the fluorspar district of southern Illinois, which once produced a majority of the country’s fluorite—used to smelt steel and create hydrofluoric acid.

Those magma injections could have played a role in creating Hicks Dome, which rises 1 kilometer above the Illinois countryside and is the closest thing the state has to a volcano. Jared Freiburg, critical minerals chief for the Illinois State Geological Survey, calls it “a crazy magmatic cryptovolcanic explosive structure.” It pops out as a magnetic anomaly in USGS airborne data, and cores drilled from the dome are rich in rare earth minerals. Geochemical tracers from the cores hint that deposits deeper in the dome were formed from carbonatites—the unusual volcanic rocks associated with the world’s best rare earth deposits. “It’s like a kitchen sink of critical minerals there,” McCafferty says.

In Nevada, a helicopter towing an induction coil measures subsurface electrical resistance and a researcher calibrates data collected by an airborne hyperspectral sensor. In Maine, geologists carry sensors to chart rocks’ radioactivity.Brett Robinson/Xcalibur Multiphysics via U.S. Geological Survey; USGS; Anji Shah/USGS

The midcontinent surveys could also help geologists assess another resource: natural hydrogen, a clean-burning fuel. Currently, all hydrogen is manufactured, but some researchers believe, contrary to conventional wisdom, that Earth produces and traps vast stores of the gas. The iron-rich volcanic rocks of the Reelfoot are exactly the kind that could produce hydrogen. Yaoguo Li, a geophysicist at the Colorado School of Mines, is developing a Department of Energy (DOE) grant proposal to prospect for hydrogen source rocks with the USGS data. “We have not done anything yet,” he says. “But I can see there’s so much we can do.”

Besides identifying resources to extract, the surveys could pay other dividends. They are pinpointing the steel casings of abandoned oil and gas wells that often leak greenhouse gases. They will help identify porous rock reservoirs, bounded by faults, that could hold carbon dioxide captured from smokestacks, keeping it out of the atmosphere. And they could also map variations in the radioactive rocks that emit radon gas, a health hazard.

These days, no mineral may be more critical than the lithium, used in cellphone and electric car batteries, that moves an ever-increasing number of the world’s electrons. Yet only one lithium mine exists in the U.S., in Nevada, and its raw lithium is sent abroad for processing. The state has potential to hold much, much more, and could become an international lithium “epicenter,” says James Faulds, Nevada’s state geologist.

Lithium is often found in igneous rocks—magma that crystallized in the crust or lava that cooled on the surface. Many of the known lithium deposits are in the state’s north, in the McDermitt caldera, a volcanic crater formed 16 million years ago by the deep-Earth hot spot currently fueling Yellowstone. Rainwater falling within the caldera or hot water from below has concentrated lithium within caldera clay deposits to levels not seen elsewhere, in other eruptions of the Yellowstone hot spot. “Why did this mineralization happen?” asks Carolina Muñoz-Saez, a geologist at the University of Nevada, Reno. She and her collaborators are studying the geochemistry of the lithium and the clays to find out whether the element was formed and concentrated during the eruption itself by superheated water or whether the concentration came later, as water infiltrated the caldera’s ash-rich rocks. The answer could lead the geologists to other, equally rich deposits.

Mountain Pass in California is the only U.S. mine producing rare earth elements. The U.S. Geological Survey hopes the Earth Mapping Resources Initiative will encourage more mining.TMY350/Wikimedia Commons

Earth MRI has already shown that lithium prospectors need not stick to calderas. Field geologists have found rocks that seem to be rich in lithium in basins bounded by tectonically uplifted blocks of crust. Nevada, famous for its “basin and range” topography, has a lot of places like that, Faulds says. Even better, the basins tend to host systems of hot brine, a potential source of geothermal power—one reason DOE is funding surveys in the state, says Jonathan Glen, a USGS geophysicist.

Just south of Nevada, DOE has similarly invested in USGS flights over California’s Salton Sea, which is being stretched apart by the movement of the Northern American and Pacific tectonic plates, leaving the crust thin and hot. “Temperatures are really high,” Glen says. “There’s huge geothermal potential.” Beyond mapping potential lithium deposits and geothermal sites, the surveys have also found new faults at the southern end of the San Andreas, and what appear to be buried volcanoes beneath the Salton Sea. “This is brand new stuff,” Glen says. “We didn’t know any of this.”

The mineral stibnite is the ore for antimony, used in batteries.Niki Wintzer/USGS

Those insights come from magnetometer, radiometric, and laser altimeter flights. But Earth MRI is also planning hyperspectral surveys that will scan the treeless, arid surface for pay dirt. Lithium and rare earth elements, for example, have strong spectral reflections; and other signatures can reveal the iron or clay minerals associated with lithium or other minerals. Beyond prospecting, the data will be valuable for spotting volcanic hazards. Those include rocks on the flanks of volcanoes that have been altered into soft clays by melting snow and heat, says Bernard Hubbard, a remote-sensing geologist at USGS. “Those become unstable—and then they collapse.”

Besides identifying the rock formations likely to hold mineral deposits, Earth MRI has accelerated USGS efforts to detect valuable resources left behind in tailings from defunct copper or iron mines. Last decade, Shah spotted the distinctive radioactive signatures of rare earths in such piles in Mineville, a hamlet in New York. With state geological agencies, USGS is compiling a national database of mine waste sites, along with methods for researchers to assess the waste’s mineral potential. “What’s the point of digging another hole in the ground if you can remine the rocks?” asks Darcy McPhee, Earth MRI’s program coordinator at USGS.

Those lingering tailings piles are a reminder of the environmental damage mining can do. For decades, the U.S. avoided environmental debates over mining by outsourcing it to other countries. The new consensus is that work should happen here, Ryker says. “But that means we have to deal with the conflict.” The survey will reveal new resources. But the rest is up to us, she says. “How much should we develop? That’s a much more complicated question.”

Those questions are now unfolding, state by state. In Nevada, lithium prospecting is booming, spurred by the Inflation Reduction Act’s mandate that electric cars must use some U.S.-sourced minerals for buyers to get a tax credit. But in Maine, legislators enacted a strict mining law in 2017, when the state’s largest landowner, the Canadian forestry company J.D. Irving, considered exploiting reserves of gold, silver, and copper found on its lands. Following the discovery of rare earth deposits at Pennington Mountain and lithium elsewhere in the state, lawmakers are now considering amending the law to allow some responsible mining.

Given the demands of green technology and the imperative to lower carbon emissions, many environmental groups are softening their stance on critical-mineral mining, Barbanell says. This exploitation doesn’t have to go on forever, she adds. Unlike coal, which must be mined indefinitely as it’s burned, the minerals used for batteries and wind turbines can almost always be recycled—as long as policymakers push for their reuse.

Slack would also welcome some mining. He retired to Maine for its natural splendor, but until recycling can cover society’s needs, critical mineral exploitation needs to happen somewhere. “We cannot have a low carbon future and green tech without mining,” he says. “It’s not an option. It’s a necessity. It’s essential.”

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