Editors’ Vox is a blog from AGU’s Publications Department.

After 18 years of data collection, quality control, processing, and archiving, the United States Magnetotelluric Array (USMTArray) data set was completed in 2024. A new article in Reviews of Geophysics introduces this unprecedented data set and a new high-resolution model of the Earth’s crust and upper mantle that was made possible because of it. Here, we asked the authors to give an overview of magnetotellurics, how the USMTArray was developed, and future directions for research.

In simple terms for a non-specialist, what is the science of magnetotellurics?

Magnetotellurics (MT) is a passive geophysical technique capable of imaging the subsurface from hundreds of meters to hundreds of kilometers depth using the Sun and global lightning as sources. The science behind MT is largely based on Faraday’s law of induction, where external magnetic field variations induce telluric (from the Latin word ‘tellus’ meaning Earth) currents in the conducting Earth. These magnetic field variations are constantly occurring and happen over a wide range of time scales ranging from milliseconds to hours. And they are tiny – typically on the order of 0.1% of Earth’s magnetic field amplitude and even during intense magnetic storms rarely exceed 1%. 

By measuring these magnetic variations, and the induced electric field variations at Earth’s surface, we can constrain the 3D distribution of conductivity in the Earth. MT is an elegant method – we exploit powerful and distant energy sources which we have no control over and can mathematically remove the stochastic source spectrum to recover reliable estimates of Earth impedance. Impedance can be thought of as the Earth filter – a complex, frequency dependent set of functions that encapsulates all the information about the 3D conductivity structure beneath our feet. Through numerical inversion of impedance data at an array of sites, we build up 3D models of electrical conductivity.

What are some of the applications of the magnetotelluric method?

MT is applied across a broad spectrum of the Earth and space sciences ranging from mineral and geothermal resource investigations, to fundamental geologic and tectonic studies, to imaging the magmatic plumbing systems of active volcanoes, and to hazard mapping centered upon geomagnetically induced currents and the risk they pose to power grids.

Studies using MT are performed on every continent and in all tectonic settings, on land and on the ocean floor, on the Antarctic ice sheet, and even on the Moon. Because of its ability to image the entire lithospheric column, MT studies have made important contributions to our understanding of continental assembly by revealing ancient orogens and rifts. Moreover, MT is uniquely able to constrain the stability of cratonic roots by mapping hydration of the mantle lithosphere. MT studies are key to understanding active tectonic processes, including constraining the water budget in subduction zones, imaging melt zones beneath orogenic plateaus, and mapping the extent of crustal extension – for example beneath the western U.S.

With the rise of computational power and 3D modeling and inversion codes, MT is now routinely used to study complex 3D systems, such as active volcanoes, geothermal systems, and mineral deposits. The sensitivity of MT to minor conductive phases – be it partial melt, clay, or conductive minerals such as graphite and metallic sulfides – make it ideal for studying these types of systems. As a result, MT is commonly employed within the resource sector at both the district and deposit scale. Many of the world’s iconic volcanoes have also been imaged with MT, where they constrain the geometry of crustal melt reservoirs – especially their volume and melt fraction which is in turn linked to the eruptibility of a subsurface magma. These analyses are especially powerful because they are sensitive to a distinct physical parameter – resistivity – of Earth materials. MT therefore provides unique and complementary information about the subsurface across a wide range of scales and is a particularly invaluable tool when other methods yield non-unique interpretations. 

One somewhat unexpected application of MT has been to space weather hazards. It was recognized a little over a decade ago that MT impedances are key to estimating surface electric fields generated during intense geomagnetic storms that can impact electric power grids. Past storms have knocked out power to vast areas and damaged critical infrastructure such as transformers. The importance of MT data to scenario analysis, in which power grid components are ‘stress tested’ against past geomagnetic storms, cannot be overstated. Regional to national-scale geoelectric hazard maps, both in the U.S. and internationally, are also informed by MT data, as are real-time geoelectric hazard estimates.

What is the United States Magnetotelluric Array (USMTArray)?

The USMTArray was an ambitious program begun in 2006 under the NSF-funded EarthScope program and completed in June of 2024 under USGS funding. The USMTArray collected long-period MT soundings on a 70-km grid across the contiguous U.S. – totaling more than 1,800 stations – each collected with uniform instrumentation, acquisition parameters, data processing, archiving, and metadata. Funded throughout its 18-year lifetime by three different federal agencies (the NSF, NASA, and USGS working closely with the Incorporated Research Institutions for Seismology and Oregon State University), the data – time series, response functions and metadata – were released incrementally to the public without data embargo or usage restriction.

In broad terms, how was the USMTArray developed?

The USMTArray had humble beginnings – being mentioned in early planning documents as having value in understanding subduction zones and characterizing volcanic systems. Funded by NSF in 2003, the MT component of EarthScope was modeled after the much larger seismic component, with a transportable array of instruments to march across the U.S. on a 70-km spaced grid and a backbone array of seven instruments to study deep mantle structure. The USMTArray started off small and before dedicated instruments were even available. In 2006, a pilot study collected the first 30 stations in eastern Oregon using borrowed instruments, while subsequent years expanded what became known as the ‘northwest’ footprint, a 331-sites array completed in 2011 encompassing the Yellowstone-Snake River Plain, the Northern Rocky Mountains, the Cascades magmatic arc, and the northern Basin and Range province. Subsequent footprints in the midcontinent and the eastern U.S. continued to expand coverage.

What were some of the challenges in developing the USMTArray?

The biggest challenge by far was money. Within the EarthScope program, the USMTArray was never funded at the level needed to cover the contiguous U.S. The MT component was instead carried out as a series of footprints in areas deemed most scientifically advantageous. This limitation, however, led to one of the big successes of the USMTArray – active community engagement. Siting workshops held in 2008 and 2013 brought together participants from academia, government, and industry to discuss and prioritize where the array would go next, while a community working group provided scientific and operational guidance throughout the life of the array. The success of the USMTArray was recognized early on by the community governance of the EarthScope facility activities, with the ‘full-48’ concept endorsed in 2009, leading to modest increases in funding and an acceleration of station completions. In 2018, by the end of NSF-sponsored activities, roughly 2/3 of the contiguous U.S. had been covered. Seeing the array to completion, however, required additional funding, a challenge met by NASA (2019-2020) and the USGS (2020-2024), in large part due to recognition of the importance of USMTArray data to space-weather hazards and supported through executive orders in 2016 and 2019.

Another notable challenge that we faced while developing the USMTArray operations was the absence of established data sharing practices within the magnetotelluric community. Indeed, the concept of FAIR data was only introduced in 2016. Back in 2006 when this program commenced, the concepts of open data and systematic data sharing were largely unfamiliar, and no widely adopted, sustainable data formats existed. Available data formats were lacking in flexibility, consistency, and self-descriptive metadata. As the project progressed, our team developed such formats and accompanying databases, which have now reached maturity and are helping to drive more sustainable MT data‑sharing practices internationally.

How has the development of the USMTArray advanced the scientific field?

The USMTArray, along with parallel advances in modeling capabilities and increased computational power, ushered in a jump to 3D MT and to interrogating the Earth at regional to national scales. National-scale conductivity models, such as those developed from the USMTArray, now join the ranks of other data sets like magnetic, gravity, and seismic, and are a new lens with which to view the architecture of the North American continent. Numerous contributions to continental architecture and assembly and to understanding active tectonic processes have come from the USMTArray.

The USMTArray also serves as a framework for more detailed studies, allowing Principal Investigators (PIs) to derisk future surveys and industry to investigate anomalous or unexpected structure. Studies of the Cascadia subduction zone and the adjacent magmatic arc and geothermal energy prospectivity studies in the Oregon Cascades and Great Basin have been built upon the USMTArray while new MT surveys along the eastern seaboard are collecting high-resolution MT data to improve space-weather hazard maps over areas identified as particularly at risk from the analysis of USMTArray data.

Beyond the data and models derived from them, the USMTArray has motivated methodological advances, led to an investment in MT instrumentation and open-source software for researchers within the NSF-supported National Geophysical Facility, and served as a model for other regional and continental scale MT experiments.

What are some of the future directions for research in continental scale magnetotellurics?

With completion of the USMTArray, and the 3D conductivity models derived from it, there are numerous avenues for future research. Most models of continental evolution, for example, were developed prior to the advent of this rich data set. Critically evaluating such models in light of this new data set is paramount, and initial studies are already forcing a reexamination of certain paradigms.  

Multi-disciplinary studies incorporating geochronology, geochemistry, and rapidly evolving seismic models is another promising area as is the coupling of geophysical models to geodynamic models to examine the evolution of newly imaged model structure. Similarly, advancements in integrated and joint inversion are promising directions to leverage the wealth of public data sets available at regional to continental scales.  

Geology doesn’t stop at national borders or the land-sea interface – additional opportunities exist for cross-border arrays and onshore/offshore MT studies. Investigation of subduction zone processes and rifted continental margins by their very nature demand an amphibious approach.

On the applied front, resource assessments increasingly are applied at national and even global scales and demand data support at these same scales. Mineral resource assessments, for example, in the U.S., Canada, and Australia are exploring machine learning approaches to map prospectivity for various deposit types and incorporate a range of geophysical data layers to do so. Similarly, geothermal assessments can benefit from the consistent and synoptic data coverage offered by USMTArray data and models.

Finally, on the space-weather hazards front, partnering with power-system engineers to investigate data scale and uncertainty shows promise in generating accurate hazard maps and in improving upon operational, near real-time geoelectric field models. For all these future research directions the USMTArray remains both a framework and a benchmark upon which to build.

—Paul A. Bedrosian (pbedrosian@usgs.gov; 0000-0002-6786-1038), U.S. Geological Survey, United States; Anna Kelbert (anna.kelbert@cfa.harvard.edu; 0000-0003-4395-398X), Center for Astrophysics | Harvard & Smithsonian, United States; Adam Schultz (adam.schultz@oregonstate.edu; 0000-0003-1663-1547), Oregon State University, United States; and Gary D. Egbert (gary.egbert@oregonstate.edu; 0000-0003-1276-8538), Oregon State University, United States

Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.

Citation: Bedrosian, P. A., A. Kelbert, A. Schultz, and G. D. Egbert (2026), Mapping the hidden electrical anatomy of a continent, Eos, 107, https://doi.org/10.1029/2026EO265021. Published on 26 May 2026.

This article does not represent the opinion of AGU, Eos, or any of its affiliates. It is solely the opinion of the author(s).

Text © 2026. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

Editors’ Vox is a blog from AGU’s Publications Department.

The supply of magma from the Earth’s mantle is a primary source of heat to volcanic arc crust, where the heat is then dissipated through various processes. Much of this magmatic heat is dissipated as heated water, or aqueous fluid.

A new article in Reviews of Geophysics compares 11 different volcanic-arc segments where heat discharge via aqueous fluid has been well-inventoried to better understand the factors that influence this process. Here, we asked the authors to give an overview of heat discharge from volcanic arcs, how scientists measure it, and what questions remain.

Why is it important to study how heat is dissipated from volcanic arcs?

The heat from these magmas matters for geothermal energy, patterns of groundwater flow, and the patterns of volcanic activity at the surface.

Volcanic arcs are the chains of volcanoes on top of subduction zones. They can produce some of Earth’s most explosive and hazardous eruptions. But much of the magma beneath the surface never erupts. Nevertheless, the heat from these magmas—and the simple fact of their existence and abundance—still matters for geothermal energy, patterns of groundwater flow, and the patterns of volcanic activity at the surface.

What are the main modes in which heat is discharged from volcanic arcs?

Heat at volcanic arcs can be carried by magmas, transmitted through the crust conductively, and carried by water seeping slowly through the crust. At the base of the crust, magmas are probably most important, with conduction coming in second. But as magmas move upwards through the crust, some of them solidify and impart their heat to their surroundings where it is transferred by conduction. Within a few kilometers of the surface, fluids seeping through the crust begin to take up all that heat, and so if we can quantify the heat carried by those fluids, we can retrace it to its origins in magmas.

How do scientists measure these different forms of heat loss?

Scientists measure the heat carried by erupting magmas using satellites, or by adding up the erupted masses and making an estimate of how much energy was released by cooling from eruption temperatures to solid igneous rocks at Earth’s surface. Conductive heat flow is measured by drilling holes in the Earth’s crust to see how quickly it gets hotter with depth.

Measuring the heat carried by aqueous fluids in the crust is in some ways the trickiest. One approach is to find all the springs where hot or even slightly warm water is trickling out and measure the temperature and discharge to estimate how much extra heat that water is carrying.

What are the challenges and uncertainties in measuring hydrothermal heat discharge?

One challenge is that many springs are only slightly warmer than you’d expect. There is good data for many hot springs, but there are data tracking these ‘slightly warm’ springs for only a subset of arcs. Another challenge is that warm underground fluids can flow laterally, so you have to try to account for that. This is not an uncertainty in hydrothermal discharge, but one additional big uncertainty for our study, where we were trying to quantify the proportion of magmas that freeze underground versus erupting, is in the estimates of how much magma has actually erupted through time.

What are some of the factors that influence hydrothermal heat loss?

A major goal of our paper is to try to quantify these hidden magmas.

A main factor that influences hydrothermal heat loss is the magmas that solidify underground. This link is the key motivation for our study. A major goal of our paper is to try to quantify these hidden magmas—how much magma needs to intrude the crust beneath the surface to supply the hydrothermal heat fluxes that we observe? The composition of magmas influences how much heat they can release. The depth at which magmas are emplaced also matters, because magmas that intrude the shallow crust eventually cool to lower temperatures than magmas emplaced in the lower crust and therefore release more heat.

What are the remaining questions or knowledge gaps where additional research efforts are needed?

A big outstanding challenge is combining estimates from hydrothermal data of how much magma is coming into the crust – like ours – with other approaches to estimating the same thing. The magmatic systems beneath volcanoes span the crust. At the base of the crust, you have magma supply, sort of like the water main feeding your plumbing system. Despite how fundamental magma supply is, we know remarkably little about it. It’s exciting to think about how the rates of magma supply could vary through time and space and why. Applying a range of techniques—including geophysical imaging, hydrothermal budgets, gas and igneous geochemistry, and petrology—to understand these questions could really be a game changer.

—Benjamin A. Black (bblack@eps.rutgers.edu; 0000-0003-4585-6438), Rutgers University, United States; S. E. Ingebritsen (steve.ingebritsen@gmail.com; 0000-0001-6917-9369), Kyoto University, Japan; and Kazuki Sawayama (sawayama@bep.vgs.kyoto-u.ac.jp; 0000-0001-7988-3739), Kyoto University, Japan

Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.

Citation: Black, B. A., S. E. Ingebritsen, and K. Sawayama (2026), Hydrothermal heat flow as a window into subsurface arc magmas, Eos, 107, https://doi.org/10.1029/2026EO265017. Published on 28 April 2026.

This article does not represent the opinion of AGU, Eos, or any of its affiliates. It is solely the opinion of the author(s).

Text © 2026. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

The weirdest volcano in the world may be Tanzania’s towering Ol Doinyo Lengai, an active peak that squeezes out a strange, low-temperature lava called carbonatite. Carbonatites are composed of more than 50% carbonate minerals, the same substances that form the ocean’s reefs. At Ol Doinyo Lengai, they are key components of the coldest lava on the planet.

Carbonatites are found on every continent and range in age from today-ish years old (in Tanzania) to about 3 billion years old (in Greenland). What’s more, they’re a major source of critical minerals.

In a new study published in Science Advances, a team of scientists led by Carl Spandler from Adelaide University in Australia identified a compelling correlation between carbonatites and specific sections of Earth’s continents—those proximal to past subduction zones.

Carbonatites and Critical Minerals

In the United States, the federal government defines critical minerals as those essential to the nation’s economic or national security. These minerals must also have supply chains that are vulnerable to distortions such as demand surges and foreign conflict. For example, most of the world’s terbium, used for everything from naval sonar systems to indoor lighting, comes from China. The United States considers terbium a critical mineral because the possibility of political or economic conflict within China or between China and another polity could directly or indirectly threaten the world’s supply of the element.

If you wanted to identify a rock that likely hosts rare earth elements, “carbonatite would be a good place to start.”

Critical minerals are either chemical elements (like terbium) or minerals. Important elements range from the familiar, like the lithium we need for batteries, to the sesquipedalian, like praseodymium, used for high-strength magnets. (Sesquipedalian means “having to do with a very long word.”)

Praseodymium is one of the 17 rare earth elements (terbium is another), all of which are considered critical minerals. Rare earth elements are not actually rare and are often (but not always) found in carbonatites. If you wanted to identify a rock that likely hosts rare earth elements, “carbonatite would be a good place to start,” said Kathryn Goodenough of the British Geological Survey, who was not involved in this study.

Fertilizing the Mantle

Much of Earth’s mantle is rock that remains after magma has been extracted—this mantle has been depleted. But carbonatites must come from mantle that’s quite the opposite—from parts that had to have been fertilized with volatiles containing trace metals, often critical minerals of interest. The question of how the mantle source for carbonatites came to be fertilized has no definitive answer.

Just as a garden can be fertilized in many ways ranging from synthetic sprays to coplanted cover crops, Earth’s mantle can be fertilized via myriad methods. “You must have volatiles or melts rising up from deeper in the mantle that are carrying metals with them,” Goodenough said.

For example, as a slab subducts beneath another tectonic plate, a volcanic arc typically arises above the zone at which the subducting slab reaches about 100 kilometers below Earth’s surface. This is the approximate depth at which the slab releases water, triggering melting in the overlying plate.

But fluids and melts can continue to exit the subducting slab far beyond the trace of the volcanic arc. That far out, the overriding plate almost always comprises a complete section of lithosphere—crustal lithosphere on top and mantle lithosphere on the bottom. The fluids and melts from the underlying slab, rich in halogens, carbon dioxide, phosphorus, and the like, rise into the overriding plate’s mantle lithosphere, changing the rocks via a process called metasomatism, Goodenough explained.

On the other hand, mantle plumes ascending from the core-mantle boundary are thought to be fertilized from a graveyard of subducted slabs that pond in the very deepest part of the mantle.

Spandler and his colleagues focused on testing whether that first method of fertilization, subduction-driven metasomatism, spatially correlates with carbonatites and rare earth element deposits. TL;DR—it does.

Fertilized Mantle Lithosphere

GPlates is a piece of software that allows users to rewind the movements of tectonic plates, exploring how continents have shifted their locations over the past 2 billion years. Using GPlates, Spandler’s coauthors Andrew Merdith and Amber Griffin, also of Adelaide University, mapped 43 polygons that denote regions of subduction lasting 100 million years or longer. These polygons, the authors infer, mark the locations of fertilized mantle lithosphere, which they call FML. These zones are thought to contain the good stuff—the critical minerals of interest.

“If [the correlation were] 100%, I wouldn’t believe it myself.”

Spandler and his colleagues compared the locations of carbonatites and rare earth elements with the polygons. They found that 67% of carbonatites and 72% of rare earth element ore deposits lie within these polygons. This correlation, though not perfect, suggests that mantle lithosphere fertilized by subduction could provide the source for many of these curious and critical deposits.

“If [the correlation were] 100%, I wouldn’t believe it myself because geology doesn’t work that way,” Spandler said.

Two Stepping

Spandler and his colleagues argue that carbonatites form in a two-step process. He emphasized that the new paper focuses on the first step—the process that led to fertilization of the eventual sources for carbonatites and rare earth element deposits.

The second step—the trigger—generates the carbonate-rich magma itself. It’s this event that provides the heat that causes melting of the mantle, said Richard Ernst, a scientist in residence at Carleton University in Canada who was not involved in this study.

“The trigger can be almost anything,” said Spandler, because the lithosphere needs only a nudge to melt. A plume can disrupt the structure of the lithosphere, triggering carbonatite magmatism, but so can continental rifting, he said. Indeed, Ol Doinyo is one of the mountains presiding over the East African Rift (which some scientists think also sits atop a plume).

Previous work by Ernst considered whether plumes could provide at least part of the source for some carbonatites by looking at the age of the deposits and those of nearby large igneous provinces—dramatic, long-lived outpourings of hot basalt thought to result from mantle plumes. In that work, Ernst and his colleague, the late Keith Bell, found the ages of large igneous provinces correlate with the ages of nearby carbonatite deposits; in short, the examples in that paper are potentially linked in both space and time.

Where carbonatite ages match those of nearby flood basalts from large igneous provinces, Spandler said, “I suspect that may just be the trigger mechanism.”

Plume Problems

For some carbonatites, there’s a time difference between when the mantle was fertilized and when the magmas were emplaced, explained Goodenough. “We can track that in several different localities,” she said. This observation would support something like the two-step process outlined above, as opposed to plumes driving the entire sequence.

Another problem with associating carbonatite formation exclusively with plumes, Goodenough said, is that carbonatites require cool conditions that result in relatively minor mantle melting. Plumes, and the large igneous provinces they appear to produce, are hot, and a lot. Plume proponents counter this critique by arguing that carbonatites are often found near the edges of large igneous provinces, away from the hottest zones.

Ernst noted, however, that though Spandler and his colleagues have made the spatial argument for subduction, “they haven’t made the isotopic argument that requires a subduction zone mechanism [for the source].” That sets up a testable hypothesis for future studies that could make use of existing data-rich geochemical studies of deposits within FMLs.

Moreover, even newer research may link the two camps, at least in some cases, with geochemical indicators pointing to both mantle plumes and mantle lithosphere being involved in forming some carbonatites. The latter component, said Ernst, may result from subduction-based fertilization as proposed by Spandler and his colleagues.

The Future of FMLs

“This is just an example of what we could do [with GPlates],” said Spandler. “In the next decade, we’ll see these models getting much more sophisticated and applied to all sorts of things.”

Computing power has improved to allow these models to run in a reasonable time frame. Plus, there’s lots of data. “We have a much better understanding about the history of each little bit of the continental crust around the planet,” he said.

And although people rightly point out that details become fuzzy in plate models that reach into the Proterozoic and beyond, “you’ve just got to pick one model and use it,” said Goodenough. “They’ve…taken the most widely available, repeatable model out there and used that.”

And on the basis of that model, Spandler and colleagues have shown a correlation between subduction—via FMLs—and carbonatites and rare earth element deposits. If someone comes up with another explanation, Spandler said, “that’s fine as well.”

—Alka Tripathy-Lang (@dralkatrip.bsky.social ), Science Writer

Citation: Tripathy-Lang, A. (2026), Ancient subduction may have seeded today’s critical mineral deposits, Eos, 107, https://doi.org/10.1029/2026EO260173. Published on 29 May 2026.

Text © 2026. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.