A magnetotelluric array deployed for nearly two decades has revealed an exceptionally resistant rock body, known as the Piedmont Resistor, which may be a remnant of the breakup of Pangaea.
A new 3D map of the electrical structure beneath the United States reveals not only how electric currents flow deep underground, but also ancient episodes in the continent’s history. According to a report in Science, one of the most striking findings is an ancient continental rift, buried beneath the eastern front of the Appalachian Mountains, and dubbed by researchers the Piedmont Resistor. It is a body of rock that is highly resistant to electrical conduction, and appears to be a remnant of processes that occurred when the supercontinent Pangaea broke up about 200 million years ago. (science.org)
The discovery was made by the United States Magnetotelluric Array, a large-scale measurement array designed to map the electrical properties of the crust and upper mantle beneath North America. According to the U.S. Geological Survey, the dataset was collected from 2006 to 2024 and includes more than 1,700 long-cycle measurement stations spread across the United States at intervals of about 70 kilometers. The data was used to build the National Impedance Map and a 3D conductivity model of the United States. (USGS)
The technique the researchers used is called magnetotellurics. It measures natural variations in the Earth’s electric and magnetic fields at the surface. Because different rock types, fluids, minerals and temperature affect the way electricity travels underground, the measurements can identify deep geological structures that are not necessarily exposed by standard seismic methods. According to the Center for Astrophysics | Harvard & Smithsonian, the array provides a three-dimensional picture of electrical resistivity from the surface to the mantle, so it provides data that is different from that provided by earthquake waves.cfa.harvard.edu)
A remnant of Pangaea beneath the eastern United States
The body, known as the Piedmont Resistor, stands out because it conducts electricity very poorly compared to its surroundings. It reportedly extends along the eastern Appalachian Mountains and is considered one of the most striking discoveries of the survey. The proposed explanation is that it is an ancient lithospheric relic, created or reshaped during extensive volcanic activity associated with the breakup of Pangaea and the opening of the Atlantic Ocean. Previous studies have described the Piedmont Resistor as a particularly unusual body, with a lithosphere that is more resistant and deeper than would be expected in an area that has undergone tectonic extension and opening (EGL).
The term “lost continent” does not mean that a continent like Atlantis has been found, but rather a fragment of ancient continental material, which is absorbed into the geological structure of North America and is not visible on the surface. Its scientific importance is that it provides evidence of how supercontinents break up, how continental margins are formed, and how deep structures can be preserved for hundreds of millions of years below the surface.
The new map isn't just about the distant past. According to the U.S. Geological Survey, the data allows researchers to identify boundaries between rock types, fluids at depth, areas where geothermal resources and vital minerals may be found, and even ancient structures that connected together over billions of years to create North America as it is today.
Not just geology: also a risk to the electricity grid
The new conductivity map also has very practical implications. During geomagnetic storms, energy from the sun can cause electrical currents in the ground, which can penetrate power lines and transformers. According to Harvard & Smithsonian . A storm like the one that caused the major power outage in Quebec in 1989 can have different effects in different areas, depending on the electrical structure of the ground. In Maine, for example, researchers calculated that during that storm, geoelectric fields reached a strength of 22.79 volts per kilometer, a level that could endanger electrical infrastructure.
This means that deep geology is not just an academic matter. Areas where the ground conducts or resists current in a certain way can increase or decrease the risk to power grids during solar storms. According to the USGS, combining USMTArray data with NOAA and USGS' real-time geoelectric field map allows for a more accurate assessment of the local risk to infrastructure during geomagnetic storms.
One map, many uses
The researchers emphasize that the array of measurements will be useful for many years to come. In addition to understanding the history of the Appalachians and North America, the data may help locate critical mineral resources, assess geothermal potential, understand volcanic systems like Yellowstone, and study ancient fault zones beneath the Great Plains of the United States. According to the USGS, the array has already contributed to studies on the structure of the Appalachians, fluid circulation beneath the western United States, ancient fault zones, critical mineral potential, and deep roots of volcanoes.
The discovery of the “lost continent” beneath the Appalachians illustrates how new measurement technologies are changing our understanding of the Earth. What appears on the surface as an ancient, eroded mountain range is actually just the top layer of a much deeper system. Beneath it lie the remains of supercontinents, collision zones, ancient volcanism, and electrical features that still affect modern infrastructure.
Suggested image caption:
The United States Magnetotelluric Array measurement map shows measurement stations spread across the United States. The measurements revealed the electrical structure of the crust and upper mantle, including the Piedmont Resistor, a particularly resistant body of rock beneath the Appalachian Mountains. Credit: USGS / Public Domain
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