Ancient tectonic remnants are driving the slow erosion of North America’s foundation. (D-VISIONS/Shutterstock)
In a nutshell
- Even the oldest, most stable parts of North America’s foundation are slowly eroding. Scientists have discovered that the continent’s deep cratonic root, once thought to be permanent, is actively thinning as pieces drip into the Earth’s mantle.
- A long-dead tectonic plate is still reshaping the continent from below.
The ancient Farallon plate, now deep in the mantle, is driving mantle flows that pull down pieces of North America’s lithosphere, like honey dripping from a spoon. - This discovery rewrites the story of how continents evolve. By capturing craton thinning in real time, the study reveals that continental foundations can be reshaped by forces from deep inside the planet, offering new clues about Earth’s long-term geological recycling.
AUSTIN, Texas — Deep beneath the American heartland, the thick, ancient base that has held up the continent for billions of years is slowly dripping away. This process, normally happening too slowly for humans to notice, has been captured in unprecedented detail by scientists using advanced seismic imaging.
The oldest, most stable parts of continents are called cratons, which are essentially sturdy keels that keep the continents from bobbing around too much. Scientists have long believed these keels are practically permanent features that stay largely unchanged for billions of years.
But a startling new discovery is challenging this view. A team of researchers has found evidence that North America’s keel is actually dripping away into Earth’s deeper layers right now. It’s comparable to discovering that the foundation of a seemingly solid building is slowly melting.
Catching a continent in the act of changing its structure is rare. Usually, geologists can only study these processes after they’ve already happened, arriving at the scene long after the event. Published in Nature Geoscience, the study reveals that beneath the central United States, chunks of the continent’s deep foundation are being pulled downward into Earth’s interior. Using a technique somewhat like an ultrasound for the planet, the scientists created detailed 3D images of what’s happening hundreds of miles below our feet.
The Culprit: An Ancient Sinking Plate
Nature Geoscience, Hua et al.)
The catalyst for this continental dripping appears to be an ancient tectonic plate called the Farallon plate. This massive slab of Earth’s crust began sliding underneath North America more than 100 million years ago. Although this plate is now hundreds of miles deep in Earth’s mantle, it’s still affecting what happens closer to the surface.
As the Farallon plate continues sinking, it creates a downward suction that pulls material from surrounding areas toward it, similar to how a heavy object sinking in water creates currents that pull nearby items along. This suction is strong enough to tug at the bottom of North America’s supposedly stable foundation, causing pieces of it to stretch and eventually drip down into the planet’s deeper layers. The continental root slowly stretches and thins until pieces of it break free and sink deeper into the Earth.
The researchers named their imaging technique SATONA, which stands for Seismic Adjoint Tomography of North America. To create their underground maps, they analyzed how earthquake waves traveled through different materials beneath the continent. Just as X-rays pass differently through bones versus soft tissue, earthquake waves travel at different speeds through various types of rock.
By examining data from 206 earthquakes recorded at over 6,000 monitoring stations across North America, the team created a detailed picture of what’s beneath the surface. Their images clearly show unusual drip-like structures extending from the bottom of North America’s foundation down to depths of about 400-600 kilometers.
Chemical Changes and Volcanic Clues
Nature Geoscience, Hua et al.)
The team’s analysis of these dripping structures found they contain 20-50% more continental material than the surrounding mantle while being only slightly cooler (about 60°C). This composition strongly indicates they’re seeing pieces of the continent’s foundation being transported downward.
The process may have gotten a boost from chemical changes. When the Farallon plate sank to certain depths, it likely released water and carbon dioxide. These compounds rose up and weakened the continental foundation from below, making it easier for pieces to break off.
Historical evidence supports this theory. The team observed that ancient volcanic activity around the edges of the continent’s stable core aligns with the movement of the Farallon plate over time. Before about 100 million years ago, when the plate was further west, volcanic eruptions occurred at the western edge of the continent’s core. After the plate moved eastward about 70 million years ago, similar eruptions appeared near the eastern edge in Kentucky.
Based on their calculations, the researchers estimate the continent’s foundation may have already thinned by about 20-60 kilometers, that’s up to 37 miles of lost foundation. While that might not sound like much compared to the size of the Earth, it represents a significant change to structures previously thought to be incredibly stable.
Rethinking Continental Stability and Earth’s Evolution
Discovering that cratons can be eroded from below upends the conventional wisdom that these structures are virtually unchangeable over billions of years.
“This sort of thing is important if we want to understand how a planet has evolved over a long time,” says study author Thorsten W. Becker from the University of Texas at Austin, in a statement. “It helps us understand how do you make continents, how do you break them, and how do you recycle them [into the Earth.]”
This discovery also helps explain other geological mysteries, such as why the North China Craton, another ancient continental core, lost much of its foundation during the age of dinosaurs. The patterns seen under North America might be happening elsewhere, too, or might have happened to other continents in the past.
For most people, this might seem like a distant concern. After all, the process happens over millions of years, far too slow to notice in a human lifetime. But for scientists, it offers a window into how our planet works and evolves. The seemingly solid ground beneath our feet is actually part of a dynamic, ever-changing system.
What we’re witnessing beneath North America is a rare glimpse into the complex workings of our planet. It shows that even the most stable parts of continents aren’t permanent. This continental evolution, happening right beneath our feet, reminds us that Earth remains an active planet, where nothing, not even the foundations of continents, lasts forever.
Paper Summary
Methodology
The researchers used full-waveform adjoint tomography to create detailed images of Earth’s interior beneath North America. They analyzed data from 206 earthquakes recorded at 6,099 seismic stations, focusing not just on when seismic waves arrived but also on matching their detailed shapes. The team processed their data in two stages: a coarse-grid stage using all earthquakes and a fine-grid stage using 110 higher-quality ones. They employed a mini-batch algorithm to complete 216 iterations using just four graphics processing units, analyzing both body waves and surface waves to independently determine seismic velocities throughout the region.
Results
The study revealed a clear lithosphere-asthenosphere boundary at about 200 kilometers depth beneath the central and eastern United States, marking the bottom of the cratonic root. Below this boundary, the team observed high-velocity anomalies extending down to the mantle transition zone (410-660 kilometers). Analysis showed these anomalies contain 20-50% more cratonic composition than surrounding mantle while being only about 60°C cooler, indicating they are pieces of continental lithosphere being transported downward. Computer models demonstrated that these dripping structures are likely driven by mantle flow induced by the sinking Farallon slab in the lower mantle. Based on their measurements, the researchers estimate North America’s cratonic lithosphere has already thinned by approximately 20-60 kilometers.
Limitations
The resolution of the seismic imaging decreases with depth, particularly for P-wave velocities below the mantle transition zone. Some horizontal anomalies in the model might be artifacts introduced to reconcile inaccuracies in the starting model assumptions. The conversion from seismic velocities to temperature and composition relies on assumptions about mantle and cratonic lithosphere compositions that might not apply uniformly across all regions. The estimation of lithospheric thinning is approximate, based on the assumption that S-wave velocity perturbations directly correspond to the amount of cratonic material present.
Discussion and Takeaways
This study challenges long-held views about the stability of cratons, revealing they can be modified by external mantle processes, particularly deep mantle effects related to subduction. The lower part of a craton appears weaker and more prone to deformation than previously thought, especially when subjected to strong tractions from below. This mechanism helps explain other instances of cratonic modification in Earth’s history, such as the well-documented case of the North China Craton. The research demonstrates that continents are more dynamic than traditionally believed, with even their most stable foundations subject to change over geological time.
Funding and Disclosures
The research was supported by National Science Foundation grants EAR-1902400, EAR-1903108, and EAR-2045292, as well as by the Jackson School of Geosciences at the University of Texas at Austin. Seismic data from Mexican stations were provided by the Servicio Sismológico Nacional (México).
Publication Information
The study, “Seismic full-waveform tomography of active cratonic thinning beneath North America consistent with slab-induced dripping,” was published in Nature Geoscience on February 17, 2025. The research was led by Junlin Hua with co-authors from the University of Science and Technology of China, University of Texas at Austin, University of Hawai’i at Mānoa, University of Nevada, Reno, and Southern University of Science and Technology in China.
