Giant hidden heat blob slowly travels beneath the U. S.

The team argues that this deep heat source is not a leftover feature from when North America separated from Northwest Africa 180 million years ago, which had long been the prevailing view.

This region of hot material, known as the Northern Appalachian Anomaly (NAA), spans roughly 350 kilometers and sits about 200 km beneath New England.

A Deep Origin Far From Today's Location

The study, published in the journal Geology, suggests that the NAA originally formed about 1,800 km away, near the Labrador Sea where the crust began to split between Canada and Greenland. Over tens of millions of years, this pocket of warm, unstable rock slowly migrated to its current position at a pace of about 20 km per million years.

Researchers from the University of Southampton, the Helmholtz Centre for Geosciences in Potsdam (GFZ), and the University of Florence contributed to the project.

Tom Gernon, lead author and Professor of Earth Science at the University of Southampton, said: "This thermal upwelling has long been a puzzling feature of North American geology. It lies beneath part of the continent that's been tectonically quiet for 180 million years, so the idea it was just a leftover from when the landmass broke apart never quite stacked up.

"Our research suggests it's part of a much larger, slow-moving process deep underground that could potentially help explain why mountain ranges like the Appalachians are still standing. Heat at the base of a continent can weaken and remove part of its dense root, making the continent lighter and more buoyant, like a hot air balloon rising after dropping its ballast. This would have caused the ancient mountains to be further uplifted over the past few million years."

Introducing the 'Mantle Wave' Concept

The scientists based their analysis on a theoretical framework they recently proposed called 'mantle wave' theory, which was named a finalist for Science magazine's 2024 Breakthrough of the Year.

This idea describes how hot, dense rock gradually detaches from the base of tectonic plates after continents split, behaving somewhat like blobs rising and falling in a lava lamp. These slow-moving waves can travel along the underside of continents for tens of millions of years and help account for uncommon volcanic eruptions that bring diamonds to the surface, as well as elevated terrain far from plate boundaries.

By combining geodynamic computer models, seismic tomography (similar to a medical ultrasound but using seismic waves to view Earth's interior), and reconstructions of past plate positions, the researchers traced the NAA back to the period when the Labrador Sea opened and Greenland pulled away from Canada 90 to 80 million years ago.

Rock 'Drips' Slowly Moving Beneath the Continent

Professor Sascha Brune, co-author of the study and head of the Geodynamic Modelling Section at GFZ, explained: "These convective instabilities cause chunks of rock, several tens of kilometers thick, to slowly sink from the base of the Earth's outer layer known as the lithosphere. As the lithosphere thins, hotter mantle material rises to take its place, creating a warm region known as a thermal anomaly.

"Our earlier research shows that these 'drips' of rock can form in series, like domino stones when they fall one after the other, and sequentially migrate over time. The feature we see beneath New England is very likely one of these drips, which originated far from where it now sits."

Based on the team's calculations, the NAA appears to be moving southwest across the North American lithosphere at about 20 kilometers per million years. Its present size and depth, approximately 350 km wide, line up well with predictions for these slow-moving mantle instabilities. The researchers estimate that the center of the anomaly could pass beneath the New York region within about 15 million years.

A Greenland Counterpart

The study also proposes that a similar warm anomaly exists beneath north-central Greenland. This feature may share the same origin as the NAA and could be its geological counterpart, having formed on the opposite side of the Labrador Sea during the breakup.

Beneath Greenland, this deep heat source increases the temperature at the bottom of the thick ice sheet, influencing how the ice flows and melts today. As Professor Gernon noted, "ancient heat anomalies continue to play a key role in shaping the dynamics of continental ice sheets from below."

Long-Lived Processes That Shape Continents

Dr. Derek Keir, study co-author and tectonics specialist at the University of Southampton and the University of Florence, said: "The idea that rifting of continents can cause drips and cells of circulating hot rock at depth that spread thousands of kilometers inland makes us rethink what we know about the edges of continents both today and in Earth's deep past."

The results support earlier work showing that deep Earth processes can continue long after activity at the surface has quieted. These persistent instabilities can affect everything from uplift and erosion to inland volcanic patterns, even in regions considered geologically stable.

Professor Gernon added: "Even though the surface shows little sign of ongoing tectonics, deep below, the consequences of ancient rifting are still playing out. The legacy of continental breakup on other parts of the Earth system may well be far more pervasive and long-lived than we previously realized."