Roughly 124 miles (200 kilometers) beneath the Appalachian Mountains in New England lies the aptly named Northern Appalachian Anomaly (NAA), a mysterious 218-mile-wide (350-km) region of unusually hot rock.
Researchers have long believed that the NAA resulted from the plate tectonic movement that broke North America off northwest Africa 180 million years ago. In a new study published Tuesday in the journal Geology, however, a team of international researchers argue that the hot, subsurface rocks are related to when North America and Greenland split near the Labrador Sea between 90 and 80 million years ago.
The NAA “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,” Tom Gernon, lead author of the study and an Earth scientist at the University of Southampton, said in a university statement.
To reach this conclusion, the team used advanced computer simulations, seismic tomography data (like an ultrasound, but for Earth’s interior), and tectonic plate reconstructions. According to the study, the NAA may have developed around 1,119 miles (1,800 km) from its current position and, at the rate of around 12 miles (20 km) per million years, slowly moved southwestward to where it sits now.
“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,” Gernon added.
The slow-moving process is the team’s previously proposed “mantle wave” theory, which hypothesizes that hot, dense chunks of material detach from the base of tectonic plates after continents separate, like blobs in a lava lamp.
“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,” Gernon explained. “This would have caused the ancient mountains to be further uplifted over the past few million years.”
As the blobs slowly “drip” from the lithosphere—the layer including Earth’s crust and upper part of the mantle—hotter mantle rocks rise to fill up the space, which creates a thermal anomaly. The same team’s earlier work also revealed that these blobs can move over time.
“The feature we see beneath New England is very likely one of these drips, which originated far from where it now sits,” said Sascha Brune, study co-author and head of the Geodynamic Modelling Section at GFZ, Germany’s national research center for Earth sciences. According to the team, the center of the NAA will likely move under New York in the next 15 million years.
“The idea that rifting of continents can cause drips and cells of circulating hot rock at depth that spread thousands of kilometres inland makes us rethink what we know about the edges of continents both today and in Earth’s deep past,” admitted Derek Keir, another co-author of the study and a tectonics expert at the University of Southampton and the University of Florence.
The researchers argue that their mantle wave theory could explain a similar anomalous hot zone under north-central Greenland—basically a reflection of the NAA on the other side of the Labrador Sea.
The study is ultimately a reminder to never judge a book by its cover—or the Earth by its surface-level tectonic activity.
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