There’s a giant blob of incredibly hot rock beneath New Hampshire — and it may be part of the reason the Appalachian Mountains are still standing tall, according to new research. It has, however, been slowly moving and is on course for New York in the next 15 million years.
This hot rock blob, called the Northern Appalachian Anomaly, or NAA, sits about 124 miles (200 kilometers) beneath the mountain range in New England and measures between 217 and 249 miles (350 and 400 kilometers) wide. It is in the asthenosphere, or the semi-molten layer of Earth’s upper mantle, and is considered a thermal anomaly because its temperature is hotter than its surroundings.
Rock formations in this part of the Earth’s interior are unusual, and scientists previously thought it formed when the North American continent broke apart from northwest Africa 180 million years ago.
But new research, published July 29 in the journal Geology, suggests the anomaly is linked to when Greenland and North America separated 80 million years ago.
At a rate of 12.4 miles (20 kilometers) per 1 million years, the thermal anomaly has migrated about 1,118.5 miles (1,800 kilometers) from its point of origin as Earth’s crust ruptured near the Labrador Sea between Canada and Greenland.
The hot rock mass has long been a puzzling feature of North American geology, said lead study author Tom Gernon, professor of Earth science at the University of Southampton in the UK.
“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,” Gernon said in a statement.
Instead, the rock blob could help explain why ancient mountains such as the Appalachians haven’t eroded away as much as expected over time.
“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 said. “This would have caused the ancient mountains to be further uplifted over the past few million years.”
New insights about the blob could help scientists better understand other similar geological abnormalities across the globe — including one beneath north-central Greenland that may be a sibling of the Northern Appalachian Anomaly— as well as the impacts these rare features could have on Earth’s surface.
Waves beneath Earth’s surface
To explain the rock blob’s origin and current position, the scientists used “mantle wave” theory, which they proposed in previous research.
The idea is similar to the process that unfolds inside a lava lamp. After continents rift, or break apart, hot, dense rock detaches from the base of tectonic plates in blobs, which generate waves beneath Earth’s crust.
When continents stretch and split, space opens beneath the breaking point and is rapidly filled with semi-molten asthenosphere, Gernon said. The upwelling material rubs against the newly broken edge of the colder continent, causing the material to cool, grow dense and sink — a process called edge-driven convection. The hotter mantle substance creates a warm region known as a thermal anomaly, said study coauthor Sascha Brune, professor at the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany.
“This sudden movement disturbs the edge of the continent’s root, triggering a chain reaction,” Gernon said. “Much like falling dominoes, blobs of the root begin to drip downward one after another — a process driven by gravity known as Rayleigh-Taylor instability. These ‘drips’ migrate inland over time, away from the rift. We think this same process might explain unusual seismic patterns beneath the Appalachians.”
The convective rock currents continue to flow slowly and ripple over millions of years, leading to rare volcanic eruptions that bring diamonds to Earth’s surface or help uplift mountains, the researchers found.
“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,” study coauthor Dr. Derek Keir, associate professor of Earth science at the University of Southampton, said in a statement.
For its research, the team used seismic waves to image Earth’s interior, as well as geodynamic simulations and tectonic plate reconstructions, to track the Northern Appalachian Anomaly’s origin point.
“If we trace the wave’s path backwards from where it is now,” Gernon said, “it would have originated below the Labrador Sea rift margin at the time when the rift was forming and close to the point of continental breakup.”
Maureen D. Long, the Bruce D. Alexander ’65 Professor and Chair of Yale University’s department of Earth and planetary sciences, and her team have several active research projects studying the North Appalachian Anomaly.
While Long was not involved in this study, her research group is collecting new seismic data from arrays of seismometers in the region to capture more detailed images of the rock blob. The new model shared in the recently published study will help Long and her colleagues think through all the possible ways to interpret the images they capture, she said.
“It’s exciting to see a new and creative model proposed for the origin of the Northern Appalachian Anomaly, which still remains poorly understood despite much study,” Long wrote in an email. “While I don’t think any of our conceptual models for how the NAA might have formed, including this new one, does a perfect job of explaining the full range of observations, it’s great to see some new thinking on this that brings some novel ideas to the table.”
The future of the Appalachian Mountains
Looking forward, the team said its modeling shows that the center of the anomaly will pass beneath New York within the next 15 million years.
“What the anomaly will look like in the future is a really interesting puzzle for geologists to think about,” Long said, “but it’s not going to have any foreseeable impact on human infrastructure or on our daily lives.”
But what does the movement mean for the Appalachian Mountains? The range, formed when the North American Plate collided with other tectonic plates during the Paleozoic Era, between 541 million and 251.9 million years ago, experienced a new growth spurt when the supercontinent Pangaea broke apart around 180 million years ago, Gernon said.
The rock blob may have also contributed to uplifting the mountains during the Cenozoic Era over the last 66 million years, according to the new study.
“It is likely that this anomaly has played some role in shaping the geologic structures that lie above it,” Long said. “For example, several studies have suggested that the lithosphere (the crust and the uppermost mantle, which makes up the tectonic plate) above the NAA is particularly thin, and it’s likely that the anomaly has played a role in thinning the plate above it.”
Once the rock blob moves, the crust beneath the Appalachians would likely settle and stabilize once more, Gernon said.
“In the absence of further tectonic or mantle-driven uplift, erosion would continue to wear down the mountains, gradually lowering their elevation,” Gernon said.
Investigating other blobs
Additionally, the team believes the breakup of Greenland and North America may have created another thermal anomaly that emerged from the opposite side of the Labrador Sea. This second anomaly adds to a flow of heat at the base of a thick continental ice sheet, influencing the movement and melting of the ice, the study authors said.
“Even though the surface shows little sign of ongoing tectonics, deep below, the consequences of ancient rifting are still playing out,” Gernon said in a statement. “The legacy of continental breakup on other parts of the Earth system may well be far more pervasive and long-lived than we previously realised.”
Junlin Hua, a seismologist and professor at the University of Science and Technology of China, said he believes the mechanism in the study to explain the anomalies is novel and could be applied to other regions where rifting occurs. Hua was not involved in the study, but he recently authored research that found that the underside of the North American continent is dripping rock blobs.
“The mechanism presented in this study shows a great potential solution for the puzzle, but more relevant observational and modeling works might be needed to further confirm it, and as said in the paper, multiple mechanisms may play a role together,” Hua said. “In any case, personally, this is a great piece of work that opens a new door to improve our understanding of the region.”
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