A hot rock blob under New Hampshire, causing the Appalachian Mountains to stand tall.

A ‘giant blob’ of incredibly hot rock beneath New Hampshire, is part of the reason why 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.

The scientists have suggested 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.

Tom Gernon, professor of Earth science at the University of Southampton in the UK, has reiterated that, “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. 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. This would have caused the ancient mountains to be further uplifted over the past few million years”.

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.

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.

Team Maverick