A vast blob of hot rock moving slowly beneath the Appalachian Mountains in the northeastern US is now thought to be the result of a divorce between Greenland and Canada some 80 million years ago.
A study by an international team of researchers challenges the existing consensus in both geographical and chronological terms. It was previously thought the breaking up of the North American and African continents was responsible, some 180 million years ago.
To test their assertion, the researchers used a combination of existing data and computer modeling to link the hot blob to a geological formation in the Labrador Sea in the North Atlantic dated to around 85-80 million years ago.
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“This thermal upwelling has long been a puzzling feature of North American geology,” says earth scientist Thomas Gernon, from 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.”
Technically known as the Northern Appalachian Anomaly (NAA), the 350-kilometer- (217-mile-) wide blob of hot rock hasn’t been in any particular hurry to get to its present location, moving at a rate of around 20 kilometers every million years. At that rate, the blob should pass New York in around 10 to 15 million years or so.
However, the research team suggests this anomaly is one of the main reasons the Appalachians are still in place. The heat helps the continental crust remain buoyant, contributing to the mountains being uplifted further over the years.
The new study builds on previous work from some of the same researchers. Known as the ‘mantle wave’ theory, it posits blobs of hot rock rise in a lava-lamp style when continents break apart, triggering a variety of geological phenomena such as volcanic eruptions and formation of mountains.
“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,” says geophysicist Sascha Brune, from the GFZ Helmholtz Centre for Geosciences in Germany.
“The feature we see beneath New England is very likely one of these drips, which originated far from where it now sits.”
Further analysis and tracking of the hot rock will help to confirm its origins. Meanwhile, the same theories and techniques can be used to identify other geological features like this.
In fact, the researchers think they might have already spotted a ‘mirror’ to the NAA, under north-central Greenland and also originating from the Labrador Sea.
“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,” says Derek Keir, a geophysicist from the University of Southampton.
The research has been published in Geology.
