Mazza and her co-authors analyzed rocks from the volcanic swarm dotting Virginia and West Virginia. Two prominent examples include Mole Hill, west of Harrisonburg, Va., and Trimble Knob, in Highland County, Va. The gentle hills and knobs are long extinct. "You probably wouldn't know they were there unless you talked to a local," Mazza said.
Even though the lava chemistry is similar to hotspot volcanoes such as the Azores and Cape Verde Islands, the researchers concluded that a hotspot did not spark the Virginia volcanoes.
Here's why: First, the magma temperature is too low — roughly 2,570 degrees Fahrenheit (1,410 degrees Celsius), rather than the 2,732 F (1,500 C) measured at hotspot volcanoes, Mazza said. Second, the magma source is too shallow, she added. Third, the researchers precisely dated the eruptions to between 47 million and 48 million years ago, at least 10 million years after the hotspot passed through. "That difference is significant enough for us to think this hotspot probably wasn't the case," Mazza said.
Instead, the team proposes that magma reached the surface as pieces of the thick crust beneath Virginia peeled away like sloughing skin — a process called delamination. Afterward, magma seeped through the newly thinned crust, reaching the surface through pre-existing cracks in the overlying rock. In this model, the Virginia lava is the chemical cousin of eastern Atlantic volcanoes, because their sources are both deeply buried leftovers from the breakup of Pangaea, the supercontinent.
"The upwelling is allowing these volcanoes to sample a part of the mantle that is also seen over in the eastern part of the Atlantic," Mazza told Live Science's Our Amazing Planet.
The shedding crust under Virginia could underlie topographic changes in the Appalachians, such as their recent face-lift. The mountains are more rugged than they should be, given their age and the tectonic quiescence of the East Coast.
"I hope this project is a good stepping stone for interpreting what is going on in the crust and the mantle," Mazza said.