Virginia’s volcanic past: Mole Hill and Trimble Knob
By John Kelly,
Virginia has two extinct volcanoes, Mole Hill and Trimble Knob. What are the odds of us Virginians dying in a pyroclastic flow?
— Jim Ward, Alexandria
Rest assured that Virginia’s two best-known volcanoes — Mole Hill in Rockingham County and Trimble Knob in Highland County — are in no danger of blowing their tops. Elizabeth Johnson, assistant professor in the department of geology and environmental science at James Madison University in Harrisonburg, said their erupting days are long over.
Well, long over in human terms. On the geologic scale, their period of activity — which was roughly a 2-million-year span 47 to 49 million years ago, during the Eocene epoch — was fairly recent.
“These are particularly interesting volcanoes, not because we’ re worried they’re going to erupt tomorrow,” she said. “They are very young geologically. They’re the youngest volcanoes anyone knows about on the East Coast of the United States.”
And what may be most interesting is this: Geologists aren’t sure why they erupted in the first place.
Volcanoes typically form where tectonic plates — those big, sliding pieces of the Earth’s crust — come together and rub against one another. But 50 million years ago, the nearest plate boundary was where it is today: out in the Atlantic Ocean.
“It’s a bit of a mystery why these things would erupt during that time,” Johnson said. “We’re interested in trying to figure out why that happened.”
Scientists may not know the why, but they’re pretty sure about the how: 49 million years ago, molten rock known as magma bubbled below the Shenandoah Valley at a depth of 40 to 65 kilometers, roughly 20 to 30 miles. Conduits in the mantle, the layer under the Earth’s crust, allowed the magma to rise. The magma exploded upwards at the surface through cracks in the crust. Mole Hill and Trimble Knob are actually remnants of the insides of the volcanoes, where the magma cooled. They were once surrounded by softer rock that has eroded substantially in the intervening millennia.
Geologists know they are volcanoes because, while most of the Shenandoah Valley is composed of sedimentary rock, Mole Hill and Trimble Knob are composed of igneous rock, mainly basalt. But as the magma rose, it picked up other rocks along the way, bringing those to the surface. These rocks — called xenoliths (“xeno” for foreign, “liths” for rocks) — can tell scientists what the composition is like miles down.
Not every gentle dome in Virginia is an old volcano. But Johnson said there are plenty more volcanoes in the commonwealth besides Trimble Knob and Mole Hill. She’s counted in excess of 100 on maps, giving her and her students plenty to work with. Every semester they visit different ones, taking rock samples that will be sliced and scrutinized.
Are the volcanoes related in any way to last year’s Virginia earthquake? Hard to say, but probably not. The fault along which that earthquake was centered is southeast of the volcanic outcrops, on the other side of the Blue Ridge. Johnson said there are other geologic processes at work, though further study is required to determine exactly what’s going on down there.
The two volcanoes are on private property. Gerald Knicely’s parents owned a dairy farm on the north side of Mole Hill — “what we always called the front side,” he told Answer Man. His bicycle store in Dayton, Va., is called Mole Hill Cycles. Two years ago, he bought 46 acres of Mole Hill and arranged to have them protected through a conservation easement with the Virginia Outdoors Federation.
“I spent many hours tromping through the woods as a child,” he said. “I wanted to see it stay Mole Hill.”
Answer Man asked Johnson why humans are so fascinated by volcanic activity. One reason, she said, is because of a volcano’s sheer power. “People can’t do anything about it,” she said. “It’s so hot and so uncontrollable. It’s something that’s happening that doesn’t have anything to do with other parts of human life at all, with everyday life like driving to the grocery store.”
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