Volcano in the Valley: Mole Hill's active history

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ROCKINGHAM COUNTY, Va. (WHSV) — Nestled in the rolling landscape of the central Shenandoah Valley, Mole Hill is a familiar site for people in Rockingham County.

But the privately-owned landmark, situated about two-and-a-half miles west of Harrisonburg, has a unique distinction. The hill used to be an active volcano.

A sharp incline along the surface indicates where the limestone rock commonly found in the Valley ends and volcanic rock, known as basalt, begins.

"These rocks here, are so much younger, have a completely different history," said Dr. Eric Pyle, a Geoscience Education Professor at James Madison University. "It was molten material that basically found a pathway up to the surface, created a volcano, and has since eroded away as the Valley floor has eroded away."

While the surrounding rock dates back 450 million years, the rock found on Mole Hill is just 48 million years old.

Nearly 50 million years may seem long, but it's a relatively recent period in geological history.

Mole Hill formed from magma pushing up to the surface through cracks in the lithosphere. The landmark we see today is made of remnants of the cooled magma column, eroding at a slower place than the sedimentary rock around it.

"Anything that is made of tougher rock than the sedimentary rock in the Valley is not going to weather as fast and so it stands proud," said Pyle.

The rocks have a hexagonal shape, which is indicative of the magma cooling from the outside in.

But Mole Hill is not the only place in our area with these types of formations. Igneous rocks are dotted in various locations across Rockingham, Highland, Pendleton and Augusta counties — the only region on the East Coast where these types of formations are found.

"There are these little anomalies that pop up. Things that don't quite fit the rule," said Pyle.

About a decade ago, the site caught the attention of Dr. Liz Johnson, an Associate Professor of Geochemistry. Since then, she and student researchers have climbed Mole Hill's rocky surface to unearth the mystery buried beneath the surface, thanks to a grant from the National Science Foundation's GEOPRISMS program.

Before Johnson arrived, the last time a research study was conducted at Mole Hill was 1993 — leaving a lot of questions unanswered.

"Not just what is it, — is it really volcanic — but why is it here? And how did it get here," said Pyle.

Elizabeth McTaggart, a rising senior at JMU, is among the undergraduates researching Mole Hill. She's studying crystals found in the rocks, pushed up to the surface from the mantle of the volcano.

"We can't really dig that deep. The mantle is really deep inside the earth," said McTaggart. "So we can't just go and dig a hole and find out what the mantle is made of."

But are we at risk for an eruption today?

The answer is — no, because the volcano is now extinct.

But the site will still keep geologists coming back for years and years to come.

"It is cutting edge. It's thing that we don't know the answers to. It's not like a high school chemistry lab where the instructor knows what the answer is supposed to be," said Pyle.


Example of basalt found on Mole Hill
Igneous rock formations in Rockingham, Augusta, Highland and Pendleton counties
Circa ~48 million years ago. Mole Hill formed from magma pushing up to the surface through cracks in the lithosphere.
Circa Today. Mole Hill formed from magma pushing up to the surface through cracks in the lithosphere.