What Lies Beneath the Valley's Crust? Local Landmark May Have Answers

Dr. Elizabeth Johnson didn't know what she was going to do with herself when she moved to Harrisonburg two years ago to teach geology at JMU. She had just spent years studying volcanic activity in the Northwest, home to volcanoes like Mount Hood and Mount St. Helens. But there were no volcanoes in the Shenandoah Valley—or so she thought.

To her surprise, Johnson, an assistant professor of geology and environmental science, discovered that Mole Hill, a local landmark just a few miles from campus, was volcanic. That led her to seek permission from a landowner to study Mole Hill with students in her volcanoes class. 

"I just went up there one day and met the guy who owned the land, just kind of knocked on his door and got permission to take a class out there," she said. 

The students she assigned to make a presentation on Mole Hill quickly realized there was very little published research material to reference. 

"One of the only things that's been published on Mole Hill is an age date," said Johnson, who received a $20,000 grant from the Jeffress Memorial Trust to research the Earth's crust beneath the Shenandoah Valley. "If you do all the corrections, it's about 48 million years old. So it's much, much younger than any of the rocks around here." The granite formations at the top of the Blue Ridge Mountains are around 1 billion years old, she said. 

Johnson and one of her students, senior geology major Adam Wenger, began collecting samples from Mole Hill on December 19. The researchers collected chunks of rock covered with permafrost, reminding Johnson of her work on frigid Mount St. Helens in Washington. 

"We went out there and started looking around and found out that it really is neat, and that no one really knows anything that's going on," said Wenger. "And so it was a great opportunity to learn about something that hasn't already been done." 

Mole Hill 

Mole Hill is likely the weathered remains of a violent volcanic eruption that occurred nearly 50 million years ago during the Eocene time period, Johnson said. The cone, comprised almost entirely of basalt, a blackish igneous rock made from cooling magma, has undergone millions of years of erosion, leaving only a "mole hill" covered with thick, forested vegetation. 

"It started as a field, basically nothing much going on, and then formed in one drastic episode," Wenger said, pointing to a three-dimensional topographical map he created as part of the preliminary research. Mole Hill towers over the surrounding landscape. 

As magma raced toward the Earth's surface, it dislodged certain minerals like spinel, pyroxene, and "beer-bottle green" olivine from the mantle. The presence of the minerals in the basalt give clues to how far down the magma came from. For example, the mineral spinel is only stable at depths between 18 and 45 miles, Johnson said. 

Brittany Sacco, a sophomore in Johnson's class, will do a chemical analysis on the minerals found in the basalt samples. To do so, the research team sent rectangular pieces of basalt—first cut using high-speed and low-speed diamond saws at JMU—to Spectrum Petrographics in Vancouver, Wash., where they were sliced into 30-micrometer-thick rock sheets with a polished carbonate surface, called thin-sections. 

Sacco will analyze the thin-sections using a research-grade polarizing microscope with a fixed camera to record the presence of different rocks and minerals forced to the surface by ascending magma. 

Using an electron microprobe at the Smithsonian Institution's National Museum of Natural History, where Johnson is a research associate, the chemical composition of each thin-section can be measured, indicating how much silicon, calcium, sodium and other elements can be found in each mineral. 

"If you know the chemistry of the mineral, you can calculate its pressure and temperature, which will tell you how deep it comes from," Sacco said. 

Though limestone and dolostone are the most prevalent surface rocks in the area, neither are found on Mole Hill. Instead, only small amounts of sandstone, a rock foreign to the immediate region, can be found within hunks of basalt. Currently, there is no explanation for this phenomenon. But Johnson, said it could indicate there is a layer of sandstone somewhere in the valley's crust, which temporarily halted the flow of magma to the surface. 

"We're trying to create a profile of what's underneath the Shenandoah Valley by looking at the stuff that's come up," Johnson said. "This is information that people don't have right now, not yet at least." 

Why Here? 

Mole Hill is not the only geological structure in the Shenandoah Valley region created by magma breaching the Earth's surface. Similar, smaller bodies of igneous rocks are scattered throughout Rockingham County, Highland County and Pendleton County, W.Va. The volcanic intrusions closely correspond to the North Mountain fault, which is a significant fracture system created during the formation of the Allegheny Mountains between 320 and 250 million years ago. 

"The whole area got rifted when the Atlantic Ocean formed," Johnson said, describing the pulling-apart of Pangaea, or super continent, around that same time period. That event thinned the crust of the eastern coast. Since there were still about 200 million years before Mole Hill erupted, Johnson has a theory about why these volcanic episodes occurred along the Allegheny fractures. 

"The fractures may act as conduits for heat from the mantle to reach the surface, as made evident by areas like the hot springs in Highland County," Johnson said. "Some of the gases seem to be coming from the mantle. So once you have all these cracks, and if the crust is stretched thin, it's easier for heat to get transported from the mantle." 

Johnson also has a theory as to why the Mole Hill eruption happened 48 million years ago and not at any other time. On the other side of the world, around 50 million years ago, a tectonic shift sent the Indian tectonic plate crashing into the Asian plate, a geologic event violent enough to trigger repercussions around the world. 

"It's cool that we have all these little things here, and it's cool that they haven't been explored," Johnson said. "Many more students could work on this project, not just at Mole Hill. We can go out and find more, I'm pretty sure people haven't even found all the ones in Highland County." 

Alex Sharp ('10), JMU Public Affairs