Mole Hill, as shown on Jedediah Hotchkiss's 1862 map
Source: Library of Congress
Virginia has a long history of volcanic activity, particularly the basalt flows in the Triassic basins east of the Blue Ridge and similar-aged dikes in the Shenandoah Valley. The highest spot in Virginia, Mount Rogers, is rhyolite that erupted 750 million years ago.
However, Virginia also has two very young volcanos, Trimble Knob in Highland County and Mole Hill in Rockingham County. Young is a relative term - Mole Hill is 48 million years old, and Trimble Knob is 35 million years old.1 They were created in the Eocene Epoch, at a time when tectonic shifts may have created a temporary weak zone in the middle of the normally-quiet North American Plate.
Both appear to be "diatremes," formed when a rising shaft of magma intercepted shallow groundwater. The water flashed into steam and erupted through the overlying sediments, creating a thin volcanic tube in the ground. Today, we do not see the original surface where the volcanic rock erupted. Over the last 48 million years, 1,100 feet or more has eroded away.2
 diatreme, showing narrow path cutting through overlying rocks Source: Virginia Department of Mines, Minerals and Energy, Eocene Igneous Rocks Near Monterey, Virginia: A Field Study |
 diatreme increase in depth, through multiple steam explosions Source: US Geological Survey (USGS), Middle Eocene Igneous Rocks in the Valley and Ridge of Virginia and West Virginia |
Because the cooled lava in the diatreme's tube erodes slower than the surrounding limestone, the topographic relief is not flat - the volcanic plugs are hills today. Mole Hill is 350 higher than the surrounding limestone valley.
