Mole Hill is volcanic basalt that erupted 48 million years ago, and the remnant plug erodes slower than surrounding limestone west of Harrisonburg
Virginia has a long history of eruptions and volcanic activity. Some of the oldest rocks in Virginia, the core of the Blue Ridge, were the molten roots of the Greenville Mountains pushed up as the supercontinent of Rodinia. Those igneous rocks crystallized over one billion years ago, and through erosion were exposed at the surface 400 million years later.
The highest spot in Virginia, Mount Rogers, is rhyolite that erupted 750 million years ago. The supercontinent of Rodinia was starting to break up then. The initial rifting thinned the crust and allowed magma to reach the surface, That cracking of the supercontinent ended before the tectonic plate split apart to form a new ocean.
About 575 million years ago, when the Iapetus Ocean formed, basalt flowed over the 1-billion year old granite that today forms the core of the Blue Ridge. Those lava flows were later buried and metamorphosed into Catoctin greenstone, a rock that is now well-exposed in Shenandoah National Park and along the Blue Ridge Parkway.
dike of 575-million year old Catoctin basalt cutting through Grenville-age (1.1 billion year) bedrock in Shenandoah National Park
columnar basalt in Shenandoah National Park, formed after lava flows cooled roughly 575 million years ago
Basalt reached the surface of Virginia again during the Triassic Period 200 million years ago, when Pangea broke up and the crust thinned and cracked. There are Triassic-age dikes and sills well exposed in the Triassic basins east of the Blue Ridge, and also in the Piedmont, Blue Ridge, and Valley and Ridge provinces.
However, Virginia also has two very young volcanoes, 200-foot high Trimble Knob in Highland County and 150-foot high 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
The volcanic rock intruded through the sedimentary layers west of the Blue Ridge in the Eocene Epoch. At that time, tectonic shifts as various plates moved may have weakened old Paleozoic faults that are now in the middle of the North American Plate, far from the active margin at the Mid-Atlantic Ridge. Basalt erupted from 18 miles deep, moving at 40-70 miles/hour up to the surface.2
Those are not the only two locations where recent volcanic activity can be documented. Roughly 100 outcrops similar to Trimble Knob splatter the geologic map of Highland and Rockbridge counties, and west into Pendleton County, West Virginia. A 12-inch layer of lava was intruded into the limestone at Natural Chimneys at roughly the same time when the Mole Hill diatreme formed.
Mole Hill, as shown on Jedediah Hotchkiss's 1862 map
Source: Library of Congress
area of young (Eocene) volcanics in Virginia
plus older (Jurassic) dikes dating to formation of Atlantic Ocean
Source: US Geological Survey (USGS), Middle Eocene Igneous Rocks in the Valley and Ridge of Virginia and West Virginia
Both Trimble Knob and Mole Hill appear to be "diatremes," formed when a shaft of magma intercepted shallow groundwater. Near the surface, 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 35-48 million years, 1,100 feet or more has eroded away.3
Some diatremes include kimberlite "pipes" with diamonds that formed deep at the crust-mantle boundary. Kimberlite pipes has been found in Virginia near Mount Horeb Church (Rockbridge County) and Front Royal (Warren County). Five diamonds have been found in Virginia, but none of them can be associated with any source rock. however.4
The trigger for the Eocene eruptions in Virginia is unclear. The temperature of the basalt appears too cool to be caused by a "hotspot" similar to what created the Hawaiian Islands. The crust could have thinned and allowed hot magma to rise when a new continental rift began to form, perhaps associated with the New Madrid fault in Missouri. Another possibility is that a chunk of a crustal plate that collided with North America during the Taconic or Acadian orogenies finally melted and dripped into the mantle, causing a burst to erupt from the mantle that reached all the way to the surface.5
diatremes create a narrow path cutting through overlying rocks, and increase in depth through multiple steam explosions
Sources: Virginia Department of Mines, Minerals and Energy,
Eocene Igneous Rocks Near Monterey, Virginia: A Field Study and 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. With relatively-steep slopes, it is less-suitable for grazing and is completely covered by trees. Trimble Knob, in contrast, is grazed heavily by sheep, and the vegetation on the volcanic rock is not distinctive from grass in the surrounding pasture.
Trimble Knob, southwest of Monterey (Highland County)
Source: US Geological Survey (USGS) NationalMap
some of the Eocene-age volcanics (in green) near Monterey in Highland County
Source: US Geological Survey (USGS) National Geologic Map Database,
Geologic map of the Virginia portion of the Staunton 30 X 60 minute quadrangle
forest-covered Mole Hill, west of Harrisonburg (Rockingham County)
Source: US Geological Survey (USGS) National Map
distinctive geology of Mole Hill, west of Harrisonburg in Rockingham County
Source: Virginia Department of Mines, Minerals and Energy,
Publication 159: Geologic Map of the Augusta, Page, and Rockingham Counties Portion of the Charlottesville 30 x 60-minute Quadrangle