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. Those igneous rocks crystallized over one billion years ago as the supercontinent of Rodinia formed, and through erosion were exposed at the surface 400 million years later.
The highest spot in Virginia, Mount Rogers, is formed from lava 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, so Mount Rogers is forme from rhyolite rather than basalt.
About 575 million years ago, as Rodinia finally split apart, basalt eropted and flowed on top of the 1-billion year old granite and gneiss that today forms the core of the Blue Ridge. Those igneous lava flows were later buried and metamorphosed into Catoctin greenstone, a rock formation 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
About 200 million years ago during the Triassic Period, igneous rock reached the surface of Virginia again When the supercontinent of Pangea broke up, the crust thinned and cracked. Thinner overlying crust reduced the pressure that suppressed the rise of magma. At weak points, basalt rose up and broke through to the surface.
Dikes (where magma cuts through sedimentary layers) and sills (where magma intruded between the layers) are common in the Triassic basins east of the Blue Ridge. Many quarries crush the harder-than-average basalt into sizes of gravel used in road and building construction. Triassic dikes are also found in the Piedmont, Blue Ridge, and Valley and Ridge provinces, indicating where the crust cracked and reduced the overlying pressure.
Virginia also has the youngest volcanoes on the East Coast: 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 during the Eocene Epoch. At that time, as it is now, Virginia was on the passive margin of the North American plate. Volcanic eruptions were in the center of the Atlantic Ocean, on the Mid-Atlantic Ridge.
For some reason, Virgiia's chunk of crust was squeezed by tectonic shifts. The pressures may have weakened old Paleozoic faults, and basalt rose from 18 miles deep in the mantle. It moved at 40-70 miles/hour up to the surface.2
Roughly 100 outcrops similar to Trimble Knob splatter the geologic map of Highland and Rockbridge counties, and west to Ugly Mountain in Pendleton County, West Virginia. A 12-inch layer of lava was intruded into the horizontal layers of 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 had formed deep at the crust-mantle boundary. Small 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.4
The trigger for the young Eocene eruptions in Virginia is unclear. The temperature of the basalt appears too cool to have been caused by a "hotspot" similar to what created the Hawaiian Islands. The crust could have thinned and allowed hot magma to rise when a continental rift began to expand, perhaps associated with reactivation of the New Madrid fault in Missouri.
Another possibility is that a chunk of a crustal plate had subducted underneath the North American Plate during the Taconic or Neo-Acadian orogenies 300-400 million years earlier, and it finally melted and dripped into the mantle. The chemical change could have triggered a burst from the mantle, pushing up a tower of magma 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 has eroded slower than the surrounding limestone, the topographic relief is not flat. The volcanic plugs at Trimble Knob and Mole Hill stand out today due to differential erosion.
Mole Hill is clearly visible west of Harrisonburg because the surounding limestone has eroded away faster than the igneous basalt
Source: US Geological Survey (USGS) 7.5-minute map for Bridgewater, VA (2016)
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) National Map
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