Volcanoes in Virginia

Virginia has a long history of eruptions and 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. The Catoctin basalts flowed over the core of the Blue Ridge about 575 million years ago, and are now well-exposed in Shenandoah National Park.

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.

Those are not the only two locations where recent volcanic activity can be documented. Outcrops similar to Trimble Knob splatter the geologic map of Highland County, and 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.

columnar basalt in Shenandoah National Park
columnar basalt in Shenandoah National Park

Mole Hill, as shown on Jedediah Hotchkiss's 1862 map
Mole Hill, as shown on Jedediah Hotchkiss's 1862 map
Source: Library of Congress

area of young (Eocene) volcanics in Virginia
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.2 In Canada, Wyoming, and Arkansas, some diatremes include kimberlite "pipes" with diamonds that formed deep at the crust-mantle boundary, perhaps 500 million- 1 billion years ago.3

diatreme, showing narrow path cutting through overlying rocks
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
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. 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)
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
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

Links

References

1. Jonathan L. Tso, Ronald R. McDowell, Katharine Lee Avary, David L. Matchen, and Gerald P. Wilkes, "Middle Eocene Igneous Rocks in the Valley and Ridge of Virginia and West Virginia" in USGS Circular 1264, "Geology of the National Capital Region Field Trip Guidebook, 2004, http://pubs.usgs.gov/circ/2004/1264/html/trip4/index.html (last checked May 22, 2012)
2. Jonathan L. Tso and John D. Surber, "Eocene Igneous Rocks Near Monterey, Virginia: A Field Study" in Virginia Minerals, August/November 2006, Virginia Department of Mines, Minerals and Energy, http://www.dmme.virginia.gov/dmr3/dmrpdfs/vamin/vm%2049_3_4.pdf (last checked May 22, 2012)
3. Virginia Diamonds, "Virginia Places" (last checked May 22, 2012)
forest-covered Mole Hill, west of Harrisonburg (Rockingham County)
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
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

dike of 575-million year old Catoctin basalt cutting through<br>
Grenville-age (1.1 billion year) bedrock in Shenandoah National Park
dike of 575-million year old Catoctin basalt cutting through Grenville-age (1.1 billion year) bedrock in Shenandoah National Park

Rocks and Ridges - The Geology of Virginia
Virginia Places