Volcanoes in Virginia

Mole Hill is volcanic basalt that erupted 48 million years ago, and the remnant plug erodes slower than surrounding limestone west of Harrisonburg
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, 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 as the supercontinent of Rodinia broke up.

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.

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.

columnar basalt in Shenandoah National Park, formed after lava flows cooled roughly 575 million years ago
columnar basalt in Shenandoah National Park, formed after lava flows cooled roughly 575 million years ago

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.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

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

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

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. Daniel H. Doctor, Wil Orndorff, Joel Maynard, Matthew J. Heller, Geralamo C. Casile, "Karst geomorphology and hydrology of the Shenandoah Valley near Harrisonburg, Virginia," Field Guides for the GSA Southeastern Section Meeting, Blacksburg, Virginia, 2014, Geological Society of America, 2014, p.13 (last checked August 11, 2014)
3. 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)
4. "Diamonds In Virginia," Virginia Minerals, Virginia Department of Mines, Minerals, and Energy, Volume 42 Number 4 (November 1996), http://www.dmme.virginia.gov/commercedocs/VAMIN_VOL42_NO04.pdf; W. Dan Hausel, Diamonds & Mantle Source Rocks in the Wyoming Craton, W. Dan Hausel Geological Consulting LLC, July 30, 2007, http://gemstonebookstore.pbworks.com/f/DIAMONDS+in+the+WYOMING+CRATON.pdf (last checked August 11, 2014)
5. "When Was the Last Time Volcanoes Erupted on the East Coast?," Scientific American, January 2, 2014, http://www.scientificamerican.com/article/recent-east-coast-volcano/ (last checked August 11, 2014)

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

Thermal Springs in Virginia
Virginia Diamonds
Rocks and Ridges - The Geology of Virginia
Virginia Places