It does not look like a volcano. Mole Hill rises out of farmland west of Harrisonburg as a perfectly rounded, tree-covered knob - the kind of low hill that prompts no questions in a landscape full of low hills. Drive past it and you might not notice. But beneath the trees is dark basalt, the kind of rock you find at active volcanoes in Hawaii and Iceland, and the geological story it tells is improbable. Mole Hill is the eroded core of a volcano that erupted 47 million years ago in the middle of a continent, far from any tectonic plate boundary that should have produced it. It is one of the youngest volcanoes on the East Coast of North America - and one of the strangest.
Modern volcanism, by the standard textbook account, happens at plate boundaries - where plates collide, separate, or slide past each other. The Cascade volcanoes of the Pacific Northwest and the volcanic arc of Indonesia are the classic examples. Virginia is nowhere near a plate boundary now and was not 47 million years ago either. Yet between 48 and 35 million years ago, during the Eocene epoch, small volcanic eruptions punctuated the Valley and Ridge province of Virginia and West Virginia. Mole Hill is the best-preserved remnant. Trimble Knob in Highland County and intrusive igneous rocks near Ugly Mountain in Pendleton County, West Virginia, are geological siblings - the same eruption sequence, the same puzzle.
Geologists have spent decades arguing about what caused these eruptions. Mantle plumes - rising hot rock from deep in the Earth - have been proposed and largely rejected. Edge-driven convection has been considered. The current leading explanation is more subtle: a large-scale change in plate motion between 53.5 and 37.5 million years ago caused the North American Plate to extend slightly. That extension allowed small volumes of magma to form through decompression melting - the same way magma forms beneath mid-ocean ridges. The melt rose along old suture zones - ancient cracks in the crust dating back to the formation and breakup of the supercontinent Pangaea. The Earth's old scars provided pathways for magma to reach the surface. Mole Hill was one of the first pulses, around 48 million years ago.
The basalt at the crest of Mole Hill is technically olivine-spinel basalt - dark gray to nearly black, medium grained, with abundant pale green pyroxene crystals and minor yellow-brown olivine crystals visible to the naked eye in fresh samples. Trace amounts of more evolved magma compositions are present too: trachyandesite, trachydacite, and rhyolite, all unusual companions to a basalt. The volcanic rock intrudes through the Ordovician Beekmantown Group - a layer of carbonate rocks deposited in a shallow tropical sea some 480 million years ago, more than ten times older than the lava that pushed through it. Standing on Mole Hill, you are standing on a relatively young volcanic remnant set into truly ancient seafloor.
Today Mole Hill is a rounded monadnock - an isolated hill rising above a relatively flat valley floor - covered in deciduous forest, surrounded by Rockingham County farmland. The summit is about 1,893 feet above sea level, several hundred feet above the surrounding land. The hill is mostly private property; there is no developed trail or formal access from public roads. Drivers along the back roads west of Harrisonburg pass it constantly without realizing what it is. The geological significance is the kind that does not advertise itself. The volcano stopped erupting 47 million years ago. The lava cooled. Erosion did the rest. What is left is a small hill that geologists travel from across the country to study.
Located at 38.4486N, 78.9533W in Rockingham County, Virginia, west of Harrisonburg in the Shenandoah Valley. The hill rises to 1,893 feet, about 400-500 feet above the surrounding farmland - a small but distinct rounded landmark. Recommended viewing altitude is 4,500 to 6,500 feet for views of the hill in context with the broader Valley. Massanutten Mountain runs to the east, the Allegheny Front to the west. Nearest airport is Shenandoah Valley Regional (KSHD) about 13 nm south. Watch for valley haze in summer.