Rock Topics

Igneous Rocks (Part 2)

Rhyolite and Tuff

• Rhyolite lavas and tuffs are abundant at Mount Rogers in the southern Blue Ridge.

RhyolitesRhyolite in Grayson Highlands State Park and the Mount Rogers area (Left photograph by Robert Whisonant; Right photograph by Parvinder Sethi)

MapAncient rhyolite lava in the Blue Ridge can be found in Grayson Highlands State Park in Grayson County.  This rock in the left photograph shows columnar jointing, which indicates that the lava flow cooled quickly.  The right photograph shows a close view of a rhyolite from Mount Rogers.

Basalt and Diabase

• Basalt and diabase (fine-grained gabbro) are common in the Mesozoic Basins in the Piedmont.

• Basalt dike swarms are common in the Piedmont.

Diabase QuarryDiabase quarry and sample (Photographs by Robert Whisonant)

MapDiabase is quarried in Culpeper County.  The diabase is part of the Culpeper Basin, one of the several Mesozc Basins in Virginia’s Piedmont. 



Eocene Age Rocks

• An unusual occurrence of igneous rocks exists in Highland County, in the Valley and Ridge of western Virginia.  In this area, there are over 60 stocks, dikes, and sills of Eocene age.  The rocks are mainly rhyolite porphyry, basalt, and breccias.

57.8 to 36.6 million years ago

Highland Intrusions
Intrusion (left) and Trimble Knob in Monterey, Highland County, Virginia (Photographs by Stan Johnson)

MapUnusual igneous rocks are found in the Valley and Ridge in and around Highland County.  An igneous rhyolite dike cuts through sedimentary limestone (left).  Trimble Knob (right) is a dome of basalt that intruded into the shale in the area.