Geology of the Blue Ridge in Virginia

the Blue Ridge was buried 6-10 miles deep at one time, before being exposed by erosion over the last 300 million years
the Blue Ridge was buried 6-10 miles deep at one time, before being exposed by erosion over the last 300 million years
Source: Department of Justice

The core of the Blue Ridge is the basement rock of Virginia, the bedrock that is thought to lie beneath all other rocks of the state from the edge of the ocean to the borders with West Virginia/Kentucky.

That basement igneous rock formed during the Grenville Event (or events, perhaps involving multiple continental collisions) between 1.1-1.0 billion years ago, as tectonic plates combined to form the supercontinent of Rodinia.1

The heat and pressure of the Grenville collision(s) melted whatever rocks previously existed in the area. Colliding chunks of continental crust created mountains rising high on the surface, and pushing other rocks deep enough to melt and blend. The molten rock slowly solidified into crystalline granite and gneiss underneath the mountains. As the mountains on the surface eroded away, the granite roots were pushed upwards gradually, displaced by heavier rocks in the mantle.

The roots of the Grenville mountains were exposed by uplift/erosion at the surface, and rain/wind carved ridges and valleys into the surface. At one time the one-billion-year old crystalline rocks at the core of the Blue Ridge were called the Pedlar formation. Today, geologists often lump the different components created by the Grenville orogeny and refer to the "basement" bedrock. That basement is exposed at the core of the Blue Ridge, but hidden underneath the Piedmont, Coastal Plain, and Continental Shelf to the east.

Ancient erosion created roughly 2,000 feet of topographic "relief" (the elevation difference between tops of the mountain ridges and the valleys) by 600 million years ago. Sediments accumulated in the low spots on top of the igneous bedrock, and today those sediments are known as the Swift Run formation.2

Then the crust of the Rodinia supercontinent cracked, the continental plates pulled apart, and the Iapetus Ocean formed on the edge of the North American tectonic plate. As the plates broke up, the crust thinned and molten basalt rose in places from 40-70 miles below the surface. The eroded surface that had developed with ridges and valleys was buried by 2,000 feet of basalt flows. The landscape was leveled and black, with basalt reaching to the ocean. That basalt is known today as the Catoctin formation; places with "pillow basalt" show where it erupted and then cooled underneath the ocean.3

At the time of the basalt flows, Virginia was twisted 90 degrees from its present orientation and was located south of the Equator. In the last 600 million years, the crustal plate including Virginia has drifted north and rotated.4

After the basalt flows, the Iapetus Ocean continued to widen. The continental plates drifted apart during the Cambrian Period, and sediments washed from Virginia's surface down to the oceanfront shoreline. 500 million years ago, those sediments formed beaches similar to Virginia Beach today. Further offshore, rivers eroding the North American craton carried layer after layer of sediment into deeper waters, creating a continental shelf of mud and quartz (silica) sand.

Initially, large sand particles eroded off the Catoctin basalts and basement rocks and settled in the Iapetus Ocean. Today, that sandstone is known as the Weverton Formation. As the mountains eroded to a lower elevation, or perhaps as sea level rose and increased the distance from the eroding mountains, rivers lacked the energy to carry anything larger than small particles of silt and mud to bury the Weverton Formation. Instead of more sand, silt-rich sediments known today as the Harpers Formation were deposited on top of the Weverton sandstone.

the Blue Ridge was thrust to the west when Africa collided with North America
the Blue Ridge was thrust to the west when Africa collided with North America 250 million years ago
before being buried, then uplifted and exposed by erosion
Source: US Geological Survey Bulletin 1839-I, J, Middle Eocene Intrusive Igneous Rocks
of the Central Appalachian Valley and Ridge Province: Setting, Chemistry, and Implications for Crustal Structure

When rising sea level enabled the Iapetus Ocean to drown the shoreline of Virginia, barrier islands developed that were comparable to those on the Eastern Shore or at Cape Hatteras today. The Antietam Formation, sand-sized particles on top of the Harpers Formation, is the remainder of those barrier islands.

Then, for whatever reason, the continental plates reversed direction. Africa and North America started to move together again, and the Iapetus Ocean began to close. The chunk of continental crust known as Avalonia moved closer to the shoreline of Virginia, and a "foreland basin" developed to the west of the continent's edge. In that basin, zillions of tiny sea creatures - with tiny shells of limestone - lived and died in the salt water.

As the Iapetus Ocean closed, "terranes" or island arcs and finally Africa crunched into Virginia and the rest of eastern North America, during the Taconic, Acadian, and Appalachian (Alleghenian) orogenies. Those collisions with additional chunks of crust would bury those sediments and beaches deep enough to lithify into rocks known today as the Chilhowee Group, with the Weverton Formation near the bottom, the Antietam Formation near the middle, and the Harpers Formation near the top.

The Catoctin basalt itself would be pushed down six-ten miles, deep enough to be metamorphosed by heat/pressure that realigned atoms and created new minerals such as epidote. Epidote is a green mineral, and is so common in the metamorphosed basalt that the Catoctin formation in Shenandoah National Park is often referred to as "greenstone." The sediments on top of the basalt lava flows were also compressed and metamorphosed, converting sandstone into layers of quartzite. Thanks to cycles of deposition and erosion, the crystalline basement bedrock has moved up and down into the earth almost like an elevator.5

As part of the Alleghenian orogeny, when Africa collided with North America 300 million years ago and the supercontinent of Pangea formed, the edge of Virginia was cracked and pushed west. A slice of buried billion-year old granites, with metamorphosed sediments on top, was thrust 40-60 miles from its original position. As Africa pressed against North America, the younger carbonate bank and sandy beaches that had formed on the edge of the Iapetus Ocean (exposed today in the Shenandoah Valley) ended up being shoved underneath the older slice of "basement" that was lifted up and pushed inland.

It has been 300 million years since the collision with Africa cracked and transported the edge of Virginia. Since then, Pangea has broken up, like the predecessor supercontinent Rodinia. The ocean formed when Africa and North America parted is known as the Atlantic Ocean, and 300 million years of erosion has exhumed the slice of "basement."

That now-exposed basement bedrock forms the Blue Ridge. The core of the Blue Ridge mountains in Shenandoah National Park (including Old Rag Mountain) was once located somewhere near I-95.

how Cambrian/Ordivician sediments (exposed in Shenandoah Valley) may lie underneath the Grenville-age rocks (exposed as the Blue Ridge)
how Cambrian/Ordivician sediments (exposed in Shenandoah Valley) may lie underneath the Grenville-age rocks (exposed as the Blue Ridge)
Source: Virginia Department of Mines, Minerals and Energy, Virginia Minerals
Oil And Gas Exploration And Development In Virginia, 1979-1988 (February 1990)

Differential erosion has created the modern topography of Virginia. The Blue Ridge was exposed in its new location as softer rocks around it have eroded away faster than the basement gneiss and hard Catoctin greenstone.

To the west of the Blue Ridge, softer Cambrian and Ordovician limestones in the Valley and Ridge physiographic province have eroded to form the Shenandoah Valley, New River Valley, and others which are sometimes lumped together as the "Valley of Virginia." East of the Blue Ridge, the geologic formations in the Piedmont have eroded faster than the harder and older igneous bedrock of the Blue Ridge. The result: a ridge with 1,000-3,000 feet of topographic relief, wit a valley to the west and a gently sloping plain to the east.

old Blue Ridge crystalline basement/volcanics thrust over Cambrian/Ordivician carbonates/shale (now exposed in Shenandoah Valley)
old Blue Ridge crystalline basement/volcanics thrust over Cambrian/Ordivician carbonates/shale (now exposed in Shenandoah Valley)
Source: Virginia Department of Mines, Minerals and Energy, Publication 066: Part 1:
Interpretive seismic profile along Interstate I-64 in central Virginia from the Valley and Ridge to the Coastal Plain

Links

the General Assembly used the watershed divide at the crest of the Blue Ridge to define county boundaries
the General Assembly used the watershed divide at the crest of the Blue Ridge to define county boundaries
Source: Earth Resources Observation and Science (EROS) Center, US Geological Survey (USGS),
Landsat State Mosaics

References

1. Richard T. Williams, William M. Dunne, and Lynn Glover III, "Global Geoscience Transect 20- Central Appalachians: Cratonic North America to the Atlantic Abyssal Plain," International Geology Review, Volume 41, Issue 8, 1999, Pages 711 - 738 http://www.informaworld.com/smpp/content~db=all~content=a910212761 (last checked June 17, 2011)
2. Gathright, T.M., II, 1976, Geology of the Shenandoah National Park, Virginia, Virginia Division of Mineral Resources Bulletin 86, http://www.dmme.virginia.gov/commercedocs/BUL_86.pdf, p.47 (last checked June 23, 2011)
3. Richard P. Tollo, James McLelland, Louise Corriveau, Mervin J. Bartholomew (ed.), Proterozoic tectonic evolution of the Grenville orogen in North America, The Geological Society of America Memoir 197, 2004, p.454 http://books.google.com/books?id=uT4HISRCon8C&pg=PA454; John C. Reed, Jr., "Ancient Lavas In Shenandoah National Park Near Luray, Virginia," Geological Survey Bulletin 1265, 1969, http://www.cr.nps.gov/history/online_books/geology/publications/bul/1265/sec2.htm (last checked June 23, 2011)
4. Richard Diecchio and Richard Gottfried, "Regional Tectonic History of Northern Virginia" in Circular 1264: Geology of the National Capital Region - Field Trip Guidebook, http://pubs.usgs.gov/circ/2004/1264/html/trip1/index.html (last checked June 23, 2011)
5. Christopher M. Bailey, "Mind the Gap! Where is Afton Mountain?," William and Mary, http://web.wm.edu/geology/virginia/field_loc/rfgweb.pdf (last checked August 28, 2014)

erosion has carved a steeper slope on the eastern edge of the Blue Ridge in southern Virginia
erosion has carved a steeper slope on the eastern edge of the Blue Ridge in southern Virginia
Source: USGS National Elevation Dataset, NED Shaded Relief - Virginia


Rocks and Ridges - The Geology of Virginia
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