Grenville Event and Virginia Geology

The oldest rock in Virginia, the Nellysford gneiss found in Nelson County, has been a "Cheerio" of Virginia for 1.8 billion years. When the rock cooled into its current state, however, most of Virginia had not been created yet.

About 2 billion years ago - give or take a few hundred million years - various chunks of continental crust had connected with each other. Today, we see those chunks exposed in the central part of Canada and the core of the Blue Ridge Mountains. The correct buzzword for those old Canadian rocks is "North American craton" - but it may be easier to visualize continental plates, floating on the molten mantle 25-50 miles below the surface, as clumps of cereal ("Cheerios" or "Rice Crispies") floating in milk.

About 1.4-1.0 billion years ago, another chunk of continental crust banged into that craton. When those continental plates drifted together, it was not a gentle bump. Rocks melted in the heat and pressure, and the Grenville Mountains were uplifted. Some of the Rice Krispies joined the North American craton, and the North American plate was enlarged by the rocks that formed the first part of Virginia. At this time, all the continental crust on the earth clumped together and formed a supercontinent called Rodinia surrounded by an ocean that covered most of the globe.

The Grenville Mountains and the rest of Rodinia were barren; there was no vegetation on the land yet. Algae photosynthesizing in the ocean had already transformed the atmosphere, poisoning most of the earliest forms of life with the waste product of photosynthesis - oxygen. However, the evolutionary process had not resulted in land plants yet, so a billion years ago all of Virginia was bare rock or lakes/rivers.

As the Grenville Mountains were pushed up by the collision of continental plates, rain eroded the exposed rock. Rivers formed. Erosion carved valleys. Sediments washed downstream to build shorelines. Up to 15 miles of rock eroded away, as uplifts were countered by erosion. Today, erosion has exposed the once-buried-deep roots of the mountains that were formed by continental plate collisions over a billion years ago.¹

The Grenville Event - events, perhaps involving multiple continental collisions - formed the old "basement" rocks in Virginia. It built the core of the Blue Ridge mountains now exposed in Shenandoah National Park at Old Rag and Mary's Rock, plus the State Farm granite northwest of Richmond and under the women's prison in Goochland County. Erosion occured throughout the uplift; as rocks were exposed, they immediately began to wash away. About one billion years ago, the erosion had been so effective that topographic "relief" (the difference between the mountain peaks and the valleys) may have been roughly 2,000 feet in Virginia.

Old Rag, in Shenandoah National Park

Tha relief is close to what you see today from the Skyline Drive on the crest of the Blue Ridge, from overlooks west towards the Shenandoah Valley or east towards the Piedmont. The Skyline Drive may be nearly 4,000 feet high in places... but the adjacent territory is also about 2,000 feet above sea level, so the "relief" (the difference in elevation between two points) is usually 1,000-2,000 feet.

Several hundred million years after the Grenville Event, continental plates drifted apart. The mantle underneath the crust bubbled a little (or a spoon swirled...). Multiple cracks formed in the crust, and then one developed as a "spreading center" with magma welling up in the middle of that crack. The rising magma pushed one plate away from the other. When the spreading center first developed, debris from ridges on both side of the crack eroded into it. At some point, salty ocean water filled the crack. Instead of being in the middle of Rodinia, smushed between two continents, Virginia finally emerged on the coastline of the North American craton.

This bumping and bouncing by continents is still going on today, incidentally. The rift valley in East Africa is a crack that's still all dry land, but you can see the ocean invading between Africa and Arabia in the crack that we call the Red Sea. In geology, present conditions are a key to the way rocks have moved in the past. The shape of the continents and oceans change, but the geologic processes that build/destroy mountains have been constant for billions of years.

When the continents reversed direction and the crust of Rodinia thinned and then cracked, the Grenville-era granitic rocks on the land were covered by a series of volcanic outpourings of basalt. If you used a straw to blow air gently underneath the Cheerios in your bowl of milk, you can create equivalent eruptions of milk to coat the cereal, the same way the basalt coated the edge of the North American craton.

Basalt is a volcanic rock that is relatively low in quartz (silica), in contract to granite, and can flow fast. It appears the individual basalt flows in Virginia may have moved at rapid (even supersonic) speed up through the Grenville-age rocks. They reached the surface in many cases, poured out, and coated both the hard bedrock and the sediments that had eroded previously. The Grenville-age hills and valleys were covered by about 2,000 feet of basalt, leveling much of the landscape in what became Shenandoah National Park. So the park is covered with "Catoctin basalt" today that erupted nearly 600 million years ago.

Underneath, the core of the Blue Ridge is the Grenville-era rocks, particularly the Pedlar and Lovingston "granites." They're not exactly pure granites in their mineral composition, and they have been metamorphosed (changed by heat and pressure) slightly. However, if you can think of rocks as Cheerios... then we won't bother to split hairs here.

NOTE: You may see the Grenville-age rocks called "charnockite," referring to a granite-like rock with lots of pyroxene (a mineral) and grains of blue quartz. As Keith Frye decribes in Roadside Geology of Virginia, the name comes from a tombstone for the founder of Calcutta, Job Charnock. The tombstone was carved from this sort of rock, so obviously one rocktype in the Blue Ridge core also occurs in India.

At roughly the same time as the basalts were erupting, a volcano spewed a vast amount of lava near what became the North Carolina border. Later, that hard volcanic rock would be buried and exposed again by erosion to become Virginia's highest point, Mount Rogers.

The basalt lava east of the Blue Ridge appear to have erupted under water. What water? It would have been the Iapetus Ocean, filling in one of the cracks that formed as the Cheerios and Rice Krispies drifted apart.

[Iapetus? Not the Atlantic Ocean? In Greek myth, Iapetus was the father of Atlantis. As you'll see in a moment, this ocean 600 million years ago was the "father" to the Atlantic Ocean of today. When continental plates bump and grind and then separate, it helps to give different names to the continental masses and to the oceans.]

So imagine you could hop in a time machine at Peaks of Otter on the Blue Ridge Parkway, or on Old Rag Mountain in Shenandoah National Park. If you could go back 600 million years, you'd see a flat Virginia plain if you looked in one direction towards what's now the Shenandoah Valley. You'd see the Iapetus Ocean in the other direction. Manassas, Charlottesville, Lynchburg, and Danville would have been beachfront property, or maybe underwater. And you would not be looking west towards the flat plain, or east towards the Iapetus Ocean. Virginia was twisted 90 degrees from the present orientation, "aligned parallel to the equator that passed through the central part of the United States, roughly coincident with the present-day longitude of Kansas."² You would have been south of the Equator. The future Shenandoah Valley lay to the north of the future Blue Ridge Parkway, and the coastline of the Iapetus Ocean lay to the south.

Don't stop now - keep on going to: The Orogeny Zones and Virginia Geology

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