The Rest of the Story About Virginia Geology

The short story: Africa collided with North America and pushed up the Appalachian Mountains. The Appalachians we see today are just the roots of older mountains - after 200 million years of erosion, the height of the old Appalachians is low compared to the younger Rockies or the Andes or the Alps.

But that is only part of the story.

After the collision (Appalachian or Alleghanian orogeny), those mountains pushed up by the Africa-America collision eroded to the west. The sediments filled up the center of the North American continent in layer after layer. The sediments permanently raised the bed of the continent's center, after numerous large seas had inundated it previously and deposited limestones, coal, shale, and sandstones, to form the bedrock of Indiana, Illinois, Missouri. The mountains also eroded to the east, depositing sediments on what today is Morocco and Mauritania.

The supercontinent formed after the African collision (known as Pangea) was unstable, just as the combined continental plates known as Rodinia had been unable to stay together after the Grenville event. About 200 million years ago, the tectonic stresses pulled Africa and North America in different directions again. Pangea developed a series of parallel cracks. These cracks became valleys where the crust thinned, and basalt lava erupted into them from below.

Most of the parallel cracks finally filled with sediments, and are known today as Triassic Basins. However, one particular crack became the primary release point for the heated rock below. A rift valley with a "spreading center" formed, and the continents split apart along this seam. Sediments continued to flow into this particular crack from the east and the west, but the rift valley deepened faster than erosion could fill it.

This major rift continued to grow a few inches per century. The rift valley finally grew so wide that it forced the continents completely apart - and as the salt water flowed into the valley, the Atlantic Ocean was born. Initially the Atlantic was only a narrow strip of salt water, but once again Virginia was a coastline state and no longer trapped in the center of s supercontinent.

Over time, several thousand miles of lava built up between the still-erupting rift (known now as the Mid-Atlantic Ridge) and the old North American sediments. The Atlantic continues to grow as the North American plate - carrying Virginia with it - drifts westward away from Europe and Africa at 2-3 centimeters/year... the rate at which fingernails grow.

After the rift valley and the Atlantic Ocean formed, the mountains pushed up by the colliding plates in the Appalachian Orogeny continued to erode... and are still eroding today. For 200 million years, sediments have been carried to the east as well as to the west. Most sediments that eroded to the east from the Taconic-time and Acadian-time mountains ended up as part of the coontinental shelf in the Iapetus Ocean. Those sediments were scrunched and metamorphosed in the Appalachian/Alleghanian collision, forming the basement rock of the Piedmont east of the Blue Ridge.

Since Pangea split up and the Atlantic Ocean opened, a new continental shelf has developed to the east of the old Apppalachian Mountains. Erosion has carved away at the rocks lifted up by the collision with Africa, and rivers have carried sediments downstream. Where topographic relief is lower, mudflats/beaches have been created at the mouths of rivers. On the eastern edge of the continent, along the new Atlantic Ocean, the sediments have created a massive new Coastal Plain. It sits on top of the ancient, now-metamorphosed rock layers that were compressed into the Piedmont basement, when the Iapetus Ocean closed.

Today, much of the surface of Virginia east of Interstate 95 consists of these Coastal Plain sediments. Virginia's rivers have created a wide trail of eroded dirt as the continental plate drifts to the west, like a wedding dress trailing behind a bride going down the aisle. (Of course, not all Virginia rivers drain to the east. Some sediments went down the New River drainage to the Gulf of Mexico, helping to form the birdsfoot delta of the Mississippi River.)

As the last range of mountains from the collision with Africa has eroded away, the old rocks underneath have been exposed after millions of years. Differential erosion has created the modern topography of Virginia. The Blue Ridge has been exhumed by erosion, exposed in its new location as softer rocks around it have eroded away faster than the basement gneiss and hard Catoctin greenstone.

Mount Rogers, with its hard volcanic rock laid down 600-700 million years ago when the supercontinent of the Grenville orogeny split up, has become the highest point in Virginia. The hard sandstones deposited after each orogeny, such as the Clinch Formation deposited at the end of the Taconic orogeny, have become the higher ridges of the Appalachian mountains.

Softer shales and limestones have eroded faster and formed the valleys to the west of the Blue Ridge, while more-resistent sandstones have formed ridges. The Shenandoah Valley formed because the Cambrian and Ordovician limestones have eroded away faster than the volcanic Blue Ridge to the east, or the quartz-rich North Mountain to the west. In the middle of the Shenandoah Valley, some layers of quartz-rich rocks have not disappeared yet - but Massanutten Mountain is gradually eroding away. Its presence shows the relative resistance of sandstone, compared to the softer carbonate rocks to the west and in Page Valley to the east of Massanutten.

Spectacular mountains have been created and eroded away three times, and maybe more, in Virginia. You can see the roots, and the ruins, of those events if you look at the rocks outside your car window and in your backyard. The modern rivers etching away at the current Virginia landscape are just a gentler, kinder version of the tremendous tectonic forces that have raised and destroyed mountains as large as the Himalayas in Virginia.

Back to: The Orogeny Zones and Virginia Geology

Source: Virginia Department of Mines, Minerals and Energy