OK, Africa collided with North America and pushed up the Appalachian Mountains. But you thought the Appalachians of today were just the roots of older mountains - that's why their height is so low compared to the Rockies or the Andes or the Alps, right?
You're right.
About 200 million years ago, after the collision ("Appalachian 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 combined continents (known as Pangea this time) were unstable, just as the combined continents (known as Rodinia then) 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 (see image to the right) 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.
Over time, however, several hundred miles of lava have 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 your fingernails grow.
After the rift valley and the Atlantic Ocean formed, the muntains pushed up by the colliding plates in the Appalachian Orogeny continued to erode... and are still eroding today. Sediments were carried to the east as well as to the west, and have accumulated at the mouths of rivers to create mudflats/beaches. On the eastern edge of the continent, along the new Atlantic Ocean, the sediments created a massive new Coastal Plain on top of the ancient, now-metamorphosed rock layers that were underneath the new Atlantic Ocean.
Today, much of Virginia east of Interstate 95 is covered with these Coastal Plain sediments. Virginia's rivers have created a wide trail of eroded dirt as the continent 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.)
When those mountains from the collision with Africa eroded away, the old rocks underneath have been exposed after millions of years. Mount Rogers, with its hard volcanic rock laid down 600 million years ago when the supercontinent of the Grenville orogeny split up, has become the highest point in Virginia. The hard sandstones, such as the Clinch Formation deposited at the end of the Taconic orogeny, have become the higher ridges of the Appalachian mountains.
The softer shales and limestones have eroded faster and formed the Appalachian valleys. The Shenandoah Valley formed because the Cambrian and Ordovician limestones have eroded away faster than the volcanic Blue Ridge to the east, or the quart-rich mountains to the west. In the middle of the Shenandoah Valley, layers of quartz-rick rocks have not disappeared yet. Massanutten Mountain is gradually eroding away, showing the relative resistance of the sandstone compared to the softer rocks.
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
