Rocks and Ridges - Where Did Virginia Get Its Mountains and Valleys?

North American Tectonic Plate Geology helps to explain why hikers can see a vista of mountains from the top of Dragon's Tooth near Roanoke, why the Chesapeake Bay Bridge has two tunnels... and why the western part of the state is mountainous but the eastern part of Virginia is flat.

Obviously, Virginia is on the eastern edge of the North American continent - but we are not on the eastern edge of the North American tectonic plate. Virginia is closer to the middle of the tectonic plate than to the eastern edge.

The eastern edge of the tectonic plate is the Mid-Atlantic Ridge, in the middle of the Atlantic Ocean. The western edge is the San Andreas fault on the California coast. There are constant earthquakes on the edges of the plate, where rock is oozing up to the surface (in the Mid-Atlantic Ridge) or rubbing against other plates (in California). Since Virginia is near the center, friction against other plates is minimal here - that's the main reason we have so few earthquakes here.

Virginia wasn't always where it is today. Plate tectonics theory says we're moving west. We're moving towards Japan at maybe one inch (2.5cm) per year.1 That is the rate at which the Mid-Atlantic Ridge is spreading - and, coincidentally, the rate at which our hair and fingernails grow. When you get a haircut, the length of your hair that is cut equals the distance we have travelled towards Asia.

Measured in English units, one inch per year is almost a foot per decade, or nearly a yard each century. Since the Godspeed, Discovery, and Susan Constant first arrived in 1607, Jamestown has moved about 10 yards westward with continental drift. Since the first immigrants from Asia walked into Virginia (or maybe they sailed from southwestern France?) about 12,000 years ago, Virginia has drifted about the length of a football field. In the last million years, Virginia moved 15 miles to the west through continental drift.

East Coast with offshore topography
onshore topography and offshore bathymetry of Virginia
(can you distinguish the wide Continental Shelf, the narrow Continental Rise,
and the wider Abyssal Plain extending eastward towards the Mid-Atlantic Ridge?)
Source: NOAA's National Geophysical Data Center (NGDC) - Natural Hazards Viewer

view from Dragon's Tooth The topography of Virginia was formed through a series of continental collisions that created vast mountain ranges equivalent to the Himalayas being uplifted between China and India now. (The Indian Plate is still moving several times faster than the North American Plate - perhaps Virginia was also in the fast lane during the Paleozoic Era.) The collisions that created tall mountain ranges in Virginia were followed by erosion that etched away those mountains and piled up layers of sediments. Virginia may not have glacier-covered peaks today, but we had glaciers in the past that carved into the ancestral mountains.

Virginia is not flat like Kansas today. To understand why there are individual ridges in some places and valleys in others, it helps to know the history of those collisions. And since the locations of towns, highways, even soccer fields is determined in part by the geology of Virginia, it helps to understand the bedrock and the topography if you want to understand the people and the culture of the state. Hey, could you have mountain music or "mountain dew" (moonshine) without mountains?

Not everyone is a geologist, or even a geologist wanna-be. If you find yourself slightly uncomfortable with the idea that the solid earth is not solid, then put the science into context with more-familiar terminology by reading Using Cereal Bowls and Car Crashes to Understand Virginia Geology. (For the solidly-scientific perspective, see The Geology of Virginia from William and Mary, and The Geological Evolution of Virginia and the Mid-Atlantic Region from James Madison University.)

topographic view of Virginia

Additional Links

References

1. This Dynamic Earth - Understanding Plate Motions, USGS, pubs.usgs.gov/gip/dynamic/understanding.html#anchor6715825 (last checked July 17, 2006)


Geography of Virginia