Nice & Curious Questions

Edwin S. Clay III and Patricia Bangs


 

The Petersburg Pluton

and Volcanoes in Virginia

 

“Geologists have sometimes been accused of finding beautiful places to work and then finding a project to work on there,” wrote Robert Badger in his preface to "Geology Along the Skyline Drive." “To that charge, I plead guilty.” As a graduate student in the early 1980s at Virginia Tech, he had a pregnant wife and had to forego exotic climes such as Alaska or Hawaii for his field work. Instead, he chose to study the high ground in the Blue Ridge Mountains. There he encountered evidence of our state’s vanished volcanoes.

 

Such clues are scattered across the state. In Richmond, boulders in the James River are remains of a vast area of molten rock known as a pluton -- a term in geology that refers to ancient magma -- liquid rock that hardens below the surface. The Petersburg pluton, as it is called, is a formation that stretches over parts of six counties. It’s 60 miles long and averages 10 miles wide, according to James Beard, curator of earth science at the Virginia Museum of Natural History in Martinsville. If all its magma had erupted at once, Beard suggests, it would have covered the entire state with ash 100 feet thick! ("Those Rocks in the River Have a Hot Story To Tell," -- Richmond Times Dispatch, Oct. 7, 2004)

 

Further west, just outside Harrisonburg, stands Mole Hill in Rockingham County. It’s 1,900 feet high and rises 500 feet above the valley floor. The rock that supports it is the plug, or neck, of a 50-million-year- old volcano and one of two such known formations in Virginia. The other, Trimble Knob, in Monterey in Highland County is much less imposing at 200 feet. (Keith Frye, "Roadside Geology of Virginia," pps. 78, 140.)

 

Actually, 50 million years is quite young for a volcano in our part of the world. According to geologist Badger, some of the volcanic rocks in Shenandoah National Park date to between one and 1.2 billion years. They are granites and gneisses that crystallized as they cooled in magma chambers below ancient volcanoes.

 

To understand our volcanic history through the millennia, it helps to grasp the basics of plate tectonics, which most geologists believe explain the formation of Virginia’s topography. According to this theory, Badger writes, the earth’s crust is divided into large moving plates which contain either continents or the floors of oceans. When they move, the friction causes earthquakes. If they collide, volcanoes and mountains are formed. The Appalachians, as well as the Himalayas and the Andes owe their existence to this process.

 

So, while Virginia appears to be at the eastern edge of the North American continent,, if you define “North America” as the plate we sit on, the state is actually in the center of the Mid-Atlantic Ridge, a plate that extends from somewhere in the Atlantic Ocean to California.

 

Since plates seldom sit still, the Old Dominion is on the move. It is drifting west at about the rate of one inch per year. Since the Jamestown settlers arrived in 1607, it has moved 10 yards. Over the 12,000 years that humans have inhabited the area, it has drifted the length of a football field. (Virginia Places -- Geology.)

 

Geologists theorize that over the past billion or so years, four collisions between plates have helped form the topography of Virginia through volcanic activity and other events. The first, known as the Grenville Orogeny, occurred about one billion years ago and is responsible for the oldest rock in the state – magma that crystallized to form rocks such as granite found in places like Old Rag Mountain near Luray. Three other collisions may have occurred at about 450, 350 and 300 million years ago, resulting in the creation of the Blue Ridge Mountains. (Robert L. Badger, "Geology Along Skyline Drive," p. 9.)

 

Just as plates collide, they separate, as well. These “rifting” events, as they are called, also result in volcanic eruptions, as well as the formation of ocean basins. Geologists believe two such events also contributed to Virginia’s landscape. The first occurred about 560 million years ago and resulted in much of the volcanic rock found in the Shenandoah Mountains. The next event fine-tuned our topography by opening the Atlantic Ocean basin about 200 million years ago.

 

No human eyes witnessed these events. There were probably not even trees, shrubs, tall grasses or other land plants when lava flowed in the Shenandoahs 570 to 565 million years ago. Geologists estimate the age of rocks through radioactive dating, fossil evidence (a prehistoric worm known as Skolithos is found in a layer of sedimentary rock, once beach sand, which covers older volcanic rock in the Shenandoahs) and the study of geologic rock layers. It can be an inexact science; no one can dispute the results.

 

As Badger admits, the volcanic forces and other geological events that molded our terrain make it a stunning place to work (and live). You probably don’t need to be an earth scientist to appreciate that!

 

NEXT: Families in the Mansion: Life in the Governor’s House  

 

-- January 30, 2006

 

 

 

 

 

 

 

 

 

 

 

 

About "Nice & Curious"

 

In 1691, a group of English wits, calling themselves the Athenian Society, founded a publication entitled, "The Athenian Gazette or Causical Mercury, Resolving All the Most Nice and Curious Questions proposed by the Ingenious." The editors accepted questions posed by readers on any and all topics, and sought the most ingenious answers.

 

Inspired by their example, Edwin S. Clay III, president of the Virginia Library Association and Director of the Fairfax County Public Library, created an occasional column on Virginia facts that may require "ingenious answers" of the type favored by those 17th-century wags.

 

If you have a query, e-mail him at [email protected].

 

Fairfax County Public Library staff Patricia Bangs, Lois Kirkpatrick and MaryAnn Sheehan assist in the writing, editing and research of the column.