“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!
January 30, 2006
Nice & Curious
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Last Modified:
Friday, June 27, 2008
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