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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
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