Tuesday, October 12, 2004
Mount Rogers: Blast from the past
Rocks on Mount Rogers provide clues to events at Mount St. Helens.
The East Coast was rocked by a violent volcanic eruption on the scale of the 1980 Mount St. Helens disaster.
Then came the continental collision that moved mountains.
Fortunately, this all took place millions of years ago. But the evidence remains, and it can be viewed right here in Southwest Virginia.
Mount Rogers technically isn't a volcano, but it's got rock that came from a large eruption 750 million years ago, said Robert Tracy, a Virginia Tech professor who teaches a course on volcanoes. The volcano likely was in North Carolina, but the tectonic plate shifts that created the Appalachian mountain range also moved the volcanic rocks here.
For geologists like Tracy, the rocks are a virtual photo into the past. To experts, the rocks - called rhyolite - can yield insight into how volcanoes work. That insight has been especially handy in recent weeks, as media interest in the impending eruption at Mount St. Helens has created a demand for comment from expert sources like Tracy.
"One of the nice things is they [the volcanic rocks on Mount Rogers] have preserved in them original features from when they were erupted," Tracy said. "That kind of magma erupts in a very violent explosion and makes ash that falls around the volcano in a thick pile."
The resulting pressure from the pile compressed the ash into rock. Scientists aren't sure exactly where the volcano was located, but Tracy said it probably was in North Carolina, near what's now Raleigh.
Despite the eruption's similarities with the 1980 explosion at Mount St. Helens, the reasons behind it are nearly opposite, though both have to do with plate tectonics, the basis for much of modern geology.
The Earth's surface is made up of crustal plates, which are constantly moving, though very slowly. The earthquakes and volcanoes in California, Oregon and Washington result from a plate in the Pacific Ocean moving underneath the plate that makes up North America. As it moves deeper, it melts and becomes magma, liquid rock that drifts back toward Earth's surface.
The volcano that produced the rock on Mount Rogers was formed not because plates were coming together, but because they were moving apart. The movement allowed another way for magma to reach the surface.
That the rock is found here and not at its original location is also because of plate movement. About 280 million years ago, 470 million years after the volcano's eruption, the Appalachian mountains were created as the result of a collision between the continents of Africa and North America. Rocks were shoved over one another, resulting in the mountains being pushed up.
The volcanic rock from the eruption ended up near the top of what is now Mount Rogers.
"There's a good metaphor for this," Tracy said. "Take a rug, hold one edge steady and push the other edge. You can take an 8-foot rug and shrink it down to a foot. Obviously it will be more crumpled and rumpled than it was when you started out. That's basically what happened to the rocks around here."
About 100 million years after the collision, the plate with Africa pulled away, creating the Atlantic Ocean. Since then, the mountains here have been worn down by rain and snow, with areas of softer rock giving way to valleys, and areas of harder rock remaining as ridges and peaks.
"High ridges are typically made up of rock that's tougher or more resistant, usually hard sandstone or granite," Tracy said. "Valleys are more shales and limestones. The reason Mount Rogers is such a high mountain is not because it's a volcano but because the rocks at the top are very hard and resist erosion."
Interestingly, the Mount Rogers area is home not only to volcanic rock, but also rock that came from glaciers. Near the town of Konnarock, people can find rocks known as conglomerates, which have lots of solidified fine grain but also bigger chunks of other types of rock. Conglomerates are formed when glaciers melt back.
Tracy said it may not be too long before volcanoes make a comeback on the East Coast.
Currently, North America is on a plate that extends from the West Coast out to the middle of the Atlantic Ocean. For the last 160 million years or so, things have been pretty quiet.
That should be about to change.
"What's now under the Atlantic will start to dive down under the East Coast," Tracy said. "It will be a mirror image of what's happening on the West Coast now. We'll start to have volcanos. It'll be like the Cascades."
That should happen sometime in the next 50 million years, which is a short time - geologically speaking.
VIRGINIA'S VOLCANO -- SORT OF
Q: How old are the rocks on top of Mount Rogers?
A: About 750 million years old.
Q: Where did they come from?
A: A volcano erupted in a violent explosion and created those rocks.
Q: Created them how?
A: Remember all the ash from the 1980 Mount St. Helens eruption? This eruption was similar. The ash fell around the volcano in a pile and then was compressed into rock.
Q: So Mount Rogers used to be a volcano?
A: No. The rocks there came from a volcanic eruption, but it likely took place somewhere off to the east or southeast, possibly near what's now Raleigh, N.C.
Q: How did the rocks end up here then?
A: About 280 million years ago the Appalachian mountains were created when the continents of Africa and North America collided. When that happened, the East Coast crinkled like a rug squished against a wall, and the volcanic rocks were shoved westward and ended on top of what later became Mount Rogers.
Q: Where can I see these rocks?
A: They're out in the open at Massie Gap in Grayson Highlands State Park, where anybody can hike up and check them out.