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Volcanic Fire and Glacial Ice, Summaries of Geology

Geology

Although volcanic activity ended long ago, volcanic rocks form the high mountains in the. Mount Rogers NRA, which are Mount Rogers,. Whitetop Mountain, and Pine ...

Typology: Summaries

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Volcanic Fire
and Glacial Ice
Geologic Wonders
of the George Washington and
Jefferson National Forests
No. 4 in a Series
Mount Rogers National Recreation Area
For additional reading (technical)
Miller, J.M.G., 1994, The Neoproterozoic Konnarock
Formation, southwestern Virginia, USA; Glaciolacustrine
facies in a continental rift, in Deynoux, M[ax], Miller,
J.M.G., Donnack, E.W., Eyles, N., Fairchild, I.J., and Young,
G.M., eds., Earth’s glacial record: New York, Cambridge
University Press, p. 47–59.
Rankin, D.W., 1993, The volcanogenic Mount Rogers
Formation and the overlying glaciogenic Konnarock
Formation—Two Late Proterozoic units in southwestern
Virginia: U.S. Geological Survey Bulletin 2029, 26 p.
Rankin, D.W., Miller, J.M.G., and Simpson, E.L., 1994, Geology
of the Mt. Rogers area, southwestern Virginia Blue Ridge
and Unaka belt, in Schultz, Art, and Henika, Bill, eds.,
Fieldguides to southern Appalachian structure, stratigraphy,
and engineering geology: Virginia Polytechnic Institute
and State University Department of Geological Sciences
Guidebook no. 10, p. 127–176.
Cover: Buzzard Rock, a shoulder of Whitetop Mountain, from
near the peak of Whitetop Mountain; volcanic rocks are in the
foreground. Photograph by Sandra H.B. Clark, U.S. Geological
Survey.
Printed on recycled paper
The United States Department of Agriculture (USDA) and
Department of the Interior (DOI) prohibit discrimination in all
their programs on the basis of race, color, national origin, sex,
religion, age, disability, political beliefs, sexual orientation, or
marital or familial status. (Not all prohibited bases apply to all
programs.)
Persons with disabilities who require alternative means for
communication of program information (Braille, large print,
audiotape, etc.) should contact the USDA’s TARGET CENTER
at (202) 720–2600 (voice and TDD).
To file a complaint of discrimination, write USDA, Director,
Office of Civil Rights, Room 326–W, Whitten Building, 1400
Independence Ave., SW., Washington, DC 20250–9410 or call
(202) 720–5964 (voice and TDD).
USDA and DOI are equal employment opportunity employers
and providers.
For more information online
Visit the USGS at
http://www.usgs.gov
and the Forest Service at
http://www.fs.fed.us/r8/gwj/
U.S. Department of the Interior
U.S. Geological Survey
2007
in cooperation with
U.S. Department of Agriculture
Forest Service
Southern Region
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Volcanic Fire

and Glacial Ice

Geologic Wonders

of the George Washington and

Jefferson National Forests

No. 4 in a Series

Mount Rogers National Recreation Area

For additional reading (technical)

Miller, J.M.G., 1994, The Neoproterozoic Konnarock Formation, southwestern Virginia, USA; Glaciolacustrine facies in a continental rift, in Deynoux, M[ax], Miller, J.M.G., Donnack, E.W., Eyles, N., Fairchild, I.J., and Young, G.M., eds., Earth’s glacial record: New York, Cambridge University Press, p. 47–59. Rankin, D.W., 1993, The volcanogenic Mount Rogers Formation and the overlying glaciogenic Konnarock Formation—Two Late Proterozoic units in southwestern Virginia: U.S. Geological Survey Bulletin 2029, 26 p. Rankin, D.W., Miller, J.M.G., and Simpson, E.L., 1994, Geology of the Mt. Rogers area, southwestern Virginia Blue Ridge and Unaka belt, in Schultz, Art, and Henika, Bill, eds., Fieldguides to southern Appalachian structure, stratigraphy, and engineering geology: Virginia Polytechnic Institute and State University Department of Geological Sciences Guidebook no. 10, p. 127–176. Cover: Buzzard Rock, a shoulder of Whitetop Mountain, from near the peak of Whitetop Mountain; volcanic rocks are in the foreground. Photograph by Sandra H.B. Clark, U.S. Geological Survey. Printed on recycled paper The United States Department of Agriculture (USDA) and Department of the Interior (DOI) prohibit discrimination in all their programs on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or familial status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact the USDA’s TARGET CENTER at (202) 720–2600 (voice and TDD). To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326–W, Whitten Building, 1400 Independence Ave., SW., Washington, DC 20250–9410 or call (202) 720–5964 (voice and TDD). USDA and DOI are equal employment opportunity employers and providers. For more information online Visit the USGS at http://www.usgs.gov and the Forest Service at http://www.fs.fed.us/r8/gwj/ U.S. Department of the Interior U.S. Geological Survey 2007 in cooperation with U.S. Department of Agriculture Forest Service Southern Region

In addition to containing the highest point in Virginia (Mount Rogers, elevation 5,729 feet), the Mount Rogers National Recreation Area (NRA) of the Jefferson National Forest is a window on the history of ancient volcanic eruptions and glacial movement.

How do we know there were volcanoes in

the Mount Rogers NRA?

Although volcanic activity ended long ago, volcanic rocks form the high mountains in the Mount Rogers NRA, which are Mount Rogers, Whitetop Mountain, and Pine Mountain. The vol- canic rocks are predominantly light-colored, fine- grained, high-silica (SiO 2 ) rocks (rhyolite) of the Mount Rogers Formation (Rankin, 1993). The Mount Rogers volcanic center was the site of different types of eruptions; some erup- tions released lava that flowed like a very thick, hot liquid, whereas other eruptions were explosive like the dramatic and destructive 1980 eruption of Mount St. Helens in Washington (fig. 1). Explo- sive eruptions from Mount Rogers produced hot, high-density, fast-moving flows of volcanic ash that formed a sheet of volcanic rock more than one-third of a mile thick. Textures of volcanic rocks provide informa- tion on the type of eruptions that formed them. For example, many volcanic rocks on Whitetop Moun- tain (elevation 5,520 feet) are fine grained and generally featureless, but some rocks have rounded, mineral-filled holes indicating where bubbles of volcanic gases once escaped (fig. 2 A ). Other rocks have fine laminations that formed as layers of hot volcanic ash cooled and ash grains welded together (fig. 2 B ). In other rocks, you can see angular, bro- ken volcanic fragments enclosed in a fine-grained rock that was once molten lava (fig. 2 C ). Figure 1. Pumice and ash eruption at Mount St. Helens, Wash. Some eruptions at Mount Rogers probably looked like this. U.S. Geological Survey photograph by Austin Post, May 18, 1980; for more photographs, see http://vulcan. wr.usgs.gov/ Photo/framework. html. Figure 2. Textures of volcanic rocks from Whitetop Mountain. A , Rock containing rounded, mineral-filled holes formed by escape of gas bubbles. B , Fine laminations formed as ash grains welded together. C , Angular volcanic fragments enclosed in a fine-grained rock. Photographs by Sandra H.B. Clark, U.S. Geological Survey. C B Former A holes

Were the glaciers part of the most recent ice

age?

The Konnarock glaciers were much older than the most recent ice age. Many ice ages have taken place throughout the history of the Earth, and they have been recorded in rocks. The best known is the most recent ice age that began a million or more years ago and covered parts of North America with thick ice sheets; it ended about 10,000 years ago. Although the Mount Rogers area experienced local effects of that ice age, the continental ice sheets did not extend as far south as Virginia. The exact age of the Konnarock glacial depos- its in the Mount Rogers area is not known, but the age can be bracketed between the ages of rocks above and below them. The Konnarock Formation overlies the Mount Rogers Formation, which is 760 million years old (Late Proterozoic) and underlies rocks that are 540 million years old (Cambrian); thus, the Konnarock glacial deposits formed between 760 million and 540 million years ago. It is not known whether the Konnarock glaciation was part of a widespread glacial advance or a local glacia- tion restricted to high elevations.

Where can you see the volcanic rocks and

glacial deposits?

Mount Rogers, Whitetop Mountain, and Pine Mountain are mountains of volcanic rock—rem- nants of an ancient volcanic field. You can see volcanic rocks nearly anywhere you look along roads and trails in these mountains. In contrast, the glacial and lake deposits that solidified into rock can be found in the valleys and on the lower slopes north, south, and west of Whitetop Mountain. To see volcanic and glacial rocks near roads, take Route 16 south from the Mount Rogers National Recreation Area headquarters (fig. 4) for 11 miles to the intersection with Route 603 in Troutdale. Turn right on Route 603, which goes through outcrops of rhythmite (maroon and green lakebed deposits) with dropstones (evidence of floating ice from glaciers). Deposits of rhythmite can be seen in outcrops about 7.7 miles from Troutdale on the right side of Route 603 just beyond a bridge. Rhythmite containing pebbles dropped from float- ing glacial ice can be seen at the east end of a road- cut on Route 603 about 0.1 mile east of its intersec- tion with Route 600 (fig. 4). At the junction of Routes 600 and 603, turn left (south), stay on Route 600, and go about 6. miles to Route 89; turn right (sharp turn). Follow Route 89 to parking at its end near the summit of Whitetop Mountain. Several kinds of volcanic tex- tures can be seen in rocks near the parking area and along Route 89 on the mountain slope. Figure 4. Routes to localities where you can see volcanic, glacial, and lake deposits in the Mount Rogers National Recreation Area (NRA), Jefferson National Forest, southwestern Virginia. Index map shows simplified physiographic provinces in Virginia. Valley and Ridge Map area Appalachian Plateaus Piedmont Blue Ridge VIRGINIA Atlantic Coastal Plain N Troutdale 91 81 TN Scale 0 5 miles Appalachian Trail Marion Chilhowie Damascus Whitetop Mountain Mt. Rogers Mt. Rogers NRA Headquarters 91 81 762 Glade Spring Grayson Highlands VA State Park Konnarock Exit 35 107 Exit 45 762 Exit 29 Mt. Rogers NRA/Jefferson National Forest Buzzard Rock Pine Mountain 11 89 600 603 58 16 603 740 16 600