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Three Fingered Jack is a deeply eroded basaltic shield volcano. Glaciers have carved about 2,600 feet (800 m) of relief into the volcano. The volcano is estimated to be between 500,000 and 250,000 years old.
Summit cone of Three Fingered Jack. Photo courtesy of the Oregon Department of Geology and Mineral Industries.
Sources of Information:
Wood, C.A., and Kienle, J., 1993, Volcanoes of North America: Cambridge University Press, New York, 354 p.