Mount Bachelor (is a broad shield volcano capped by a steep-sided summit cone. It is part of the 25-km-long Mount Bachelor volcanic chain, which contains numerous scoria cones with related lava flows and two other shield volcanoes. The chain covers about 250 km2 and constitutes a total magma volume of 30-50 km3.
Most eruptions in the Mount Bachelor volcanic chain were effusive but minor explosive activity built cones of agglutinated spatter, bombs, and scoria. Eruptions began about 18,000-15,000 years ago and constructed a shield volcano capped by Sheridan Mountain. Kwolh Butte, the shield that underlies Mount Bachelor, and most of the summit cone of Mount Bachelor were formed by about 12,000 years ago. The most recent eruption was about 8,000 to 10,000 years ago on the lower north flank of Mount Bachelor. It produced a scoria cone and lava flows. Ash from the climatic eruption of Mount Mazama blankets all volcanic deposits of the Mount Bachelor volcanic chain, indicating activity had ceased by 6,845 years ago.
Photograph of Mount Bachelor by Kyle Jones, August 1986.
Additional information about Mount Bachelor is presented on the Cascade Volcano Observatory homepage of the U.S. Geological Survey.