The amphitheater at the summit of Broken Top. Olivine basalt lava flows are in the foreground. These flows surround the mountain. Photo courtesy of the Oregon State Highway Tourism Division.
The amphitheater at the summit of Broken Top. Olivine basalt lava flows
are in the foreground. These flows surround the mountain. Photo
courtesy of the Oregon State Highway Tourism Division.
Broken Top is a complex stratovolcano. The base of the volcano is a shield made of basaltic andesite. The cone is made mostly of mafic lava flows with lesser amounts of andesite, dacite, and rhyodacite lava flows and tephra. The original crater formed by subsidence and was filled by lava flows. Later, the interior of the volcano was exposed by glacial erosion. Broken Top has not erupted in the last 10,000 years.
Sources of Information:
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.
Wood, C.A., and Kienle, J., 1993, Volcanoes of North America: Cambridge University Press, New York, 354 p.
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