Broken Top

Broken Top is an extinct, glacially eroded stratovolcano in Oregon, part of the extensive Cascade Range. Located southeast of the Three Sisters peaks, the volcano, residing within the Three Sisters Wilderness, is 20 miles (32 km) west of Bend, Oregon in Deschutes County. Eruptive activity stopped roughly 100,000 years ago, and currently, erosion by glaciers has reduced the volcano's cone to where its contents are exposed. Bend and Crook Glaciers are the two named glaciers on the peak.

Geology

Due to extensive erosion, the contents of Broken Top's cone are exposed, which allows volcanologists to classify Broken Top as a complex stratovolcano. Its initial eruptions during the Pleistocene, consisting of basaltic andesite lava, contributed to a base shield. The center of this edifice consists of agglomerate, which was oxidized, and intrusive dikes and geologic sills. Erratic eruptions continued afterward, erupting silicic lava. Much of the current cone is occupied by flows of andesite, dacite, rhyodacite, and pyroclastic flows; these deposits extending from the subordinate volcano to the summit. The volcanic crater, now 0.8 km, was most likely created through subsidence.

Wilderness

Broken Top resides in the Three Sisters Wilderness, which makes up 242,000 acres (980 km²) of the Willamette National Forest. About the area, there are geologic features such as Collier Glacier, Rock Mesa, Collier Cone, and Yapoah Crater.

Permits for average entry are distributed, free, from Memorial Day (May 31)â€"Halloween (October 31). To gain access to the Obsidian area of the wilderness, special entry is required, for both day and overnight hikers. These passes are distributed only at McKenzie Ranger District in Willamette Forest and Sisters Ranger District in Deschutes Forest. The administration disallows any vehicle entry.

See also

• Cascade Volcanoes
• Geology of the Pacific Northwest

References

External links

• "Broken Top". SummitPost.org. Retrieved 2011-05-07. 
• "Broken Top". SkiingtheBackcountry.org. Retrieved 2009-12-07.