- Status: South Sister is monitored by the Cascade Volcano Observatory and is at a background level of activity.
- Volcanic History Overview: South Sister is the highest and youngest of the Three Sisters volcanoes that dominate the landscape of the central Oregon Cascades. The main edifice of South Sister is constructed of andesitic and dacitic lava flows capped by a symmetrical summit cinder cone of probable latest-Pleistocene age. The late Pleistocene or early Holocene Cayuse Crater on the SW flank of Broken Top volcano and other flank vents such as Le Conte Crater on the SW flank of South Sister mark mafic vents that have erupted at considerable distances from South Sister itself. Late-Holocene eruptions formed a chain of dike-fed rhyodacitic lava domes and flows on the volcano's SE-to-SW flanks about 2000 years ago. Satellite radar interferometry (InSAR) data obtained by U S Geological Survey scientists detected continuing long-term slight uplift of the ground surface over a broad region centered 5 km west of South Sister volcano that began in 1997.
- Location: Western US, OR
Elevation: 3157 m
Recent Eruption: Chain of rhyodacitic lava domes and flows on the volcano's SE-to-SW flanks 2000 years ago.
- Hazard Assessments: Scott, et.al., 2001, Volcano Hazards in the Three Sisters Region, Oregon, USGS Open-File Report 99-437.
- Link to monitoring data: The Pacific Northwest Seismic Network
Volcanic Alert Level: NORMAL Aviation Color Code: GREEN