Available Weekly Reports
24 March-30 March 2004
On 23 March at around 1000 a seismic swarm began at South Sister that lasted ~48 hours. Over 300 volcano-tectonic earthquakes up to M 1.9 were recorded. The earthquakes were located in the NE quadrant of the area of on-going uplift.
Sources: US Geological Survey Cascades Volcano Observatory (CVO) , Pacific Northwest Seismic Network
24 March 2004Back to Top
2 May 2001Back to Top
Summary of eruption dates and Volcanic Explosivity Indices (VEI).
|Start Date||Stop Date||Eruption Certainty||VEI||Evidence||Activity Area or Unit|
|[ 1853 Jul ]||[ Unknown ]||Uncertain|
|50 BCE (?)||Unknown||Confirmed||3||Radiocarbon (uncorrected)||North & south flanks (Devils Hill)|
|350 BCE (?)||Unknown||Confirmed||4||Radiocarbon (uncorrected)||SW flank (Rock Mesa)|
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 3157-m-high 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.