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South Sister

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  • Country
  • Subregion Name
  • Volcano Type
  • Last Known Eruption
  • 3157 m
    10355 ft
  • 44.103°
  • -121.768°
  • Elevation
  •  
  • Latitude
  • Longitude

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



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2004: March |
2001: May |

24 March 2004              Back to Top

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

2 May 2001              Back to Top

The USGS Cascades Volcano Observatory reported on 8 May that between 1996 and 2000 they detected slight uplift of the ground surface over a broad area 5 km W of South Sister volcano. The uplift occurred over an area ~15-20 km in diameter and the maximum amount of uplift at its center is ~10 cm. CVO personnel believe the uplift may reflect intrusion of a small volume of magma at ~7 km depth beneath the ground surface. They stated that if the intrusion of magma continues it could eventually lead to a volcanic eruption, but precursory activity would most likely occur beforehand. There is no precursory activity that suggests an eruption is imminent; seismic activity at the volcano is near or below background levels, gas emissions are low, and no unusual surface changes have been observed.

Sources: US Geological Survey Cascades Volcano Observatory (CVO) , Associated Press



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.