44.10 N, 121.77 W
summit elevation 3157 m
South Sister is the highest and youngest of the Three Sisters volcanoes in central Oregon Cascades. The volcano contains cinder cones and lava flows.
Uplift continued at South Sister volcano, but at a reduced rate. The area of uplift is 20 km in diameter and is centered 5 km W of South Sister volcano. Only five earthquakes were located in the uplift area in 2005. The uplift in 2004-2005 was only half that recorded previously.
2004 Seismic Swarm
On 23rd March a seismic swarm produced 1000 earthquakes in 48 hours. The earthquakes were centred in the area which had been experiencing uplift.
1996-2001 Volcano inflation
In September 2001 a European satellite circling about 500 miles above Earth took photos that included precise measurements of the surface topography. Comparing it with earlier images, researchers found that the area centered three miles west of South Sister had continued bulging upward at more than an inch a year. That brings its total swelling to about 5 inches in five years.
Water sampling has shown conclusively that magma is present under the volcano. Also a small amount of carbon dioxide also has been detected in the atmosphere above the region, which is consistent with the idea that magma is responsible for the uplift.
If it continues, it could eventually culminate in an eruption. This may be in 5, 10 or 20 years. Nobody can tell if and when an eruption will occur. However this is the most significant volcanic event in the Cascades Range since the eruption of Mt St Helens in 1980.
1853?, 2000 years ago.