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South Sister volcanoSouth Sister (also known as Charity) is the highest and youngest of the Three Sisters volcanic group in the central Cascades, Oregon. It dominates the landscape and is a popular destination for climbers and hikers.
Background:The lavas from South Sister range from basaltic andesite to rhyolite and rhyodacite. The main edifice of South Sister is less than 55,000 years old and was formed as a shield of andesitic and dacitic lava flows.
The first eruptive phase has been termed the Rock Mesa eruptive cycle. There are tephra deposits from flank vents on the south and southwest flanks and a thick rhyolite lava flow.
At the top of South Sister is a symmetrical summit cinder cone, which probably is about 10,000 years old. A second eruptive cycle, called the Devils Hill eruptive cycle, was caused by the intrusion of a dike of new silica-rich magma and eruptions from about 20 vents on the southeast side, to form a chain of rhyodacitic lava domes and flows on the volcano's SE-to-SW flanks. This activity lasted up until about 2000 years ago.
It is suggested that the magma reservoir under the South Sister has also fed some flank vents at great distance, such as the Cayuse Crater on the SW flank of Broken Top volcano and Le Conte Crater on the far SW flank of South Sister.
South Sister has an intact summit crater about 0.25 mi (0.4 km) in diameter with a small crater lake, Teardrop Pool, the highest lake in Oregon.
There are 2 small glaciers, Lewis and Clark glaciers, near the crater rim.
March 2004 seismic swarm
On 23-24 March 2004, a seismic swarm with ca. 1000 earthquakes within 48 hours occurred in the same area that had been experiencing uplift.
Satellite radar interferometry (InSAR) data from the USGS detected continuing slight uplift of a broad region centered 5 km west of South Sister volcano that began in 1997. ...more
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