North Sister is a shield volcano made of basaltic andesite. The shield is about 5 miles (8 km) in diameter. The summit cone is made of cinders, lava flows, dikes, sills, and a central plug. The flanks of North Sister have been cut by Pleistocene and Holocene glaciers.
Middle Sister formed after North Sister. Basalt was erupted from fissures on the flanks of the volcano. Andesite lava flows were erupted from a vent near the base of the volcano. Domes and flows of dacite and rhyodacite also make part of Middle Sister. The most recent eruptions were from vents high on the north and south sides of the volcano. The produced long dacite lava flows. Middle Sister has also been eroded by glaciers.
South Sister started erupting as activity waned at Middle Sister . South Sister is the youngest of the Three Sisters. A summit crater was formed by a late Pleistocene eruption. The main cone is made of andesite, dacite, and rhyodacite. The most recent eruption was about 1,900 years ago. Glaciers carved cirques in the volcano but did not remove the summit crater.
Additional information about Three Sisters is presented on the Cascade Volcano Observatory homepage of the U.S. Geological Survey.
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Uplifting is still ongoing at South Sister. On March 23 activity increased
in the NE quadrant of the uplifting area: a 48-hour long period of seismic
swarm, with more than 300 volcano-tectonic earthquakes (< M 1.9), was recorded.
On 8 May, the USGS Cascades Volcano Observatory detected a slight uplift of the ground surface over a broad area 5 km W of the South Sister volcano. The uplift may reflect the intrusion of a small volume of magma at ~7 km beneath the ground surface. CVO reported that if the intrusion of magma continues it could over time result in a volcanic eruption. However, there is no precursory activity that suggests an eruption is imminent.
This information was summarized from the GVP/USGS Weekly Volcanic Activity Report
Sources of Information:
Scott, W.E., 1987, Holocene rhyodacite eruptions on the flanks of South Sister volcano, Oregon: Geological Society of America Special paper 212, p. 35-53.
Scott, W.E., Gardner, C.A., Sarna-Wojcicki, A.M., 1989, Guidebook for field trip to the Mount Bachelor-South Sister-Bend area, central Oregon High Cascades: U.S. Geological Survey Open-file Report, 89-645, p. 1-68.
Taylor, E.M., MacLeod, N.S., Sherrod, D.R., and Walker, G.W., 1987, Geologic map of the Three Sisters Wilderness, Deschutes, Lane, and Linn Counties, Oregon: U.S. Geological Survey Map MF-1952.
Wood, C.A., and Kienle, J., 1993, Volcanoes of North America: Cambridge University Press, New York, 354 p.
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