North Sister volcano, along with Middle and South Sister part of the Three Sisters Group in central Oregon Cascades. The group forms a prominent landmark in the Central Oregon Cascades.
North Sister is the glacially eroded remnant of a andesitic-dacitic stratovolcano, exposing the volcano's central plug.
Middle Sister volcano, also over 3000 m in elevation, is located only 2 km to the south. It is basaltic-to-rhyolitic in composition and less eroded than North Sister, but there are no known eruptions in the past 10,000 years.
440 AD ± 150, 40 AD ± 200, 800 BC (?), 7350 BC ± 1000 (all radiocarbon dated)
effusive and explosive
North Sister was built over the remnants of the basaltic shield of Little Brother volcano to the NW. The main volcanic edifice of North Sister was built before 55,000 yrs ago, but activity continued along N-S-trending fissures north of the volcano, at least until about 10,000 years ago.
Volcanic activity past the last Ice Age from the North Sister area is restricted to a group of cinder cones north and NW of the North Sister. They erupted a series of fresh-looking blocky lava flows visible on both sides of McKenzie Pass. The youngest of these flows came from Collier Cone and has been radiocarbon-dated to about 1600 years ago. It reached a length of 13.5 km to the west and is one of the main geologic features in area of the McKenzie Pass.
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