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North Sister Field
"North and Middle Sister volcanoes anchor the northern end of the Three Sisters volcano group that dominates the landscape of the central Oregon Cascades. Glaciers have deeply eroded the Pleistocene andesitic-dacitic North Sister stratovolcano, exposing the volcano's central plug. North Sister was constructed over the remnants of the basaltic Little Brother shield volcano to the NW. Construction of the main edifice ceased at about 55,000 yrs ago, but N-S-trending fissures north of the volcano were active until at least the latest Pleistocene. Middle Sister volcano, also over 3000 m in elevation, is located only 2 km to the south. The basaltic-to-rhyolitic Middle Sister and its flank vents is less-eroded, but Holocene activity in the North Sister area is restricted to a group of cinder cones north and NW of the North Sister that have produced a series of fresh-looking blocky lava flows on both sides of McKenzie Pass. The youngest lava flow, from Collier Cone, which was erupted about 1600 years ago and traveled 13.5 km to the west, is a prominent feature of the McKenzie Pass area."  -Smithsonian Volcano Archive
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