South Sister Volcano (elevation 10,358 ft/3157m) is the tallest and geologically youngest of the Three Sisters group and the third-tallest mountain in Oregon. South Sister has been active in historic time,last erupting pumice and obsidian flows from flank vents around 2000 years ago. Scientists are currently monitoring a bulge of molten rock that is slowly rising from below South Sister's western flank. South Sister is one of the most popular climbing destinations in the Northwest, attracting hundreds of climbers to its summit on late summer weekends via a physically demanding 6-mile trail up its south side.
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