South Sister Mountain from Hosmer Lake, Oregon. South Sister?is the youngest?and tallest volcano of a trio of volcanic peaks known as the Three Sisters,?of the?Cascade Volcanic Arc?and the?Cascade Range?in?Oregon, each of which exceeds 10,000?ft (3,048 m) in elevation.?The region was a volcanic center in the?Pleistocene?epoch, with eruptions from 600,000 - 700,00 to 170,000 years ago, an explosively active complex known as the Tumalo volcanic center. Its eruptive products range from basaltic and