South Sister, also known as "Charity," is the youngest and tallest volcano of the trio. Its eruptive products range from basaltic andesite to rhyolite and rhyodacite. It is a stratovolcano overlying an older shield structure, no more than 50,000 years old, which last erupted about 2000 years ago. The first such episode, termed the Rock Mesa eruptive cycle first spread tephra from flank vents from the south and southwest flanks, followed by a thick rhyolite lava flow. The second cycle, the Devils Hill eruptive cycle, was similar in result, but was caused by the intrusion of a dike of new silicic magma which erupted from about twenty vents on the southeast side, with a smaller line on the north side.
South Sister has an uneroded summit crater about 0.25 mi (0.4 km) in diameter, which holds a small crater lake known as Teardrop Pool, the highest lake in Oregon. The slopes of South Sister have a number of small glaciers including the Lewis, Clark, Lost Creek and Prouty Glaciers, near the crater rim. The standard climbing route up the South Ridge of South Sister is a long, steep, non-technical hike that can be easily completed in a day by reasonably fit hikers. Popular starting points are the Green Lakes or Devil's Lake trailheads.
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