Eruption History

The cluster of glaciated stratovolcanoes called the Three Sisters extends for 20 km (12 mi) along the crest of the Cascade Range in Oregon, 35 km (20 mi) west of the city of Bend and 100 km (60 mi) east of Eugene.

South Sister is the southernmost and tallest of the Three Sisters. Early eruptions began between 50 to 45 ka, when South Sister produced numerous rhyolite lava flows and domes. Between 37 and 30 ka, the broad cone was built of rhyolite to andesite domes and flows, and construction of the cone ended with an explosive eruption that left pyroclastic deposi

Middle Sister formed between 40 and 14 ka, but mostly between 25 and 18 ka, closely in time with South Sister. It has erupted basaltic andesite, andesite, and dacite lavas from its central-vent area, as well as dacites from six flank vents and andesites from three more.