The USGS Cascades Volcano Observatory reported on 8 May that between 1996 and 2000 they detected slight uplift of the ground surface over a broad area 5 km W of South Sister volcano. The uplift occurred over an area ~15-20 km in diameter and the maximum amount of uplift at its center is ~10 cm. CVO personnel believe the uplift may reflect intrusion of a small volume of magma at ~7 km depth beneath the ground surface. They stated that if the intrusion of magma continues it could eventually lead to a volcanic eruption, but precursory activity would most likely occur beforehand. There is no precursory activity that suggests an eruption is imminent; seismic activity at the volcano is near or below background levels, gas emissions are low, and no unusual surface changes have been observed.

Geological Summary. The north-south-trending Three Sisters volcano group dominates the landscape of the Central Oregon Cascades. All Three Sisters stratovolcanoes ceased activity during the late Pleistocene, but basaltic-to-rhyolitic flank vents erupted during the Holocene, producing both blocky lava flows north of North Sister and rhyolitic lava domes and flows south of South Sister volcano. Glaciers have deeply eroded the Pleistocene andesitic-dacitic North Sister stratovolcano, exposing the volcano's central plug. Construction of the main edifice ceased at about 55,000 yrs ago, but north-flank vents produced blocky lava flows in the McKenzie Pass area as recently as about 1600 years ago. Middle Sister volcano is located only 2 km to the SW and was active largely contemporaneously with South Sister until about 14,000 years ago. South Sister is the highest of the Three Sisters. It was constructed beginning about 50,000 years ago and was capped by a symmetrical summit cinder cone formed about 22,000 years ago. The late Pleistocene or early Holocene Cayuse Crater on the SW flank of Broken Top volcano and other flank vents such as Le Conte Crater on the SW flank of South Sister mark mafic vents that have erupted at considerable distances from South Sister itself, and a chain of dike-fed rhyolitic lava domes and flows at Rock Mesa and Devils Chain south of South Sister erupted about 2000 years ago.

Sources: Associated Press, US Geological Survey Cascades Volcano Observatory (CVO)