Three Sisters

Eruptions in the Cascade Range during the past 4000 years. USGS GIP 64

Mount Jefferson (pointed peak in middle) and Three Sisters (to the right) in Oregon, south-facing aerial view as seen from Mount St. Helens.

Global Positioning System receiver (dome on pole) and seismometer (box on ground in distance) at a volcano monitoring station for Three Sisters in Central Oregon. Solar panel mounted to pole.

Three Sisters volcanoes (left to right - South, Middle, and North) viewed from the south, Oregon. Foreground rhyolite Newberry flow is one of most recent eruptive units (2.2 ka).

During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the flanks of Mount St. Helens. Pyroclastic flows typically move at speeds of over 60 miles per hour (100 kilometers/hour) and reach temperatures of over 800 Degrees Fahrenheit (400 degrees Celsius).

Three Sisters, Oregon simplified hazards map showing potential impact area for ground-based hazards during a volcanic event.

Radially dipping strata of the cone include subequal proportions of lava flows and flow breccias and of proximal agglutinate and scoria falls, nearly all having 54 to 56 percent SiO2. Numerous mafic dikes and sills intrude the cone, and an apron of compositionally similar mafic lavas extends far outward from the cone. Skyline crags behind buttress at right are top of a