Description of the Three Sisters
Aerial view of North and Middle Sister Photo by Ken McGee
The Three Sisters areas is located in the Central Oregon High Cascades. This is part of the Cascade Range which extends from northern California to northern Washington (USGS, 2002a). The area has a temperate maritime climate and the mountains act as major barriers, intercepting eastward-flowing moisture (USGS, 2002d). As a result, there are warm, dry summers and higher precipitation in the winter. Areas of higher elevation, like the Three Sisters, receive most of their precipitation as snowfall.
The Three Sisters are comprised of North Sister, Middle Sister, and South Sister. Formerly, they were called "Faith", "Hope", and "Charity" (Fukuchi, 1999).
North Sister is 3,074 meters in height and is a stratovolcano atop a shield volcano (USGS, 2002d). North Sister is deeply dissected and has probably been inactive for at least 100,000 years (Jacobs, 1996). This mountain consists mostly of crumbling rock, so reaching the summit is difficult.
Middle Sister is 3,062 meters in height and is a stratovolcano. Middle Sister is younger than North Sister and was active in the late Pleistocene but not in postglacial time (USGS, 2002d). This mountain is less eroded than North Sister, but more so than South sister. The erosion contributes to its more pointed appearance.
South Sister is 3,157 meters in height and is a stratovolcano. South Sister is older than 25,000 years. However, eruptions of rhyolite from flank vents have occurred as recently as 2,000 years ago (USGS, 2002d). South Sister is the least eroded and maintains its conical shape. It has a well-preserved summit crater that is usually filled with snow. During the summer, the snow melts and forms the highest lake in Oregon (Fukuchi, 1999).
Aerial view of Three Sisters from the southeast Photo by William E. Scott, USGS
The Three Sisters area has the most extensive glaciation in the Central Oregon High Cascades and is the most developed system of glaciers thus far in North and South America. The area has 17 named glaciers that cover approximately 7.5 kilometers (USGS, 2002d). The largest glacier, the Collier, is located between the North and Middle Sisters on their western slope. The presence of the glaciers has led to glacial erosion which has worked to reveal the inner structure of these mountains (USGS, 2002b).
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