Three Sisters (Oregon)
The Three Sisters looking North
|Elevation||South 10,358 ft (3,157 m)
Middle 10,047 ft (3,062 m)
North 10,085 ft (3,074 m)
|Prominence||South 5,598 ft (1,706 m)
Middle 1,127 ft (344 m)
North 2,725 ft (831 m)
|Location||Lane / Deschutes counties, Oregon, U.S.|
|Coordinates||South Coordinates: 
|Topo map||USGS South Sister and North Sister|
|Type||Stratovolcanoes and shield volcanoes|
|Age of rock||Quaternary|
|Volcanic arc||Cascade Volcanic Arc|
|Last eruption||about 50 BC|
|Easiest route||hiking or scrambling, plus glacier travel on some routes|
The Three Sisters are three volcanic peaks of the Cascade Volcanic Arc and the Cascade Range in Oregon, each exceeding 10,000 feet (3,000 m) in elevation. They are the third, fourth, and fifth highest peaks in the state of Oregon and are located in the Three Sisters Wilderness, about 15 miles (24 km) southwest from the nearest town of Sisters.
Although they are often grouped together and seen as one unit, the three mountains were formed under differing geologic situations, and the petrologic composition of each mountain can vary significantly.
South and Middle Sister do not require any rock climbing skills to ascend. South Sister in particular is a well known "first mountain" for many people. Although it does not require any technical skills or much in the way of mountaineering knowledge, many environmental dangers remain the same, and rescue operations are common during the climbing season. Jon Krakauer. writer of Into Thin Air, has frequently noted South Sister as his inspiration for getting into mountaineering. In his admissions essay to Amherst College, he mentioned that his interest in alpinism "really grew by the time [he] was eight and climbed the South Sister in Oregon".
The Three Sisters are located on the boundary of Lane and Deschutes County, and the Willamette and Deschutes National Forest. The three peaks have 15 named glaciers among them, nearly half of all the 35 named glaciers in Oregon. The Sisters were named Faith, Hope, and Charity by early settlers, but "these names have not prevailed", and instead they are named North Sister, Middle Sister, and South Sister.
The Three Sisters are the centerpiece of a region of closely grouped volcanic peaks, an exception to the typical 40-to-60-mile (64 to 97 km) spacing between volcanoes elsewhere in the Cascades. Nearby peaks include Belknap Crater, Mount Washington, Black Butte, and Three Fingered Jack to the north, and Broken Top and Mount Bachelor to the south. The region was a volcanic center in the Pleistocene epoch, with eruptions between 700,000 and 170,000 years ago from an explosively active complex known as the Tumalo volcanic center. Basaltic lava flows from North Sister overlay the newest Tumalo pyroclastic deposits, making the age of North Sister, the oldest of the three, less than 170,000 years.
North Sister, also known as "Faith," is the oldest and most eroded of the three, with towering rock pinnacles and glaciers. It has not erupted since the late Pleistocene. It is the most dangerous climb of the Three Sisters due to its level of erosion and thus rockfall.
North Sister is a shield volcano consisting primarily of basaltic andesite. It is estimated that it last erupted over 100,000 years ago and is considered extinct. The North Sister possesses more dikes than any similar Cascade peak. Many dikes were pushed aside by the intrusion of a 300-metre (980 ft)-wide plug dome that now forms the mountain's summits of Prouty Peak and the South Horn. The remainder of the peak is chiefly loose debris held in place by the dike system.
Middle Sister also known as "Hope," is a stratovolcano consisting primarily of basalt, but it also has erupted andesite, dacite, and rhyodacite. Having last erupted approximately 50,000 years ago, it is considered extinct. The mountain is shaped as a cone that has lost its east side to glaciation. The Hayden and Diller glaciers continue to cut into the east face while the Renfrew Glacier sits on the northwestern slope. The large but retreating Collier Glacier descends along the north side of Middle Sister and cuts into North Sister's west side. Middle Sister is the smallest and most poorly studied. It is also the middle in age, but its most recent flows are dated to 14,000 years ago, slightly older than South Sister's.
Although the climb is ostensibly non-technical, roped ascents are common due to exposure. Climbers typically start at Camp Lake between South and Middle Sister.
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 that erupted from about 20 vents on the southeast side and from a smaller line on the north side.
South Sister has an uneroded summit crater about 0.25 miles (0.40 km) in diameter that 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 completed in a day by reasonably fit hikers; Jon Krakauer noted in Into the Wild that his first climb of South Sister with his father inspired him to take up mountaineering. Popular starting points are the Green Lakes and Devil's Lake trailheads.
In 2000 satellite imagery showed that there was a deforming tectonic uplift 3 miles (4.8 km) west of the South Sister. The ground likely began to bulge in late 1997, when magma started to pool about 4 miles (6.4 km) underground. There was concern that the volcano was awakening. A map at the Lava Lands Visitor Center of the Newberry National Volcanic Monument south of Bend shows the extent of the uplift, which reaches a maximum of 11 inches (28 cm). In 2004 an earthquake swarm occurred with the epicenter in the area of uplift. The hundreds of small earthquakes subsided over several days. By 2007 the uplift had slowed somewhat, though the area was still considered potentially volcanically active. In February 2013 scientists determined that the uplift had slowed to a rate of about 0.3 inches (7.6 mm) per year, compared to up to 2 inches (51 mm) per year in the early 2000s.
|Wikimedia Commons has media related to Three Sisters.|
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- "Bulge at Oregon's South Sister volcano barely growing". The Register-Guard (Eugene, Oregon). Associated Press. 2013-02-05. Retrieved 2013-05-26.
- "Three Sisters, Oregon Information Statement". U.S. Geological Survey. 2007-04-11. Retrieved 2008-04-02.
- Bishop, Ellen M.; John E. Allen (2004). Hiking Oregon's Geology. Seattle, Washington: Mountaineers Books. ISBN 978-0-89886-847-0.
- Harris, Stephen L. (2005). "Chapter 13: The Three Sisters". Fire Mountains of the West: The Cascade and Mono Lake Volcanoes (Third ed.). Missoula, Montana: Mountain Press. ISBN 0-87842-511-X.
- McArthur, Lewis A. (1974). Oregon Geographic Names (Fourth ed.). Portland, Oregon: Oregon Historical Society. ISBN 0-87595-102-3.
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- Scott, W.E.; R.M. Iverson, S.P. Schilling, and B.J. Fisher (2001-02-15). "Volcano Hazards in the Three Sisters Region, Oregon". Open-File Report 99-437. U.S. Geological Survey. Retrieved 2008-05-01.
- "South Sister". NGS data sheet. U.S. National Geodetic Survey. Retrieved 2008-04-02.