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.|
|Topo map||USGS South Sister and North Sister|
|Type||Complex volcano, made up of stratovolcanoes and a shield volcano|
|Age of rock||Quaternary|
|Volcanic arc||Cascade Volcanic Arc|
|Last eruption||50 BC (?)|
|Easiest route||Hiking or scrambling, plus glacier travel on some routes|
The Three Sisters form a complex volcano comprised by three volcanic peaks of the Cascade Volcanic Arc and the Cascade Range in the U.S. state of 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 10 miles (16 km) south of the nearest town of Sisters. Diverse species of flora and fauna inhabit the area on and around the mountains, which is subject to frequent snowfall, occasional rain, and extreme temperature differences between seasons. The mountains, particularly South Sister, are popular for climbing and scrambling.
Although they are often grouped together and seen as one unit, the three mountains evolved under differing geologic situations, and the petrologic composition of each mountain can vary significantly. Whereas North Sister is extinct and Middle Sister is inactive, South Sister last erupted about 2,000 years ago and still could erupt, threatening life within the region. After satellite imagery detected uplift near South Sister in 2000, the United States Geological Survey made plans to improve monitoring in the immediate area.
Geography and geology
The Three Sisters are located on the boundaries of Lane and Deschutes counties and the Willamette and Deschutes national forests in the U.S. state of Oregon, about 10 miles (16 km) south of the nearest town of Sisters. The three peaks, the third, fourth, and fifth highest peaks in Oregon, have 15 named glaciers among them, nearly half of 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.
A complex volcano that extends for 20 miles (32 km), the Three Sisters are located at latitude 44.103° N and longitude 121.768° W. Part of the Cascade Volcanic Arc and the Cascade Range, the volcanoes formed during separate time periods from several varieties of magma, and the amount of rhyolite found in the lava of the younger two mountains is unusual relative to nearby volcanoes. Like other Cascade volcanoes, the Three Sisters were fed by magma chambers produced by the subduction of the Juan de Fuca Plate under the western edge of the North American Plate. They were also affected by the changing atmosphere of the Pleistocene epoch, a geologic period during which an Ice Age occurred where glaciers ate away at mountains as they retreated. Specifically, Three Sisters join several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends from north–south.
The Three Sisters form 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. These 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 active 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, placing the age of North Sister, the eldest, at less than 170,000 years.
Wilderness and climate
The Three Sisters Wilderness covers an area of 281,190 acres (1,137.9 km2), making it the second largest wilderness area in Oregon. Created by the U.S. Congress in 1964, it borders the Mount Washington Wilderness to the north and shares a southern edge with the Waldo Lake Wilderness. The Three Sisters Wilderness includes 260 miles (420 km) of trails, in addition to many forests, lakes, waterfalls, and streams, such as the source of Whychus Creek. More specifically, the Three Sisters and Broken Top account for about a third of the Three Sisters Wilderness—an area known as the Alpine Crest Region. Rising from about 5,200 feet (1,600 m) to 10,358 feet (3,157 m) in elevation, it features the wilderness area's most-frequented glaciers, lakes, and meadows.
Weather in the area varies because of separation created by the mountain barrier. The west is wetter and the east is drier, and precipitation increases with elevation. The moisture, falling mostly as winter snow, originates from warm air pockets over the Pacific Ocean and cools as it ascends the mountains. In the western section, precipitation ranges from 80 to 125 inches (200 to 320 cm) annually, varying from 40 to 80 inches (100 to 200 cm) in the east. Temperature extremes reach 80 to 90 °F (27 to 32 °C) in summers and −20 to −30 °F (−29 to −34 °C) during the winters.
In the wilderness, part of the National Wilderness Preservation System, motor vehicles and means of transport are prohibited. These include bicycles, wagons, motorboats, and helicopters. From Memorial Day to October 31, permits are required for entry to the wilderness.
Flora and fauna
While Douglas fir and ponderosa pine trees predominate in the western and eastern parts of the Cascades, respectively, flora in the Three Sisters Wilderness also consists of lodgepole pine, Pacific silver fir, subalpine fir, western hemlock, western white pine, Englemann spruce, and western redcedar. White fir, once a common species in the area, has declined due to recent blights of mountain pine beetle. At higher altitudes, alpine meadows are common, and they feature the flowers lupine, red Indian paintbrush, heather, arnica, larkspur, sunflowers, columbines, and many other genera. Also common at high altitudes are mountain hemlock and whitebark pine trees.
The local fauna of the Three Sisters includes fish such as brook and rainbow trout, birds such as blue and ruffed grouse, and larger species like the Columbian black-tailed deer, mule deer, Roosevelt elk, and American black bear. In addition to predator bobcats, cougars, and coyotes, raccoons, martens, weasels, and American minks can be found throughout the Three Sisters area.
North Sister, also known as "Faith," is the oldest and most eroded of the three, with towering rock pinnacles and glaciers. A stratovolcano atop an ancient shield volcano named Little Brother, North Sister is 5 miles (8.0 km) wide. Consisting primarily of basaltic andesite, it is more mafic (rich in magnesium and iron) than the other two volcanoes. Its deposits are rich in palagonite and red and black cinders, and grow progressively more iron-rich with decreasing age. North Sister's lava flows demonstrate similar composition throughout its eruptive history, which lasted for a long period of time. Estimates place the volcano's last eruption at more than 100,000 years ago, so the volcano is considered extinct. The North Sister possesses more dikes than any similar Cascade peak, and many dikes were pushed aside by the intrusion of a 980-foot (300 m)-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.
North Sister has not erupted since the late Pleistocene. Extensive erosion exposed the volcanic edifice, including magma pathways such as dikes and sills. At one point, the volcano stood more than 11,000 feet (3,400 m) in elevation, but this erosion claimed a quarter to a third of its original volume.
Middle Sister also known as "Hope," is a stratovolcano consisting primarily of basalt, but it also has erupted andesite, dacite, and rhyodacite. The smallest and most poorly-studied of the trio, it is also the middle in age, its most recent flows dating to 14,000 years ago, making them slightly older than South Sister's. These flows, thick and rich in dacite, extended from the northern and southern sides. They stand in contrast to older, andesitic lava remains which reach as far as 4.35 miles (7.00 km) from the volcano's base.
The mountain has a cone shape that lost its eastern side to glaciation, but its western portion remains near-intact. 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, cutting into North Sister's west side. Erosion from Pleistocene and Holocene glaciation exposed a plug near the center of 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 2,000 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.
Unlike its sister peaks, South Sister has an uneroded summit crater about 1⁄4 mile (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.
Climbing and recreation
The Three Sisters are a popular climbing destination for hikers and mountaineers. The United States Forest Service requires permits for use from Memorial Day until the beginning of September, and dogs must be kept on leashes on the Green Lakes, Moraine Lakes, South Sister, Soda Creek, Todd Lake, and Crater Ditch trails from July 15 through September 15. Horses are prohibited, and ice axes are required during the winter climbing season for safety reasons.
Due to extensive erosion, which causes rockfall, North Sister is the most dangerous climb of the three mountains. However, one of its peaks, Little Brother, can be scrambled. The common trail lasts 11 miles (18 km) round-trip, gaining 3,165 feet (965 m) in elevation and lasting about 6–7 hours for the average climber. After passing through forest, lava flows, and meadows, from the summit scramblers can see various cinder cones as well as North Sister and nearby volcanoes.
Middle Sister can also be scrambled, for a round-trip of 16.4 miles (26.4 km) and an elevation gain of 4,757 feet (1,450 m). The trip lasts about 12 hours, beginning with a route through dense forest and ending on Middle Sister's southern summit. In addition to views of North Sister, the summit offers sights of South Sister, Hayden Glacier, and other Cascade Volcanoes including Mount Hood.
The standard climbing route up the south ridge of South Sister runs for 12.6 miles (20.3 km) round-trip, and rises from 5,446 feet (1,660 m) at the trailhead to 10,358 feet (3,157 m) at its summit. Traveling over cinder cones, domes, and lava flows, the hike does not become demanding until the last mile as it ascends to the summit. In his book Into the Wild, Jon Krakauer notes that his first climb of South Sister with his father inspired him to pursue mountaineering. Popular starting points are the Green Lakes and Devils Lake trailheads. Although this path 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.
Recent history and potential hazards
Neither North nor Middle Sister is likely to resume volcanic activity, while South Sister could become active. If South Sister were to erupt, it would pose a threat to nearby life. The proximal danger zone extends from 1.24 to 6 miles (2.00 to 9.66 km) from the volcano's summits. Tephra would accumulate to 1 to 2 inches (25 to 51 mm) in the city of Bend, and lahars (volcanically induced mudflows) and pyroclastic flows would run down the flanks of the mountain, threatening any life in their paths.
South Sister was found to be potentially active when in 2000, satellite imagery showed a deforming tectonic uplift 3 miles (4.8 km) west of the mountain. The ground began to bulge in late 1997, when magma started to pool about 4 miles (6.4 km) underground. Scientists were concerned 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 an epicenter in the area of uplift, and the hundreds of small earthquakes subsided after several days. By 2007 the uplift had slowed somewhat, though the area was still considered potentially 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.
Because of the uplift at South Sister, the United States Geological Survey planned to increase monitoring of the Three Sisters and their vicinity by installing a global positioning system (GPS) receiver, sampling airborne and ground-based gases, and adding seismometers.
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