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Three volcanic peaks in Oregon, U. S.

The Three Sisters are closely spaced volcanic peaks in the U. S. state of Oregon. They are part of the Cascade Volcanic Arc, a segment of the Cascade Range in western North America extending from southern British Columbia through Washington and Oregon to Northern California. Each over 10000 feet (3048 m) in elevation, they are the third , fourth and fifth highest peaks in Oregon. Located in the Three Sisters Wilderness at the boundary of Lane and Deschutes counties and the Willamette and Deschutes national forests, they are about 10 mile (16 km) south of the nearest town, Sisters. Diverse species of flora and fauna inhabit the area, which is subject to frequent snowfall, occasional rain, and extreme temperature variation between seasons. The mountains, particularly South Sister, are popular destinations for climbing and scrambling. Although they are often grouped together as one unit, the three mountains have their own individual geology and eruptive history. Neither North Sister nor Middle Sister has erupted in the last 14,000 years, and it is considered unlikely that either will ever erupt again. South Sister last erupted about 2,000 years ago and could erupt in the future, threatening life within the region. After satellite imagery detected tectonic uplift near South Sister in 2000, the United States Geological Survey improved monitoring in the immediate area.

The Three Sisters are at the boundary of Lane and Deschutes counties and the Willamette and Deschutes national forests in the U. S. state of Oregon, about 10 mile (16 km) south of the nearest town of Sisters. The three peaks are the third , fourth , and fifth highest in Oregon, and contain 16 named glaciers. Their ice volume totals The Sisters were named Faith, Hope and Charity by early settlers, but are now known as North Sister, Middle Sister and South Sister, respectively.

The Three Sisters Wilderness covers an area of , making it the second largest wilderness area in Oregon. Designated by the United States Congress in 1964, it borders the Mount Washington Wilderness to the north and shares its southern edge with the Waldo Lake Wilderness. The area includes 260 mile (418 km) of trails and many forests, lakes, waterfalls, and streams, including the source of Whychus Creek. The Three Sisters and nearby Broken Top account for about a third of the Three Sisters Wilderness, and this area is known as the Alpine Crest Region. Rising from about 5200 feet (1585 m) to 10358 feet (3157 m) in elevation, the Alpine Crest Region features the wilderness area's most frequented glaciers, lakes, and meadows.

Weather varies greatly in the area due to the rain shadow caused by the Cascade Range. Air from the Pacific Ocean rises over the western slopes, which causes it to cool and dump its moisture as rain (or snow in the winter). Precipitation increases with elevation. Once the moisture is wrung from the air, the air descends on the eastern side of the crest, which causes it to become warmer and drier. On the western slopes, precipitation ranges from annually, while precipitation over the eastern slopes varies from in the east. Temperature extremes reach in summers and during the winters. The Three Sisters have about 130 snowfields and glaciers ranging in altitude from with a cumulative surface area of about The Linn and Villard Glaciers are north of the North Sister summit, while the Thayer Glacier is on its eastern slope. The Collier Glacier is nestled between North Sister and Middle Sister and flows to the northwest. The Renfrew and Hayden Glaciers are on the northwestern and northeastern slopes of Middle Sister, respectively, while the Diller Glacier is on its southeastern slope. The Irving, Carver and Skinner Glaciers lie between Middle Sister and South Sister. Finally, around the summit of South Sister, in a clockwise direction, are the Prouty, Lewis, Clark, Lost Creek, and Eugene Glaciers. In September 2012, a lightning strike caused a fire that burned 41 square miles (106 km2) in the Pole Creek area within the Three Sisters Wilderness, leaving the area closed until May 2013. In August 2017, officials closed 417 square miles (1,080 km2) in the western half of the Three Sisters Wilderness, including 24 mile (39 km) of the Pacific Crest Trail, to the public because of 11 lightning caused fires, including the Milli Fire. As a result of the increasing incidence of fires, public officials have factored the role of wildfire into planning,

The Three Sisters join several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends north–south. Constructed towards the end of the Pleistocene Epoch, these mountains are underlain by more ancient volcanoes that subsided due to parallel north–south faulting in the surrounding region. Part of the Cascade Volcanic Arc and the Cascade Range, each of the three volcanoes formed at different times from several variable magmatic sources. The amount of rhyolite present in the lavas of the younger two mountains is unusual relative to nearby peaks. This area features andesitic and mafic cinder cones such as Lava Butte, as well as rhyolite domes. Cinder cones accumulate from the airfall of many pyroclastic rock fragments of various sizes, while the viscous rhyolite domes extrude onto the surface like toothpaste. The Tumalo volcano spread ignimbrites and plinian deposits in ground eruptions across the area, similar to the eruption of Vesuvius that destroyed Pompeii. These deposits spread from Tumalo to the town of Bend. Basaltic lava flows from North Sister overlay the youngest Tumalo pyroclastic deposits, indicating that North Sister was active more recently than 260,000 years ago.

North Sister, also known as "Faith", is the oldest and most highly eroded of the three, with zigzagging rock pinnacles between glaciers. It is a shield volcano that overlays a more ancient shield volcano named Little Brother. North Sister is wide, and its summit elevation is 10090 feet (3075 m). The oldest lava flows on North Sister have been dated to roughly 311,000 years ago, though the oldest reliably dated deposits are approximately 119,000 years old. Estimates place the volcano's last eruption at 46,000 years ago, in the Late Pleistocene. North Sister possesses more dikes than any similar Cascade peak, caused by lava intruding into pre existing rocks. Many of these dikes were pushed aside by the intrusion of a 980 feet (299 m) wide volcanic plug. The dikes and plug were exposed by centuries of erosion. The smallest and least studied of the three, Middle Sister began eruptive activity 48,000 years ago, and it was primarily built by eruptions occurring between 25,000 and 18,000 years ago. One of the earliest major eruptive events (about 38,000 years ago) produced the rhyolite of Obsidian Cliffs on the mountain's northwestern flank. Thick and rich in dacite, the flows extended from the northern and southern sides. They stand in contrast to older, andesitic lava remains that reach as far as from its base. With an elevation of 10052 feet (3064 m), It is a predominantly rhyolitic stratovolcano overlying an older shield structure. Its modern structure is no more than 50,000 years old, and it last erupted about 2,000 years ago. Scientists became concerned that the volcano was awakening, but examination of interferograms, or diagrams of patterns formed by wave interference, revealed that only small amounts of deformation occurred. A 2002 study inferred that the composition of intruding magma was basaltic or rhyolitic. 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 In 2004, an earthquake swarm occurred with an epicenter in the area of uplift; the hundreds of small earthquakes subsided after several days. According to a 2011 study, this swarm may have initiated a significant decrease in the peak uplift rate, which dropped by 80% after 2004. By 2007 the uplift had slowed, though the area was still considered potentially active. A study published in 2010 described the magma intrusion as having a volume of about Scientists determined in 2013 that the uplift had slowed to a rate of about per year, compared to up to per year in the early 2000s. The agency completed a GPS survey campaign, including planning, documentation, and data processing and archiving, at South Sister in 2001, as well as annual InSAR radar observations from 1992 to 2001, and it followed up with campaign GPS and tilt leveling surveys in August 2004. As of 2009, semi permanent GPS networks have been deployed every year at the Three Sisters, continually showing that inflation persists at South Sister.

The ecology of the Three Sisters reflect their location in central Oregon's Cascade Range. The westside slopes from lie in the Western Cascade Montane Highlands ecoregion, where precipitation is abundant. Forests here consist predominantly of Douglas fir and western hemlock, with minor components of mountain hemlock, noble fir, subalpine fir, grand fir, Pacific silver fir, red alder, and Pacific yew. Vine maples, rhododendron, Oregon grape, huckleberry, and thimbleberry grow beneath the trees. Douglas fir is dominant below 3500 feet (1067 m), while western hemlock dominates above. Timberline at the Three Sisters occurs at 6500 feet (1981 m), where the forest canopy opens and the subalpine zone begins. These forests lie in the Cascade Crest montane forest ecoregion. Mountain hemlock trees dominate the forest in this area, while meadows sustain sedges, dwarf willows, tufted hairgrass, lupine, red paintbrush, and Newberry knotweed. Tree line occurs at 7500 feet (2286 m). The vegetation in this harsh alpine zone consists of herbaceous and shrubby subalpine meadows. This zone has a large winter snowpack, with low temperatures for much of the year. There are patches of mountain hemlock, subalpine fir, and whitebark pine near the treeline, as well as wet meadows supporting Brewer's sedge, Holm's sedge, black alpine sedge, tufted hairgrass, and alpine aster. Near the peaks of the Three Sisters, there are extensive areas of bare rock. The lower eastern slopes of the Three Sisters below 5200 feet (1585 m) lie in the Ponderosa Pine/Bitterbrush Woodland ecoregion. This ecoregion has less precipitation than the western slopes and has soil derived from Mazama Ash (ash erupted from Mount Mazama). Stream flow changes little throughout the year due to the region's volcanic hydrogeology. These slopes support nearly pure stands of ponderosa pine. Understory vegetation includes greenleaf manzanita and snowberry at higher elevations and antelope bitterbrush at lower elevations. Mountain alder, stream dogwood, willows, and sedges grow along streams. Local fauna includes birds such as blue and ruffed grouse, small mammals like pikas, chipmunks, and golden mantled ground squirrels, and larger species like the Columbian black tailed deer, mule deer, Roosevelt elk, and American black bear. Bobcats, cougars, coyotes, wolverines, martens, badgers, weasels, bald eagles, and several hawk species are many of the predators throughout the Three Sisters area.

The Three Sisters area was occupied by Amerindians since the end of the last glaciation, mainly the Northern Paiute to the east and Molala to the west. They harvested berries, made baskets, hunted, and made obsidian arrowheads and spears. Traces of rock art can be seen at Devils Hill, south of South Sister. The first Westerner to discover the Three Sisters was the explorer Peter Skene Ogden of the Hudson's Bay Company in 1825. He describes "a number of high mountains" south of Mount Hood. Ten months later in 1826, the botanist David Douglas reported snow covered peaks visible from the Willamette Valley. As the Willamette Valley was gradually colonized in the 1840s, Euro Americans approached the summits from the west and probably named them individually at that time. Explorers, such as Nathaniel Jarvis Wyeth in 1839 and John Frémont in 1843, used the Three Sisters as a landmark from the east. The area was further explored by John Strong Newberry in 1855 as part of the Pacific Railroad Surveys. In 1862, to connect the Willamette Valley to the ranches of Central Oregon and the gold mines of eastern Oregon and Idaho, Felix and Marion Scott traced a route over Scott Pass. This route was known as the Scott Trail, but was superseded in the early 20th century by the McKenzie Pass Road further north. Around 1866, there were reports that one of the Three Sisters emitted some fire and smoke. In the late 19th century, there was extensive wool production in eastern Oregon. Shepherds led their herds of 1,500 to 2,500 sheep to the Three Sisters. They arrived in eastern foothills near Whychus Creek by May or June, and then climbed to higher pastures in August and September. By the 1890s, the area was getting overgrazed. Despite regulatory measures, sheep grazing peaked in 1910 before being banned in the 1930s at North and Middle Sister, and in 1940 at South Sister. In 1892, President Grover Cleveland decided to create the Cascades Forest Reserve, based on the authority of the Forest Reserve Act of 1891. Cascades Reserve was a strip of land from wide around the main crest of the Cascade Range, stretching from the Columbia River almost to the border with California. In 1905, administration of the Reserve was moved from the United States General Land Office to the United States Forest Service. The Reserve was renamed the Cascade National Forest in 1907. In 1908, the forest was split: the eastern half became the Deschutes National Forest, while the western half merged in 1934 to form the Willamette National Forest. Most of the Three Sisters glaciers were described for the first time by Ira A. Williams in 1916. Collier Glacier, between North and Middle Sister, was first studied and mapped by Edwin Hodge. Ruth Hopsen Keen took a forty year photographic record of Collier Glacier, documenting part of the retreat of the glacier from 1910 to 1994. Today, the common trail covers 11 mile (18 km) round trip, gaining 3165 feet (965 m) in elevation. Middle Sister can also be scrambled, for a round trip of 16.4 mile (26 km) and an elevation gain of 4757 feet (1450 m). South Sister is the easiest of the three to climb, and has a trail all the way to the summit. The standard route up the south ridge runs for 12.6 mile (20 km) round trip, rising from 5446 feet (1660 m) at the trailhead to 10363 feet (3159 m) at its summit. It is a popular climb during August and September, with up to 400 people each day.