Three Sisters (Oregon)
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 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 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 nor have erupted in the last 14,000 years, and it is considered unlikely that either will ever erupt again. last erupted about 2,000 years ago and could erupt in the future, threatening life within the region. After satellite imagery detected ground inflation near in 2001, the United States Geological Survey improved monitoring in the immediate area.
Geography
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 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.Wilderness
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 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 to in elevation, the Alpine Crest Region features the wilderness area's most-frequented glaciers, lakes, and meadows.Physical geography
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. 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. The Collier Glacier, despite a retreat and a 64% loss of its surface area between 1910 and 1994, is generally considered to be the largest glacier of the Three Sisters at. Eliot Glacier on Mount Hood is now two-and-a-half times larger than the Collier Glacier. According to sources, the Prouty Glacier is sometimes considered to be larger than the Collier glacier.
When Little Ice Age glaciers retreated during the 20th century, water filled in the spaces left behind, forming moraine-dammed lakes, which are more common in the Three Sisters Wilderness than anywhere else in the contiguous U.S. The local area has a history of flash floods, including an event on October 7, 1966, caused by a sudden avalanche; this flash flood reached the Cascade Lakes Scenic Byway. Concerned about the hazard of similar flooding events, scientists in the 1980s from the U.S. Geological Survey identified that Carver Lake on South Sister could flood and breach its natural dam, producing a large mudflow that could endanger wilderness visitors and the town of Sisters. Studies at Collier Lake and Diller Lake suggested that both had breached their dams in the early 1940s and in 1970, respectively. Other moraine-dammed lakes within the wilderness area include Thayer Lake on North Sister's east flank and four members of the Chambers Lakes group between Middle and South Sister.
Before settlement of the area at the end of the 19th century, wildfires frequently burned through the local forests, especially the ponderosa pine forests on the eastern slopes. Due to fire suppression over the past century, the forests have become overgrown, and at higher elevations, they are further susceptible to summertime fires, which threaten surrounding life and property. In the 21st century, wildfires have been larger and more common in the Deschutes National Forest. In September 2012, a lightning strike caused a fire that burned in the Pole Creek area within the Three Sisters Wilderness, leaving the area closed until May 2013. In August 2017, officials closed in the western half of the Three Sisters Wilderness, including 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, including organizing prescribed fires with scientists to protect habitats at risk while minimizing adverse effects on air quality and environmental health.
Geology
The Three Sisters join several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends north–south. The High Cascades are the most recent volcanic arc in the Pacific Northwest, which forms the Cascade Volcanoes. These volcanoes, including the Three Sisters, were fed by magma chambers resulting from the subduction of the Juan de Fuca tectonic plate under the western edge of the North American tectonic plate.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. The Three Sisters form the leading edge of a rhyolitic crustal-melting anomaly, which might be explained by a combination of mantle flow and decompression that has generated similar melting and rhyolitic volcanism nearby for the past 16 million years.
The three mountains were also shaped by the changing climate of the Pleistocene epoch, during which multiple glacial periods occurred and glaciers eroded the mountains.
The Three Sisters form the center of a region of closely grouped volcanic peaks. This is in contrast to the typical spacing between volcanoes elsewhere in the Cascades. Among the most active volcanic areas in the Cascades and one of the most densely populated volcanic centers in the world, the Three Sisters region includes nearby peaks such as Belknap Crater, Mount Washington, Black Butte, and Three Fingered Jack to the north, and Broken Top and Mount Bachelor to the south. Most of the surrounding volcanoes consist of mafic lavas; only South and Middle Sister have an abundance of silicic rocks such as andesite, dacite, and rhyodacite. Mafic magma is less viscous; it produces lava flows and is less prone to explosive eruptions than silicic magma.
The region was active in the Pleistocene, with eruptions between about 650,000 and about 250,000 years ago from an explosively active complex known as the Tumalo volcanic center. 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
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. Consisting primarily of basaltic andesite, it has a more mafic composition than the other two volcanoes. Its deposits are rich in palagonite and red and black cinders, and they are progressively more iron-rich the younger they are. North Sister's lava flows demonstrate similar compositions throughout the mountain's long eruptive history. 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 -wide volcanic plug. The dikes and plug were exposed by centuries of erosion. At one point, the volcano stood more than high, but erosion reduced this volume by a quarter to a third. The plug is now exposed and forms North Sister's summits at Prouty Peak and the South Horn.