page_title: Mount Washington (Oregon)
section_title: Geology
hierarchical_section_title: Mount Washington (Oregon) / Geology
caption_attribution_description: Mount Washington, Oregon, as seen from the east
context_page_description: Mount Washington is a deeply eroded volcano in the Cascade Range of Oregon. It lies within Deschutes and Linn counties and is surrounded by the Mount Washington Wilderness area.
  Like the rest of the Oregon Cascades, Mount Washington was produced by the subduction of the oceanic Juan de Fuca tectonic plate under the continental North American tectonic plate, forming during the late Pleistocene. Made mostly of mafic volcanic rock like subalkaline basalt and basaltic andesite, it has a volcanic plug occupying its summit cone and numerous dikes. It is surrounded by other volcanic features such as cinder cones and spatter cones. The volcano's last eruptions took place from spatter cones about 1,350 years ago, generating basaltic andesite lava deposits.
  Mount Washington has barren surroundings, which have seen little historical recreational use. A wagon road at McKenzie Pass was built in 1872, which was later paved during the 1930s. On August 26, 1923, the mountain was first ascended by six boys from the neighboring city of Bend. The surrounding area was designated the Mount Washington Wilderness by the federal government in 1964.
context_section_description: Volcanism in the Oregon segment of the Cascade Range results from intra-range rifts and the subduction of the Juan de Fuca tectonic plate under the North American tectonic plate. Mount Washington forms part of the High Cascades physiographic region in central Oregon, an arc of Pliocene to Quaternary lava flows, cinder cones, and fissure vents that trend from north to south, with occasional large stratovolcanoes. Near Mount Washington, the High Cascades form a lava field with high-alumina, diktytaxitic basalt erupted from cinder cones. These volcanoes have been eroded by glaciers and reduced to buttes in the Cascade arc, and much of the vents in the area were covered by Mount Washington. Some basalt lava flows occur on the edges of Mount Washington at the Cache Creek and Dry Creek canyons or as outcrops that form benches (long, relatively narrow strips of relatively level or gently inclined land bounded by distinctly steeper slopes above and below) about 9.9 miles (16 km) away from Washington from Patjens Lake to the McKenzie River.
  Washington is part of the informal group of volcanoes known as Oregon's Matterhorns, which includes Mount Thielsen, Three Fingered Jack, Mount Bailey, and Diamond Peak. The name originates from the spire-like appearance of the volcanoes' summits, which resembles the pinnacle of the Matterhorn in Switzerland. They all ceased eruptive activity at least 100,000 to 250,000 years ago, leading to their extensive dissection by glaciers over time. Mount Washington is also part of the Sisters Reach subsegment, which extends for 56 miles (90 km) and contains at least 466 volcanoes that were active during the Quaternary. Washington represents one of 30 mafic (rich in magnesium and iron) stratovolcanoes and shield volcanoes in the group, which include Pleistocene and Holocene eruptive centers. The volcano and its wilderness area sit on a lava platform with an altitude of 4,000 to 5,000 feet (1,200 to 1,500 m), and they are bounded by faults to the east and west. Volcanic rocks contained within the wilderness area are either composed of older basalt or younger basaltic andesite, all of which were produced during the Quaternary, probably during the past 700,000 years. Pleistocene deposits show evidence of erosion by glaciers. Holocene deposits, dated between 3,000 and 1,500 years old, encompass about half of the wilderness area, and they also occur outside the wilderness area at its northwestern and southern borders.
  Whether Mount Washington is a stratovolcano or shield volcano is debated within the literature. Wood and Kienle (1990) refer to it as a "mafic shield volcano," and the Global Volcanism Program of the Smithsonian Institution also considers it a shield volcano with a pyroclastic cone. Sherrod et al. (2004) classify Mount Washington as a "steep-sided" shield volcano. E. M. Taylor describes Mount Washington as a stratovolcano cone that reaches an elevation of 4,000 feet (1,200 m) above an older shield volcano, referring to it as "a glacially-gutted Pleistocene stratovolcano." Hildreth (2007) likewise calls it a "a glacially sculpted mafic stratocone [...] with a broad apron of mafic lavas." In his argument for classifying Mount Washington as a stratovolcano, Hildreth adds that it is made of composite materials with steep slopes and a high relief of 2,300 to 4,300 feet (700 to 1,300 m). However, Hildreth acknowledges that there is morphological continuity from steep mafic cones into transitional, cone-shaped shield volcanoes like Olallie Butte.
  Mount Washington has a diameter of about 3 miles (4.8 km). It has a summit cone, a volcanic plug comprised by cinder, lava flows, and intrusive rock that covers Washington's volcanic conduit. This plug is made of micronorite with a diameter of 0.25 miles (0.4 km). There are dikes exposed throughout the summit cone, mostly oriented from north to south, with another dike swarm trending north from the central plug. The summit formed over a platform of basaltic andesite lavas from early eruptions at Mount Washington, made of thinner flows combined with pyroclasti
caption: The deeply eroded Mount Washington seen from the east
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