Орегон штатындағы Вашингтон тауының геологиялық сипаттамасы

Deeply eroded volcano in the Cascade Range of Oregon

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 (rich in magnesium and iron) 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. The wilderness area is not heavily used, though it has a number of hiking trails. Wildlife is sparse in the region. Vegetation is mostly limited to pines and shrubs. Animal life includes deer, American black bear, cougar, various small animal species, and some fish species in the lakes.

With an elevation of above sea level, Mount Washington is located in Deschutes and Linn counties in the U. S. state of Oregon. It lies north of McKenzie Pass, The volcano forms a crest of steep, icy mountains with North Cinder Peak and Three Fingered Jack to the south of Mount Jefferson. The volcano has a pinnacle appearance resembling the spire like shape of Mount Thielsen. According to Harris (2005), the volcano resembles Cleopatra's Needle from certain angles and the Sugarloaf Mountain in Brazil from others. The proximal relief for the volcano is , while the draping relief is The volcano has an overall volume of .

Elevations in the Mount Washington area range from 3200 feet (975 m) to greater than 7700 feet (2347 m). Most precipitation falls during the winter as snow, and summers are warm and dry. During the winter season, the Cascades are covered with thick snow. As a result, Oregon Route 242 over McKenzie Pass becomes impassable from fall through late spring, while the Santiam Highway and Oregon Route 126 are maintained and remain open. Within the Mount Washington Wilderness, there are outwash plains and lateral, recessional, and terminal ground moraines. Many of these glacial deposits were created during the most recent glacial advance in the late Pleistocene, though there are Holocene glacial deposits as well. There was an ice sheet during the late Pleistocene between Mount Washington and Three Fingered Jack. The McKenzie River, fed by Clear Lake, runs south for about 15 mile (24 km) just west of Mount Washington before it turns west for another 70 mile (113 km) and meets the Willamette River near the city of Eugene.

The Mount Washington Wilderness covers an area of First named a wild area by the United States Forest Service in 1975, it was designated as one of the original wilderness areas under the 1964 Wilderness Act. Located 32 mile (51 km) to the west of Bend, the Mount Washington plot is the smallest of three wilderness areas that encompass most of the High Cascades, a physiographic region located in the central part of Oregon; the two others are the Three Sisters Wilderness and Mount Jefferson Wilderness. Used mostly by hunters, hikers, and climbers, 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 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 pyroclastic rock. The volcano has a mafic composition, with subalkaline basalt and basaltic andesite. Lava flows from Washington exhibit breccia with plagioclase and olivine, with palagonite tuff at the northeastern slope of the mountain that might indicate a past subglacial eruption during Washington's cone building phase. Mount Washington has been eroded over time and is now very dissected, with its inner contents exposed and U shaped canyons and cirques. In the late Pleistocene, large glaciers extending more than to the east and west carved cirques into the slopes of the volcano. The George Lake and Dry Creek cirques, which face north and northeast, respectively, show evidence of holding glaciers similar to those documented at Canyon Creek cirque on Three Fingered Jack with moraines. Wood and Kienle (1990) estimate that Mount Washington once extended to an elevation of , with an elevation of above the basalt lava field surrounding it.

The Global Volcanism Program considers Cache Mountain, Little Cache Mountain, Hayrick Butte, and Hoodoo Butte subsidiary cones of Mount Washington. The Mount Washington Wilderness includes a number of cinder cones, typically with elevations between Composed of gray to red cinder, they also have scoria and welded spatter. Major cinder cones in the wilderness include Belknap Crater, Twin Craters, Scott Mountain, and the Sand Mountain craters. Eruptions from a chain of spatter cones mark the most recent activity at Mount Washington. About one mile in length, they trend northeast, with their mid section parallel to depressions in the Cache Creek valley between Mount Washington and Blue Lake Crater. The northernmost vent of Mount Washington has a depth of 10 feet (3 m) and likely only ever erupted volcanic gas. The first group of four spatter cones lies 200 feet (61 m) to the south, with depths ranging from Farther south, there are seven other vents, including three small craters separated from three larger vents to the south by a graben, as well as a central crater with a small crater on the northern side of its rim.

There are four major types of basalt and basaltic andesite in the central Cascades: early high alumina olivine tholeiitic (HAOT) basalt, normal High Cascade HAOT basalt, North Sister basaltic andesites, and Mount Washington type basaltic andesites. Hughes (1990) argues that their differences could be the result of different magma sources or magma evolution in open systems. Mount Washington basaltic andesites are also considered one of the three distinct mafic rock types in the High Cascade platform, with North Sister basaltic andesites and normal basalts. The three groups all exhibit different major and trace element abundances. Mount Washington basaltic andesite is more common than North Sister basaltic andesite, with higher levels of incompatible elements and rare earth elements. It also extends to the east and west of the major Cascade arc. Otherwise, the Mount Washington and North Sister basaltic andesites display lithologic similarities including a dearth of pyroxene phenocrysts and augite microphenocrysts, and both groups were probably near primary melts. According to Hughes (1982), examples of Mount Washington basaltic andesites occur at Holocene deposits from Nash Crater, Four in One Cone, and Little Belknap Cone, as well as undated deposits at Todd Lake Volcano, Falls Creek, Broken Top, and the abutment of Tumalo Dam.

Mount Washington formed during the late Pleistocene epoch; the volcano itself is not older than a few hundred thousand years. Harris (2005) estimates that it has not erupted for more than 250,000 years, similar to Mount Thielsen; this date is supported by James, Manga, and Rose (1999). Paleomagnetic evidence suggests that the volcano and associated lava flows exhibit normal magnetic polarity. Its volcanic edifice was produced through the eruption of basaltic andesite and mafic volcanic ash, the latter being preserved as palagonite tuff along the northeastern and southwestern flanks of the summit cone. Compared to larger stratovolcanoes in the Cascade volcanic arc, Mount Washington, like the rest of Oregon's Matterhorns, had a relatively short eruptive life. Several spatter cones produced basaltic andesite on the lower northeastern side of the volcano, following a fissure that reached from Washington's summit. There were no lava flows from this eruption, nor at an eruption at the nearby Blue Lake Crater, which produced volcanic material that displayed petrographic similarities to the Mount Washington splatter cone deposits. These include similar porphyritic textures with 10 to 15 percent plagioclase phenocrysts as well as similar alignments, suggesting that they erupted simultaneously. Sherrod et al. (2004) argue that the spatter cones and activity from Blue Lake Crater represent the most recent eruptions near the McKenzie Pass and Santiam Pass region. It is still possible that new volcanic cones could be produced at the base of Mount Washington. However, according to Taylor, Causey, and MacLeod (1983), the volcano itself is unlikely to still be active.

While the area is not used as frequently as many other wilderness areas in the Oregon branch of the Cascade Range, it offers scenic lakes and trails. The Pacific Crest Trail passes over the western side of Mount Washington. Other trails include a loop around the Patjens Lakes, the Hand Lake trail, and the Benson Lake route. The U. S. National Geodetic Survey Data Sheet for Mount Washington notes that climbing the volcano is challenging and dangerous and thus is not recommended for inexperienced climbers or climbers without proper equipment.