Mount Jefferson (Oregon)

The second tallest mountain in the U.S. state of Oregon after Mount Hood, Mount Jefferson lies within Jefferson, Linn, and Marion counties, in the central part of the state. Reaching an elevation of , the volcano has a proximal relief of . It is not usually visible from the city of Portland, though it is visible on clear days from Salem and can be noticed from highways to both the east and the west of the Cascade Range. The average elevation of the terrain around Jefferson is , meaning that Jefferson's cone rises nearly above its surroundings. Wilderness Mount Jefferson's eastern segment lies within the Warm Springs Indian Reservation, and its western portion within the Mount Jefferson Wilderness, of the Willamette National Forest and Deschutes National Forests. The wilderness area covers , with more than 150 lakes. It also has of trails, including of the Pacific Crest National Scenic Trail. Mount Jefferson is the major feature of the wilderness, along with the nearby Three Fingered Jack volcano. The volcano, like much of the Oregon Cascades, was likely covered by an ice cap during the Pleistocene, While the Little Devil fire covered , the Whitewater fire reached more than in area, provoking the use of amphibious aircraft and causing trail closures. As a result of the Whitewater fire, officials closed the Mount Jefferson Wilderness during the solar eclipse of August 21, 2017. ==Climate==

Vegetation at Mount Jefferson is dominated by Douglas fir, silver fir, mountain hemlock, ponderosa pine, lodgepole pine, and several species of cedar. Vine maple, rhododendron, purple lupine, yellow lupine, Indian paintbrush, wild strawberries, and red huckleberries are also common around Mount Jefferson. Above the timber line at above sea level, mountain hemlock and whitebark pine predominate, though mountain hemlock has also invaded into subalpine meadows at Mount Jefferson, possibly as a result of fire control programs, grazing, the influence of adjacent forest areas, and climate change. Carnivorous animals at Mount Jefferson and its surroundings include American black bears, coyotes, cougars, red foxes, raccoons, American martens, stoats (also known as ermines), long-tailed weasels, American minks, North American river otters, and bobcats. Deer species include Roosevelt elk, black-tailed deer, and mule deer; insectivores include vagrant shrews, American water shrews, and coast moles. Bats at Jefferson include little brown bats and silver-haired bats, and American pikas and snowshoe hares are also present. Rodents such as yellow-bellied marmots, mountain beavers, yellow-pine chipmunks, Townsend's chipmunks, golden-mantled ground squirrels, western gray squirrels, Douglas squirrels, mountain pocket gophers, North American beavers, deer mice, bushy-tailed woodrats, water voles, Pacific jumping mice, and North American porcupines are present. Birds at Jefferson include mallards, northern goshawks, sharp-shinned hawks, red-tailed hawks, dusky grouses, grey partridges, killdeers, spotted sandpipers, California gulls, band-tailed pigeons, great horned owls, mountain pygmy owls, common nighthawks, rufous hummingbirds, Northern flickers, pileated woodpeckers, yellow-bellied sapsuckers, hairy woodpeckers, and white-headed woodpeckers. Other bird species found in the area consist of Eurasian three-toed woodpeckers, willow flycatchers, olive-sided flycatchers, tree swallows, Canada jays, Steller's jays, common ravens, Clark's nutcrackers, black-capped chickadees, mountain chickadees, chestnut-backed chickadees, red-breasted nuthatches, pygmy nuthatches, Eurasian treecreepers, American dippers, wrens, American robins, varied thrushes, hermit thrushes, Townsend's solitaires, golden-crowned kinglets, ruby-crowned kinglets, water pipits, blue-headed vireos, western tanagers, Cassin's finches, gray-crowned rosy finches, pine siskins, red crossbills, green-tailed towhees, dark-eyed juncos, white-crowned sparrows, golden-crowned sparrows, fox sparrows, and Lincoln's sparrows. Long-toed salamanders, California giant salamanders, rough-skinned newts, tailed frogs, western toads, Pacific tree frogs, northern red-legged frogs, Oregon spotted frogs, pygmy short-horned lizards, common garter snakes, and northwestern garter snakes make up some of the amphibious and reptilian animals in the vicinity. Roughly half the lakes in the Jefferson area contain rainbow trout. == Geology ==

Mount Jefferson shows normal magnetic polarity, suggesting that it formed less than 730,000 years ago. Created by the subduction of the oceanic Juan de Fuca tectonic plate under the continental North American tectonic plate in an area where the Earth's crust is thick, it is part of the Oregon High Cascades, which are influenced by the movement of the North American Plate and the extension of its continental crust. These extensional processes formed grabens, or valley-like depressions between parallel fault lines, at the eastern boundary of the central Cascades, including a deep formation. Jefferson does not lie in one of these grabens, but these tectonic processes continue, albeit at a less dramatic rate. At their peak rates, the crustal extension and depression of the Cascades area caused eruption of the Minto Lavas, made of basalt, followed by the Santiam basalts, named for their movement into the North Santiam River valley, which they filled to depths of . Though the Jefferson vicinity has produced andesitic and dacitic lavas for the past 5 to 6 million years, major volcanoes more than south of the area have erupted basaltic andesite. The central Oregon Cascades are made up of Eocene to Quaternary volcanic, volcaniclastic, igneous, and sedimentary rock. Miocene and Pliocene volcanic and sedimentary rocks have been exposed in the Jefferson area, which also sits above lava flows, cinder material, and breccia from the High Cascades that formed during and after the Pliocene. Jefferson is the largest volcano in the Jefferson Reach, which forms the strip that makes up the northern part of the Oregon Cascade Range. Stretching from Frog Lake Buttes to South Cinder Peak, this segment consists of at least 175 Quaternary volcanoes. With a width of , it differs from the adjacent northern segment of the Cascades, where volcanoes show a scattered distribution. Other unusual features of the Jefferson Reach include that the northernmost of the strip does not contain many volcanoes formed since the early Pleistocene and that it features a number of andesitic and dacitic volcanoes, which are unlike the many mafic (rich in magnesium and iron) shield volcanoes within the stretch. North of Pinhead Buttes, the volcanoes in this region are older and less tall, usually between in elevation. South of Pinhead Buttes, the Cascades becomes younger Pleistocene volcanoes, which often have glaciers. Mount Jefferson may form part of a long-lasting intracrustal melting and magma storage area that encompasses an area of , where relatively little mafic eruptive activity has occurred. The melting of the metamorphic rocks amphibolite and at deeper strata, granulite, have both produced intermediate and silicic lavas at Jefferson. The strip may still be active, as monogenetic vents near Jefferson have produced basaltic andesite since the last glacial period. Jefferson — with Mount Hood, the Three Sisters-Broken Top area, and Crater Lake — represents one of four volcanic centers responsible for much of the Oregon Cascades' Quaternary andesite, dacite, and rhyolite deposits. Some of this andesite and dacite occurs in vents that underlie the Jefferson vicinity, which also erupted during the Quaternary. Quaternary volcanic production rates in the Cascade Range from Jefferson to Crater Lake have averaged per mile of arc length per million years. In the area surrounding Mount Jefferson, monogenetic volcanoes constructed an upland area composed of basaltic lava flows and small volcanic vents. Within this region, basaltic vents occur at Olallie Butte, Potato Butte, Sisi Butte, North Cinder Peak, and South Cinder Peak, with basaltic lava flows at Cabot Creek, Jefferson Creek, and upper Puzzle Creek. There are several hundred other basaltic volcanoes within the central Oregon High Cascades, extending up to away. Mount Jefferson overlies an silicic volcanic field from the early Pleistocene. Between five and six million years old, the field reaches north from Jefferson to Olallie Butte, and it covers an area of . Scientists think that the setup of this field, where various vents have erupted lava, explains why the otherwise similar Cascades volcano at Mount Hood is three times as voluminous as Jefferson, because Hood has concentrated most of the eruptions from its magma chambers. The field is also likely underlain by a batholith, a large mass of intrusive igneous rock (also called a pluton) that forms from cooled magma deep in the Earth's crust. Mount Jefferson is a stratovolcano, made up of basaltic andesite, andesite, and dacite overlying basaltic shield volcanoes, with andesite and more silicic (rich in silica) rock forming the majority of the mountain. Rhyolite from the Quaternary can also be found at Jefferson, though it is not commonly found within the major volcanic centers of the Oregon Cascades. The volcano constitutes a small stratovolcano within the Cascades, with a current volume of , though prior to erosion and other alterations over time, it may have been as large as in volume at one time. Mount Jefferson has been significantly altered by erosion, and represents one of the most eroded stratovolcanoes in the state of Oregon. Glacial motion during the Pleistocene decreased the summit's elevation by a few hundred feet and formed a cirque (an amphitheatre-like valley carved by glacial erosion) on the western side of the volcano. This feature, known as the West Milk Creek cirque, includes the two Milk Creek glaciers and extends into the interior of Mount Jefferson, exposing tephra and pyroclastic rock in the main volcanic cone. The final two advances of glaciers during the Pleistocene removed about a third of the volcano's original volume, decreasing the overall elevation by . Currently, the Whitewater Glacier and the Milk Creek glaciers erode the mountain's eastern and western flanks, respectively, and are likely to gradually form a cleft between the northern and southern horns of the summit. Within Jefferson's main volcanic cone, more than 200 andesitic lava flows are now exposed, with mean thicknesses from , as well as an immense, pink dacitic lava flow with a thickness of . The volcano also possess a small volcanic plug (created when magma hardens within a vent on an active volcano), situated under the summit. Jefferson's main cone ranges from 58 to 64 percent silicon dioxide, and is mostly made up of andesite and dacite. The upper of Jefferson's cone formed within the past 100,000 years, and consists mostly of dacite lava flows and lava domes. While it is possible that glaciers shed material from the burgeoning lava domes, any evidence of these domes generating pyroclastic flows or lahars has not been preserved in the geological record. Basalt at Mount Jefferson contains olivine, clinopyroxene, and plagioclase phenocryst crystals, while basaltic andesite phenocrysts include plagioclase (variable among samples), clinopyroxene, olivine, orthopyroxene, and occasionally, magnetite. Dacite and rhyodacite samples show amphibole, plagioclase, orthopyroxene, clinopyroxene, magnetite, apatite, and every so often ilmenite. Andesite shows similar composition to dacite samples, though sodic plagioclases and amphiboles are not as common. Subfeatures Volcanic activity in the vicinity of Mount Jefferson tends to originate from either stratovolcanoes that erupt for thousands of years or monogenetic volcanoes, which only erupt for a brief period of time before going extinct. In a 1987 report, Richard P. Hoblitt and other USGS scientists estimated that the yearly likelihood for a major explosive eruption at Jefferson does not exceed 1 in 100,000. However, given the incomplete geologic record, imprecise dating of its known deposits, and its lack of relatively recent activity, scientists from the United States Geological Survey have commented that "It is almost impossible to estimate the probability of future eruptions at Mount Jefferson." They have designated proximal and distal hazard zones for the volcano, which extend and several tens of miles, respectively. An eruption from the volcano would threaten the immediate surrounding area, in addition to places downstream near river valleys or downwind that could be affected by ashfall. Lahars (volcanically induced mudslides, landslides, and debris flows) and tephra could extend far from the volcano, and Mount Jefferson may also produce pyroclastic flows, lava domes, and lava flows. Mazama Ash in the region reached in thickness, and at least one explosive eruption from Jefferson deposited of ash onto its surroundings within . Finer ash particles from the volcano could threaten air traffic, as a large gas plume may form; clouds from such a plume might also spawn pyroclastic flows on the flanks of the Jefferson volcano. it is unlikely they would threaten areas outside the local Jefferson vicinity. An eruption at Jefferson could create lahars that would reach Detroit Lake on the western side of the volcano or Lake Billy Chinook on the eastern side, leading to increased lake water levels (or lake dam failure) and endangering life downstream. In addition to the hazards from eruptions at Mount Jefferson, other safety threats include debris avalanches and lahars, which could be caused without an eruption as a result of the failure of glacial moraine dams; this has happened in the past at Jefferson. Even a small or mid-sized landslide could create lahars that travel far from the volcano. Flooding at one of the many lakes on the flanks of Jefferson could spawn lahars in the future. Many scientists think mudflows represent the largest threat at Jefferson. Seismic activity at Mount Jefferson is monitored by a regional network of seismic meters operated by the United States Geological Survey at the University of Washington's Geophysics Department. No frequent signs of detectable earthquake have been seen within the past two decades, but if earthquakes increased, scientists are prepared to deploy additional seismometers and other tools to monitor volcanic gas emissions and ground deformation indicating movement of magma into the volcano. == Human history ==

, Thomas Jefferson (pictured; painted in 1800 by Rembrandt Peale) A Native American name for the mountain is Seekseekqua; its English name, Mount Jefferson (originally called Mount Vancouver by the British) was decided in honor of U.S. President Thomas Jefferson by the Lewis and Clark Expedition. The expedition, which was sponsored by President Jefferson, first saw the peak from the mouth of the Willamette River on March 30, 1806. In 1937, Thayer analyzed Mount Jefferson's petrography and petrology from segments of the Western Cascades and High Cascades, which he separated into local units. He expanded on this research in a 1939 publication looking at Jefferson vicinity lava flows. Field work followed in summer during 1965, led by G. W. Walker, and a 1974 study of the volcano's glacial and volcanic history was carried out by Kenneth G. Sutton and other geologists. The first ascent of Mount Jefferson was probably accomplished by E. C Cross and Ray L. Farmer on 12 August 1888 by way of the south ridge. George J. Pearce, who accompanied Cross and Farmer on the expedition, wrote an account of the climb for the Oregonian newspaper on 22 August 1900. The first climber to reach the summit via the north face was S. S. Mohler in 1903. == Recreation ==

Mount Jefferson is remote, and can usually only be reached on foot or by horse. There are no paved roads within of the mountain, and it is relatively little-known compared to other features near the Willamette Valley. Still, the mountain and its surrounding wilderness are visited by so many hikers, backpackers, and climbers each year, especially during the summer, that they face threats to their ecological well-being. The Warm Springs Tribal Council does not permit access to the volcano's eastern side, so only the western flanks can be used by the public. The western side can be reached from the Oregon Route 22 highway. Jefferson Park, on the northern slope of the mountain, can be reached on foot by taking the Whitewater Trail and following the Pacific Crest Trail for . Located within the Mount Jefferson Wilderness, it represents a popular tourist destination for its views, lakes, and meadows, with activities including backpacking, climbing, and hiking during the summer, as well as nature photography. The area contains 26 campsites, which enforce a maximum group size of 12 people, and do not allow campfires. As a result of increased traffic to the area between 2012 and 2014, the Willamette National Forest administration enforced a campsite reservation system as of 2016, but stopped the practice in 2017 due to its failure to reduce human impact within the region. The Jefferson Lake Trail runs for round trip, with an elevation gain of . Parts of the Trail were destroyed by a fire in the wilderness area in 2003, but the surviving remnants of the trail reopened after maintenance work was completed. At Marion Lake, there are several trails, including a long route and a hike to Marion Mountain that lasts round-trip. These and other trails through the region offer views of the devastation of fires in the wilderness area in 2003 and 2006. The Whitewater Trail runs north through the wilderness area for before reaching a junction, with the right path moving to the Pacific Crest Trail. In the Maxwell trail area, hikes of all difficulty levels can be found, including the challenging Maxwell Butte Trail 3391, the round trip at Santiam Lake Trail 3491, and the slightly less demanding Duffy Lake Trail 3427. At the Pamelia Lake trail area, there are streams, lakes, and springs, as well as bathrooms, parking areas, and picnic tables. The Pamelia Limited Entry Area only allows 20 groups per day and limits their size to mitigate human impacts on the wilderness. Trails at Pamelia Lake include the Hunts Creek Trail 3440 and a segment of the Pacific Crest Trail, in addition to the Pamelia Lake Trail 3439, which rises before meeting the Hunts Creek Trail. The area is popular for backpacking, mountaineering, horseback riding, and day hiking. Other popular trails include the Firecamp Lakes Trail and Canyon Creek Meadows trails. In addition to the trails, some of the most popular areas around Mount Jefferson Wilderness include Eight Lakes Basin, Pamelia Lake, Jack Lake, Duffy Lake, Russell Lake, Santiam Lake and Wasco Lake. Mount Jefferson can be climbed, but the route is challenging, especially the pinnacle of the summit. Nearly annually, at least one climber attempting Jefferson perishes. Because of the hazards and difficulty of climbing Mount Jefferson, the U.S. National Geodetic Survey recommends that only experienced climbers try to climb it. ==See also==