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| prominence = | listing = Ultra, County high point | map = Oregon | map_caption = | map_size = 250 | location = Lane and Deschutes counties, Oregon, U.S. | range = Cascade Range | lat_d = 44.1034490 | long_d = -121.7692058 | region_code = US-OR | source = GNIS | coordinates_ref = | topo = USGS South Sister and North Sister | type = Complex volcano, made up of stratovolcanoes and a shield volcano | age = Quaternary | volcanic_arc = Cascade Volcanic Arc | last_eruption = 50 BCE (?) | first_ascent = | easiest_route = Hiking or scrambling, plus glacier travel on some routes }} The Three Sisters are a complex volcano of three volcanic peaks of the Cascade Volcanic Arc and the Cascade Range in the U.S. state of Oregon. Each exceeding in elevation, they are the third, fourth, and fifth highest peaks in the state of Oregon, and are located in the Three Sisters Wilderness, about south of the nearest town of Sisters. Diverse species of flora and fauna inhabit the area on and around the mountains, which is subject to frequent snowfall, occasional rain, and extreme temperature differences between seasons. The mountains, particularly South Sister, are popular for climbing and scrambling. Although they are often grouped together and seen as one unit, the three mountains evolved under differing geologic situations, and the petrologic composition of each mountain can vary significantly. Whereas North Sister is extinct and Middle Sister is inactive, last erupted about 2,000 years ago and still could erupt, threatening life within the region. After satellite imagery detected uplift near in 2000, the United States Geological Survey made plans to improve monitoring in the immediate area. ==Geography and geology== The Three Sisters are located on the boundaries 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, the third, fourth, and fifth highest peaks in Oregon,〔(【引用サイトリンク】publisher = U.S. Forest Service )〕 have 15 named glaciers among them, nearly half of the 35 named glaciers in Oregon. The Sisters were named Faith, Hope, and Charity by early settlers, but "these names have not prevailed", and instead they are named North Sister, Middle Sister, and South Sister.〔(【引用サイトリンク】Geology and History Summary for Three Sisters )〕 A complex volcano that extends for ,〔 the Three Sisters are located at latitude and longitude .〔(【引用サイトリンク】title=Three Sisters )〕 Part of the Cascade Volcanic Arc and the Cascade Range, the volcanoes formed during separate time periods from several varieties of magma, and the amount of rhyolite found in the lava of the younger two mountains is unusual relative to nearby volcanoes.〔 Like other Cascade volcanoes, the Three Sisters were fed by magma chambers produced by the subduction of the Juan de Fuca Plate under the western edge of the North American Plate. They were also affected by the changing atmosphere of the Pleistocene epoch, a geologic period during which an Ice Age occurred where glaciers ate away at mountains as they retreated. Specifically, Three Sisters join several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends from north–south. The Three Sisters form the centerpiece of a region of closely grouped volcanic peaks, an exception to the typical spacing between volcanoes elsewhere in the Cascades. These nearby peaks include Belknap Crater, Mount Washington, Black Butte, and Three Fingered Jack to the north, and Broken Top and Mount Bachelor to the south. The region was active in the , with eruptions between 700,000 and 170,000 years ago from an explosively active complex known as the Tumalo volcanic center. Basaltic lava flows from North Sister overlay the newest Tumalo pyroclastic deposits, placing the age of North Sister, the eldest, at less than 170,000 years. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Three Sisters (Oregon)」の詳細全文を読む スポンサード リンク
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