Mount Scott Volcano (Crater Lake National Park, Oregon, USA) 4 : 無料・フリー素材/写真
Mount Scott Volcano (Crater Lake National Park, Oregon, USA) 4 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Mount Scott is an extinct volcano in the Cascade Range, a north-south linear chain of otherwise active and potentially active volcanoes in America's Pacific Northwest. It extends from northern California to Oregon, Washington State, and into British Columbia, Canada. The Cascade Range formed as a result of tectonic subduction - the offshore Juan de Fuca Plate is diving below the North American Plate. The diving plate causes melting in the mantle. The melt rises and emerges at the surface at volcanic centers. Famous Cascade Range volcanoes include Mt. St. Helens, which had a large eruption in May 1980, Mt. Rainier near Seattle, Mt. Hood, which is the highest peak in Oregon, and Mt. Mazama, which destroyed itself 7,700 years ago in an enormous eruption that produced the modern-day Crater Lake Caldera.Mount Scott Volcano is a relatively small, eroded stratovolcano east of Crater Lake Caldera (= formerly Mount Mazama) in southwestern Oregon. The rocks are principally porphyritic dacites of Middle Pleistocene age. Published isotopic dates of Mount Scott dacites range from 355 to 422 ka.------------------------------Partial description from Bacon (2008): Porphyritic medium-gray dacite (63.5 - 67% SiO2; most ≥65%) erupted from Mount Scott and related vents to the east. Unit grades from pyroclastic breccia and agglutinate sheets high on Mount Scott itself to massive lava distally and is everywhere characterized by basaltic andesitic enclaves (≥50 centimeters; 53.5 - 56.5% SiO2), which are abundant in some exposures. Greatest concentration of enclaves in outcrops on southeast ridge of Mount Scott, in which enclaves are virtually touching, suggests ejection as rigid clasts in eruptive fountains and accumulation in near-vent agglutinate, followed by flowage.Textures result from mixtures of varying proportions of silicic melt, undercooled enclave material, cumulate crystal mush, and gabbroic microxenoliths. Phenocrysts (25 - 30%): plagioclase (≥5 millimeters), augite (≥1.2 millimeters, rarely to 5 millimeters), orthopyroxene (≥1.2 millimeters), and Fe-Ti oxides (≥0.2 millimeters) in a glassy to very fine grained groundmass. Least silicic flows contain ~10% phenocrysts and ~20% 0.1 - 0.4 millimeters plagioclase laths and pyroxene crystals, apparently derived from enclaves. Enclave fragments are ubiquitous, commonly accounting for several percent of a cut surface. Many of the abundant gabbroic and diabasic microxenoliths and crystal aggregates (≥8 millimeters) contain olivine (typically ≥1.5 millimeters, rarely to 4 millimeters). Intensely altered to residual silica + minor specular hematite locally on Mount Scott; clinopyroxene + K-feldspar present in vugs and on fracture surfaces and replaces groundmass of rock in lowest exposures in cirque. Lava flows extend 8 kilometers east of Mount Scott.K-Ar ages: 422±10 ka, west side of cirque; 416±7 ka, east flank; 355±8 ka, ~5,650 feet elevation east of Mount Scott.------------------------------Locality: Mount Scott (as seen from an overlook on the eastern rim of Crater Lake Caldera), Crater Lake National Park, southwestern Oregon, USA------------------------------Reference cited:Bacon (2008) - Geologic map of Mount Mazama and Crater Lake Caldera, Oregon. United State Geological Survey Scientific Investigations Map 2832 [accompanying pamphlet].------------------------------See info. at:en.wikipedia.org/wiki/Mount_Scott_(Klamath_County,_Oregon) |
| 撮影日 | 2012-08-04 17:24:26 |
| 撮影者 | James St. John |
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