Red Cone (Late Pleistocene cinder cone with basaltic andesite, 35-36 ka; northwest of Crater Lake Caldera, Oregon, USA) 2 : 無料・フリー素材/写真
Red Cone (Late Pleistocene cinder cone with basaltic andesite, 35-36 ka; northwest of Crater Lake Caldera, Oregon, USA) 2 / 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 | Cinder cone in the Pleistocene of Oregon, USA.Crater Lake is a large, deep, freshwater lake in the Cascade Range of America's Pacific Northwest. It formed about 7,700 years ago when an ancient volcano called Mt. Mazama had an enormous explosive ash eruption. The eruption was followed by collapse of the mountain, leaving a large depression which later filled with water. Large holes or depressions formed when a volcano destroys itself or collapses are called calderas. Crater Lake Caldera in Oregon is a world-class example of this type of volcanic feature.Shown here is Red Cone, which is a partially vegetated cinder cone volcano northwest of Crater Lake Caldera. Cinder cones are relatively small, steep-sided cones of loose igneous debris - typically scoria and vesicular basalt. Their small size is due to the geologically short lifespan of active volcanism. The rocks at Red Cone are gray-colored basaltic andesites. With oxidative weathering, the rocks become reddish-brown colored.-------------------------Partial description from Bacon (2008):Basaltic andesite of Red Cone (late Pleistocene)Light- to medium-gray porphyritic basaltic andesite (52.5 - 54% SiO2) lava flows, bombs, and cinders erupted from a vent marked by Red Cone and from a fissure vent system 1.5 kilometers north of Red Cone, northwest of the caldera.Composition is magnesian basaltic andesite, chemically primitive yet rich in incompatible trace elements. Olivine-phyric lavas represent the arc end member among primitive mafic lavas in the Crater Lake region. Recognized in the field by coarse, blocky olivine phenocrysts. Phenocrysts (5 - >20%; excluding abundant plagioclase microphenocrysts): olivine (≤5 millimeters), augite (≤4 millimeters), and plagioclase (≤2 millimeters) in an intergranular groundmass. Phenocryst assemblages vary from olivine only to olivine+augite to olivine+augite+plagioclase, phenocryst content increasing in that order. Microxenoliths (≤5 millimeters) of augite±olivine±plagioclase abundant in samples with those phases as phenocrysts.Potassium-argon age: 36±12 ka; 40Ar/39Ar plateau age: 35±4 ka.-------------------------Age: Late Pleistocene, 35-36 kaLocality: Red Cone, 3.5 to 4 kilometers northwest of the northwestern margin of Crater Lake Caldera, Crater Lake National Park, southwestern Oregon, USA (42° 59' 50.97" North latitude, 122° 09' 43.81" West longitude)-------------------------Reference cited:Bacon (2008) - Geologic map of Mount Mazama and Crater Lake Caldera, Oregon. United State Geological Survey Scientific Investigations Map 2832 [accompanying pamphlet]. |
| 撮影日 | 2012-08-04 15:41:54 |
| 撮影者 | James St. John |
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