Wizard Island Cinder Cone (Holocene, ~7.2 ka; Crater Lake Caldera, Oregon, USA) 22 : 無料・フリー素材/写真
Wizard Island Cinder Cone (Holocene, ~7.2 ka; Crater Lake Caldera, Oregon, USA) 22 / 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 | 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.The island in the lake is Wizard Island, a cinder cone volcano that formed after Mazama's caldera-forming eruption. Cinder cones are relatively small, steep-sided cones of loose igneous debris. They have few eruptions and a relatively large summit crater. Cinder cones are principally composed of scoria and vesicular basalt.------------------------------Partial description from Bacon (2008):Andesite of Wizard Island (Holocene)Porphyritic, commonly seriate textured, dark-gray to grayish-black blocky andesite (58.5-60% SiO2) lava flows and breccia. Lava forming Wizard Island was extruded from vents at base of cinder cone and flowed into lake, forming pillowed flows and glassy breccia mantling the slopes below the lake's surface. Prominent increase in slope below depth of ~75 meters indicates lake level at time of final eruptions. Edifice of Wizard Island rises ~550 meters above the lake floor, the cinder cone adding another ~200 meters. Phenocrysts (~20%; ~50% when microphenocrysts are included): plagioclase (≤4 millimeters; commonly sieved or resorbed), augite (≤1.2 millimeters), orthopyroxene (≤1.0 millimeters), Fe-Ti oxides (≤0.3 millimeters), and sparse olivine (≤0.5 millimeters, rarely to 2 millimeters; typically with plagioclase) in very fine grained or glassy groundmass rich in ≤0.4 millimeter plagioclase crystals. Some samples contain rare resorbed hornblende (≤5 millimeters). Aggregates (up to at least 5 centimeters) of plagioclase ± olivine + augite + orthopyroxene ± oxides + melt common locally, as are medium-gray subangular xenoliths (to at least 10 centimeters) of fine-grained, plagioclase rich igneous rocks. The xenoliths may be partially melted and commonly have hornfelsic textures. Partially melted quartz-bearing granitic xenoliths present locally. Samples from presently exposed Wizard Island tend to have coarser microphenocrysts and more fully crystallized groundmasses than samples from the slopes below 75 meters depth.Age constrained by inference that drowned subaerial lava flows were erupted when lake level was ~75 meters lower than today, which implies eruption within at most ~500 years of caldera collapse, or ~7,200 years before present. This age is consistent with paleomagnetic measurements and secular variation history and with the ~4,800 years before present age of [overlying] unit r [= rhyodacite of a subaqueous postcaldera dome on the eastern side of Wizard Island].------------------------------Locality: Crater Lake Caldera (view from Rim Village), 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]. |
| 撮影日 | 2012-08-04 19:18:20 |
| 撮影者 | James St. John |
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