Rhyodacite lava & pumice-fall deposit (Holocene, ~7.8 to 7.9 ka; eastern flanks of Llao Rock, Crater Lake Caldera, Oregon, USA) 2 : 無料・フリー素材/写真
Rhyodacite lava & pumice-fall deposit (Holocene, ~7.8 to 7.9 ka; eastern flanks of Llao Rock, 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 | Lava, pumice, and volcanic ash in the Holocene of Orgeon, 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 event involved an estimated 50 cubic kilometers of magma being erupted. This 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 eastern lower flanks of Llao Rock, the most distinctive cliff-face along the rim of the caldera. The grayish-colored unit in the upper left part of the picture is the Llao Rock lava flow, which erupted ~7,800 to 7,900 years ago, not long before the caldera-forming event. This is a single lava flow erupted from a vent located here at Llao Rock. The maximum reported thickness of the Llao Rock lava flow is 1,200 feet. The rocks are composed of rhyodacite (also referred to as felsite) and usually have ~70.5% total silica. About 7% of this rhyodacite lava consists of small phenocrysts composed of plagioclase feldspar, hornblende amphibole, orthopyroxene, iron & titanium oxide minerals, and augite pyroxene. The lava's groundmass is finely-crystalline (aphanitic). Masses of darker-colored andesite are common in this unit.The light-colored material is a pumice-fall deposit. It is a mix of rhyodacitic volcanic ash and pumice erupted before the cliff-forming, Llao Rock lava flow, but from the same vent and at about the same time (7,800 to 7,900 years ago).Gray-colored landslide deposits (left) partly cover the pumice-fall deposit.Locality: Llao Rock (view from caldera rim ~south of Grouse Hill picnic area), Crater Lake Caldera, Crater Lake National Park, southwestern Oregon, USA------------------------------Site-specific geologic info. mostly synthesized from: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 17:05:13 |
| 撮影者 | James St. John |
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