Llao Rock (Crater Lake Caldera, Oregon, USA) 14 : 無料・フリー素材/写真
Llao Rock (Crater Lake Caldera, Oregon, USA) 14 / 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 | Volcanic ash, pumice deposits, and lava flow 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 Llao Rock, the most distinctive cliff-face along the rim of the caldera. The tan-colored material at the top is a mix of rhyodacite pumice and ash deposited during the caldera-forming eruption at 7.7 ka. This material was deposited as pumice fall and ash fall throughout much of America's Pacific Northwest and southwestern Canada.The grayish-colored rocks making up most of the cliff are part of 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.Locality: Llao Rock (view from Merriam Point), 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 15:54:25 |
| 撮影者 | James St. John |
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