Rhyodacite vitrophyre (Llao Rock Rhyodacite, Holocene, ~7.8-7.9 ka; roadcut near Llao Rock, margin of Crater Lake Caldera, Oregon, USA) 59 : 無料・フリー素材/写真
Rhyodacite vitrophyre (Llao Rock Rhyodacite, Holocene, ~7.8-7.9 ka; roadcut near Llao Rock, margin of Crater Lake Caldera, Oregon, USA) 59 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
|---|---|
| 説明 | Flow-banded rhyodacite vitrophyre in the Holocene 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.The black glassy rocks seen here are rhyodacite vitrophyre, a rock that is close to obsidian - vitrophyre is volcanic glass having phenocrysts. Some of the whitish spots in these rocks are from devitrification. This is part of a lava flow that erupted one to two hundred years before Mazama's caldera-forming eruption.The cavities seen here are lithophysae - they formed before the rock completely solidified. The original lava flow had some subspherical structures known as spherulites, composed of glassy to cryptocrystalline material (many felsic extrusive igneous rocks have these). Expanding gases in the spherulites destroyed the material, resulting in empty spaces. The lithophysae have been elongated in the direction of the flowing lava.Stratigraphy: Llao Rock Rhyodacite, lower Holocene, ~7800 to 7900 years oldLocality: roadcut near Llao Rock, Crater Lake Caldera, Crater Lake National Park, southwestern Oregon, USA |
| 撮影日 | 2012-08-04 16:20:11 |
| 撮影者 | James St. John |
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