Partially devitrified rhyolitic obsidian with lithophysa (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 6 : 無料・フリー素材/写真
Partially devitrified rhyolitic obsidian with lithophysa (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 6 / 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 | Obsidian in the Pleistocene of Wyoming, USA.Obsidian is a glassy-textured, extrusive igneous rock. Glassy-textured rocks have no crystals at all. They form by very rapid cooling of lava or by cooling of high-viscosity lava. Most obsidians form by the latter. Obsidian can be felsic, intermediate, mafic, or alkaline in chemistry. Most are felsic to intermediate.A famous locality in North America is Obsidian Cliff at Yellowstone, Wyoming. It is a Pleistocene-aged lava flow with the chemistry of rhyolite (= a light-colored, felsic, aphanitic, extrusive igneous rock). The cliff itself shows columnar jointing. The rocks principally range from aphyric rhyolitic obsidian to partially devitrified rhyolitic obsidian. Lithophysae are sometimes present. Extremely small, microscopic crystals are present - they can be seen in thin sections. Some samples are reported to have small olivine phenocrysts. Small clusters of crystals, composed of plagioclase feldspar, pyroxene, and olivine, are sometimes present.Many of the whitish-colored spots and bands running through most Obsidian Cliff rock samples are areas of devitrification. Glass is unstable on geologic times scales and it slowly crystallizes. The light-colored spots and bands are now non-glassy. Spotted, partially devitrified obsidian is known by the rockhound term "snowflake obsidian" (see: www.flickr.com/photos/jsjgeology/16561606417). The spots are composed of silica (SiO2), but are not quartz. Rather, they are composed of a polymorph of quartz - cristobalite.The ellipsoidal cavity is a lithophysa. Such structures 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 this material, resulting in partially empty spaces.Stratigraphy: Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 kaLocality: loose boulder near the base of Obsidian Cliff, Yellowstone National Park, northwestern Wyoming, USA----------------------Age & some lithologic info. from:Wooton (2010) - Age and Petrogenesis of the Roaring Mountain Rhyolites, Yellowstone Volcanic Field, Wyoming. M.S. thesis. University of Nevada at Las Vegas. 296 pp. |
| 撮影日 | 2011-08-09 19:06:17 |
| 撮影者 | James St. John |
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