File:Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 30 (46766616582).jpg
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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, which formed by cooling and contraction. The rocks principally range from black-colored, 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: <a href="https://www.flickr.com/photos/jsjgeology/16561606417">www.flickr.com/photos/jsjgeology/16561606417</a>). The spots are composed of silica (SiO2), but are not quartz. Rather, they are composed of a polymorph of quartz - cristobalite. Why does Obsidian Cliff here not look black and glassy? The rocks are weathered, partially devitrified, and considerably lichen-covered. Classic, black, glassy obsidian can be seen in some of the boulders along the road. Stratigraphy: Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka Locality: Obsidian Cliff, eastern edge of Obsidian Creek Valley, 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. |
| Date | |
| Source | Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 30 |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/46766616582 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 01:53, 10 October 2019 | | 3,008 × 2,000 (4.85 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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