File:Devitrified rhyolitic obsidian (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka ka; Obsidian Cliff, Yellowstone, Wyoming, USA) (41624576491).jpg

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Devitrified obsidian from the Pleistocene of Wyoming, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

Black = obsidian glass Gray = devitrified obsidian

Obsidian is a glassy-textured, usually black-colored, extrusive igneous rock. It can form by very rapid cooling of lava, but usually forms by cooling of high-viscosity lava having felsic or intermediate compositions. Obsidian is a natural glass (see: <a href="https://www.flickr.com/photos/jsjgeology/16146555584">www.flickr.com/photos/jsjgeology/16146555584</a>), but it is unstable on geologic time scales. Eventually, it self-crystallizes to rhyolite. In the early stages of conversion, obsidian becomes "snowflake obsidian" (see: <a href="https://www.flickr.com/photos/jsjgeology/16561606417">www.flickr.com/photos/jsjgeology/16561606417</a>), which has whitish, snowflake-shaped spots of cristobalite (SiO2 - silica).

The sample shown above is in the advanced stages of conversion from obsidian to rhyolite. The slow process of glass converting to a finely-crystalline material is called "devitrification". The exhibit signage refers to this rock as "spherulitic rhyolite", but it's better referred to as "devitrified rhyolitic obsidian". The devitrification areas are subspherically-shaped masses - "spherulites". Spherulites having various compositions and various origins can occur in many types of rocks.

Stratigraphy: Roaring Mountain Member, Plateau Rhyolite, upper Middle Pleistocene, ~180 ka

Locality: Obsidian Cliff, northern Yellowstone National Park, northwestern Wyoming, USA
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Source Devitrified rhyolitic obsidian (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka ka; Obsidian Cliff, Yellowstone, Wyoming, USA)
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@N05/41624576491 (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.

10 October 2019

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current01:50, 10 October 2019Thumbnail for version as of 01:50, 10 October 20192,909 × 1,888 (4.33 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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