tuff - densely welded tuff - teaching hand/display specimen
There is a progression in tuffs, from unwelded volcanic ash (the source material) to welded volcanic ash (tuff) to densely welded tuff. Further welding would result in vitrophyre, a phenocryst-bearing obsidian. Densely welded tuffs are uncommon. This tuff, a member of the Resting Springs Formation, formed from a pyroclastic flow that was particularly hot in the middle. The middle was so hot it welded into a vitrophyre, essentially an obsidian with phenocrysts. On either side of the vitrophyre the tuff was hot enough to become densely welded, almost glassy, and ceramic in appearance as is this specimen.
The Resting Springs Tuff is exposed in a spectacular roadcut east of Shoshone, in Inyo County, California. It is Miocene in age. The field photo shows the outcrop, with black vitrophyre sandwiched between layers of densely welded tuff.
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