tuff - teaching student specimens of densely welded Resting Springs tuff Unit of 5 specimens
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 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. This is an unusual specimen.
Flattened pumice lapilli in this densely welded tuff appear as dark lens shapes called fiamme, Italian for flames. Farther from the center of the tuff bed, where welding was not as intense, the flattened pumice fiamme are much larger and retain the pumice texture.
A cross section through the Resting Springs Tuff is exposed in a spectacular roadcut east of Shoshone, in Inyo County, California. It is Miocene in age.
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