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. This is an unusual specimen. Note the color. Darker was closer to the center of the ash flow, at a higher temperature and is more vitreous. Potassium-argon dating gives the age of the tuff and vitrophyre as 9.5 million years.
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.
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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