tuff - teaching student specimens of gritty white tuff with almost no clasts - UNIT OF 5 SPECIMENS
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This is a gritty white tuff - a welded volcanic ash. It has almost no clasts and might challenge students who expect to see broken fragments of rocks that have been torn from the vent included in the welded ash.
Many tuffs contain angular clasts (from the Greek klastos = broken). As the ash was being blasted out of the vent, fragments of other rocks were being torn loose and blasted out with it. Angular clasts make a tuff relatively easy to identify and to separate from a rhyolite.
Both rhyolite and tuff are derived from a magma with the same composition as granite. Tuffs are variable in color as is rhyolite, but rhyolite but is entirely crystalline, though the crystals are often difficult to see without a 10x lens.
An adjacent less welded layer of this tuff, a sedimentary pumice lapilli tuff, was mined from 1919 to around 1960. It was used as the polishing agent in toothpaste, in oil-absorbing compounds, and in acoustical plaster, cleaning compounds, wood fillers and in paint. The bed dips steeply to the west and the miners simply followed it down.
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