tuff - teaching hand specimen of bright white welded volcanic ash with scattered dark clasts
This is a very white tuff - a welded volcanic ash. It has enough angular fragments of other rocks included to clue students in that it's a tuff. Another white tuff from the same mine has almost no clasts, feels gritty, and is not as bright. It would be a challenge to students. We normally have examples of both types in stock.
Many tuffs contain angular clasts (from the Greek klastos = broken) - fragments of other rocks welded in. 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 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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