tuff - teaching hand specimen of a very pure lithified white volcanic ash
This tuff was mined for years as the scouring agent in Old Dutch Cleanser. It is a pure ash with no included clasts, looks like diatomite but is more dense, and will rub off on students' fingers. It looks just like a block of solidified scouring powder.
Many tuffs contain angular clasts (from the Greek klastos = broken), which are fragments of other rocks welded in. As the ash was being blasted out of the vent, fragments of the surrounding rock were being torn loose and blasted out with it. Angular clasts make a tuff easy to separate from a rhyolite, a lava with the same composition as tuff or granite. The rhyolite is entirely crystalline, though the crystals are often difficult to see without a 10x lens. In this case, an ash has to be identified by its gritty feel and not by included angular clasts.
Students should see a variety of tuffs. Tuffs are extremely variable, though most are relatively light in color and relatively light in weight. They should be able to tell you where the included angular clasts came from - blasted from the vent, and should use the presence of clasts to separate a tuff from a rhyolite.
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