Pitchstone Plateau, Yellowstone, rhyolite with sanidine

(Left) Sample of the Pitchstone Plateau rhyolite flow, which erupted about 72,000 years ago, making it is the youngest rhyolite at Yellowstone. The blocky white crystals in this sample are the mineral sanidine, whereas the rounded crystals are quartz. The sample is about 3 inches (8 cm) across.  (Right) Sanidine crystals, each less than 1 millimeter in size, separated from the Pitchstone Plateau rhyolite flow. Individual crystals like these are used in determinine the age of the rhyolite flow.