Rhyolites are quartz-rich volcanic rocks with abundant alkali feldspar (typically sanidine) and lesser plagioclase. Rhyolites are a volumetrically minor component of some volcanic arcs and occur as vast ash flows in some areas affected by tensional tectonics.
The light gray phenocrysts at the top of the photo above are sanidine; a quartz grain is near the bottom edge of the photo. Flow banding is clearly visible in the glassy matrix.
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