Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (silica-rich) composition. It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase. Biotite and hornblende are common accessory minerals. Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas. They can also occur as breccias or in volcanic plugs and dikes.
Contributions by Vsmith, Hike395, and ClueBot NG.