Light colored fined grained volcanic rock composed of glass, quartz, K-feldspar and plagioclase, with relatively high silica (69-77 wt %). The composition reflects partial melting of continental crust. Rhyolitic volcanism is a feature of both Yellowstone (USA) and the Taupo Volcanic Zone (New Zealand) where geothermal activity is widespread.  The intrusive coarsely crystalline igneous rock equivalent is called granite.