High Quality Content by WIKIPEDIA articles! Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate colored matrix. Rhyodacite is a high silica rock and often exists as explosive pyroclastic volcanic deposits.