rhyolite

rhyolite fine-grained light-colored acidic volcanic rock . Rhyolite is chemically the equivalent of granite, and is thus composed primarily of quartz and orthoclase feldspar with subordinate amounts of plagioclase feldspar, biotite mica, amphiboles, and pyroxenes. Rhyolite lava exhibits a typical banded structure produced by its flow pattern. Rhyolite lavas occur in continental and submarine volcanoes, especially island arcs, and in igneous dikes. Rhyolite lavas are typically highly viscous and are explosively ejected from volcanoes. Rhyolites were formed in profusion in the Yellowstone Park area and throughout the southwestern portion of the United States.

rhyolite A fine-grained, extrusive, igneous rock, often with a sugary texture, consisting of essential quartz, alkali feldspar, and one or more ferromagnesian minerals. Alkali rhyolites are the most common type, being characterized by the ferromagnesian mineral biotite with or without pyroxene, and are found in calcalkaline terrains. Peralkaline rhyolites are characterized by alkali pyroxenes (aegirine, aegirine—augite) and alkali amphiboles (riebeckite, arfvedsonite), and are found as end-members of alkaline magma series on oceanic islands and rifted continental crust.