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

Igneous rock that is the volcanic equivalent of granite, whose chemical composition is similar. Rhyolites are known from all parts of the Earth and from all geologic ages; they are found mostly on the continents or their immediate margins, but small quantities have been described from remote islands.

---

rhyolite [′rī·ə‚līt]

(petrology)

A light-colored, aphanitic volcanic rock composed largely of alkali feldspar and free silica with minor amounts of mafic minerals; the extrusive equivalent of granite.

---

Rhyolite 

(also liparite), a cenotypal, extrusive rock rich in silica (68-77 percent SiO₂). It has a porphyritic texture and contains phenocrysts of quartz, potassium feldspar, plagioclase, and, less commonly, biotite or pyroxene in a glassy groundmass, usually with flow texture. A glassy variety virtually without phenocrysts is called obsidian. Paleotypal analogs are classified according to their alkali composition as keratophyres (sodic) or orthophyres (potassic). Rhyolite is the extrusive equivalent of granitoids.