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Definition: rhyolite from The Columbia Encyclopedia

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.

Summary Article: Rhyolite
from Guide to Minerals, Rocks and Fossils

Color Usually light coloured; white, grey, greenish, reddish or brownish. The colour may be even, or in bands of differing shades. Grain size Fine to very fine. Texture Frequently shows alternating layers that differ slightly in granularity or colour. Phenocrysts not uncommon (porphyritic rhyolite). Flow banding is sometimes evident, defined by swirling layers of differing colour or granularity, and by aligned phenocrysts. Structure Vesicles or amygdales may be present. (Pumice is a highly vesicular variety of rhyolite.) May contain spherulites which are spherical bodies, often coalescing, comprising radial aggregates of needles, usually of quartz or feldspar. Spherulites are generally less than 0.5 cm in diameter, but they may reach a metre or more across. They form by very rapid growth in quickly cooling magma, and by the crystallization of glass. Mineralogy As for granite, but rapid cooling results in minute crystals. Phenocrysts of quartz, feldspar, hornblende or mica occur. Field relations Flows, dykes and plugs. Rhyolite (or granite) magma is highly viscous and so flows only very slowly, so that if it is extruded it forms very short, thick flows or is confined as a plug in the throat of a volcano.

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