rhyolite

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rhyolite,

fine-grained light-colored acidic volcanic rockrock,
aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their origin, into three major classes—igneous, sedimentary, and metamorphic.
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. 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.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.