rhyolite


rhyolite
rhyolitic /ruy'euh lit"ik/, adj.
/ruy"euh luyt'/, n.
a fine-grained igneous rock rich in silica: the volcanic equivalent of granite.
[1865-70; rhyo- (irreg. < Gk rhýax stream of lava) + -LITE]

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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.

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rock
      extrusive igneous rock that is the volcanic equivalent of granite. Most rhyolites are porphyritic, indicating that crystallization began prior to extrusion. Crystallization may sometimes have begun while the magma was deeply buried; in such cases, the rock may consist principally of well-developed, large, single crystals (phenocrysts) at the time of extrusion. The amount of microcrystalline matrix (groundmass) in the final product may then be so small as to escape detection except under the microscope; such rocks (nevadites) are easily mistaken for granite in hand specimens. In most rhyolites, however, the period of such crystallization is relatively short, and the rock consists largely of a microcrystalline or partly glassy matrix containing few phenocrysts. The matrix is sometimes micropegmatitic or granophyric. The glassy rhyolites include obsidian, pitchstone, perlite, and pumice.

      The chemical composition of rhyolite is very like that of granite. This equivalence implies that at least some and probably most granites are of magmatic origin. The phenocrysts of rhyolite may include quartz, alkali feldspar, oligoclase feldspar, biotite, amphibole, or pyroxene. If an alkali pyroxene or alkali amphibole is the principal dark mineral, oligoclase will be rare or absent, and the feldspar phenocrysts will consist largely or entirely of alkali feldspar; rocks of this sort are called pantellerite. If both oligoclase and alkali feldspar are prominent among the phenocrysts, the dominant dark silicate will be biotite, and neither amphibole nor pyroxene, if present, will be of an alkaline variety; such lavas are the quartz porphyries or “true” rhyolites of most classifications.

      Certain differences between rhyolite and granite are noteworthy. Muscovite, a common mineral in granite, occurs very rarely and only as an alteration product in rhyolite. In most granites the alkali feldspar is a soda-poor microcline or microcline-perthite; in most rhyolites, however, it is sanidine, not infrequently rich in soda. A great excess of potassium over sodium, uncommon in granite except as a consequence of hydrothermal alteration, is not uncommon in rhyolites.

      Rhyolites are known from all parts of the Earth and from all geologic ages. They are mostly confined, like granites, to the continents or their immediate margins, but they are not entirely lacking elsewhere. Small quantities of rhyolite (or quartz trachyte) have been described from oceanic islands remote from any continent.

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Universalium. 2010.

Look at other dictionaries:

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  • rhyolite — [rī′ə līt΄] n. [Ger rhyolit < Gr rhyax, stream (of lava) + lithos, stone] a light colored igneous rock with a fine grained, granitelike texture …   English World dictionary

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  • rhyolite — ALASKA VOLCANO OBSERVATORY GLOSSARY Volcanic rock (or lava) that characteristically is light in color, contains 69% silica or more, and is rich in potassium and sodium. GLOSSARY OF VOLCANIC TERMS A volcanic rock containing greater than 68% silica …   Glossary of volcanic terms

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  • rhyolite — rhyolithe ou rhyolite [ rjɔlit ] n. f. • 1883; du gr. rhuô, de rhein « couler », et lithe ♦ Géol. Lave volcanique de composition granitique, à texture souvent porphyrique, dont la pâte est partiellement vitreuse. ● rhyolite nom féminin Roche… …   Encyclopédie Universelle

  • rhyolite — noun Etymology: German Rhyolith, from Greek rhyax stream, stream of lava (from rhein) + German lith lite Date: 1868 a very acid volcanic rock that is the lava form of granite • rhyolitic adjective …   New Collegiate Dictionary

  • rhyolite — noun An igneous, volcanic (extrusive) rock, of felsic composition, with aphanitic to porphyritic texture …   Wiktionary

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