| Igneous rock | |
 | |
| Composition | |
|---|---|
| Classification | Felsic |
| Primary | Quartz andalkali feldspar (sanidine and sodicplagioclase) |
| Secondary | Biotite andhornblende |
| Texture | Glassy oraphanitic, sometimesporphyritic |
Rhyolite (/ˈraɪ.əlaɪt/RY-ə-lyte)[1][2][3][4] is the mostsilica-rich ofvolcanic rocks. It is generallyglassy or fine-grained (aphanitic) intexture, but may beporphyritic, containing larger mineralcrystals (phenocrysts) in an otherwise fine-grainedgroundmass. Themineral assemblage is predominantlyquartz,sanidine, andplagioclase. It is theextrusive equivalent ofgranite.
Its high silica content makes rhyoliticmagma extremelyviscous. This favorsexplosive eruptions overeffusive eruptions, so this type of magma is more often erupted aspyroclastic rock than aslava flows. Rhyolitic ash-flowtuffs are among the most voluminous of continentaligneous rock formations.
Rhyolitic tuff has been used extensively for construction.Obsidian, which is rhyoliticvolcanic glass, has been used for tools from prehistoric times to the present day because it can be shaped to an extremely sharp edge. Rhyoliticpumice finds use as anabrasive, inconcrete, and as asoil amendment.
Rhyolite is anextrusive igneous rock, formed from magma rich insilica that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content ofmafic minerals, and it is typically very fine-grained (aphanitic) orglassy.[5]
An extrusive igneous rock is classified as rhyolite whenquartz constitutes 20% to 60% by volume of its total content of quartz,alkali feldspar, andplagioclase (QAPF) and alkali feldspar makes up 35% to 90% of its total feldspar content.Feldspathoids are not present. This makes rhyolite the extrusive equivalent of granite. However, while theIUGS recommends classifying volcanic rocks on the basis of their mineral composition whenever possible, volcanic rocks are often glassy or so fine-grained that mineral identification is impractical. The rock must then be classified chemically based on its content of silica andalkali metal oxides (K2O plusNa2O). Rhyolite is high in silica and total alkali metal oxides, placing it in the R field of theTAS diagram.[6][7][8][9]: 140–146
The alkali feldspar in rhyolites issanidine or, less commonly,orthoclase. It is rarelyanorthoclase. These feldspar minerals sometimes are present as phenocrysts. The plagioclase is usuallysodium-rich (oligoclase orandesine).Cristobalite andtrydimite are sometimes present along with the quartz.Biotite,augite,fayalite, andhornblende are common accessory minerals.[5]
Due to their high content of silica and lowiron andmagnesium contents, rhyoliticmagmas form highly viscouslavas.[9]: 23–26 As a result, many eruptions of rhyolite are highly explosive, and rhyolite occurs more frequently aspyroclastic rock than aslava flows.[10]: 22 Rhyolitic ash flowtuffs are the only volcanic product with volumes rivaling those offlood basalts.[9]: 77 Rhyolites also occur asbreccias or inlava domes,volcanic plugs, anddikes.[11][12][9]: 71–72 Rhyolitic lavas erupt at a relatively low temperature of 800 to 1,000 °C (1,470 to 1,830 °F), significantly cooler than basaltic lavas, which typically erupt at temperatures of 1,100 to 1,200 °C (2,010 to 2,190 °F).[9]: 20
Rhyolites that cool too quickly to growcrystals form a natural glass or vitrophyre, also calledobsidian.[13] Slower cooling forms microscopic crystals in the lava and results in textures such as flowfoliations,spherulitic,nodular, andlithophysal structures. Some rhyolite is highlyvesicularpumice.[5]
Peralkaline rhyolites (rhyolites unusually rich in alkali metals) includecomendite andpantellerite.[14] Peralkalinity has significant effects onlava flow morphology andmineralogy, such that peralkaline rhyolites can be 10–30 times more fluid than typicalcalc-alkaline rhyolites. As a result of their increased fluidity, they are able to form small-scale flow folds,lava tubes and thin dikes. Peralkaline rhyolites erupt at relatively high temperatures of more than 1,200 °C (2,190 °F). They comprisebimodalshield volcanoes athotspots andrifts (e.g.Rainbow Range,Ilgachuz Range andLevel Mountain inBritish Columbia, Canada).[15]
Eruptions of rhyolite lava are relatively rare compared to eruptions of less felsic lavas. Only four eruptions of rhyolite have been recorded since the start of the 20th century: at theSt. Andrew Strait volcano inPapua New Guinea andNovarupta volcano inAlaska as well as atChaitén andCordón Caulle volcanoes in southernChile.[16][17] The eruption of Novarupta in 1912 was the largest volcanic eruption of the 20th century,[18] and began with explosive volcanism that later transitioned to effusive volcanism and the formation of a rhyolite dome in the vent.[19]
Rhyolite magmas can be produced byigneous differentiation of a more mafic (silica-poor) magma, throughfractional crystallization or by assimilation of melted crustal rock (anatexis). Associations ofandesites,dacites, and rhyolites in similar tectonic settings and with similar chemistry suggests that the rhyolite members were formed by differentiation of mantle-derivedbasaltic magmas at shallow depths. In other cases, the rhyolite appears to be a product of melting of crustal sedimentary rock.[10]: 21 Water vapor plays an important role in lowering the melting point of silicic rock,[10]: 43 and some rhyolitic magmas may have a water content as high as 7–8 weight percent.[20][21]: 44
High-silica rhyolite (HSR), with a silica content of 75 to 77·8%SiO2, forms a distinctive subgroup within the rhyolites. HSRs are the mostevolved of all igneous rocks, with a composition very close to the water-saturated graniteeutectic and with extreme enrichment in mostincompatible elements. However, they are highly depleted instrontium,barium, andeuropium. They are interpreted as products of repeated melting and freezing of granite in the subsurface. HSRs typically erupt in largecaldera eruptions.[22]
Rhyolite is common alongconvergent plate boundaries, where a slab ofoceanic lithosphere is beingsubducted into theEarth's mantle beneath overriding oceanic orcontinental lithosphere. It can sometimes be the predominant igneous rock type in these settings. Rhyolite is more common when the overridinglithosphere is continental rather than oceanic. The thicker continental crust gives the rising magma more opportunity to differentiate and assimilate crustal rock.[23]
Rhyolite has been found on islands far from land, but such oceanic occurrences are rare.[24] Thetholeiitic magmas erupted at volcanic ocean islands, such asIceland, can sometimes differentiate all the way to rhyolite, and about 8% of the volcanic rock in Iceland is rhyolite. However, this is unusual, and theHawaiian Islands (for example) have no known occurrences of rhyolite. Thealkaline magmas of volcanic ocean islands will very occasionally differentiate all the way to peralkaline rhyolites, but differentiation usually ends withtrachyte.[25]
Small volumes of rhyolite are sometimes erupted in association withflood basalts, late in their history and where central volcanic complexes develop.[26]
The name rhyolite was introduced intogeology in 1860 by the German traveler and geologistFerdinand von Richthofen[27][28][29] from the Greek wordrhýax ("a stream of lava")[30] and the rock name suffix "-lite".[31]
InNorth American pre-historic times, rhyolite was quarried extensively in what is now easternPennsylvania. Among the leading quarries was theCarbaugh Run Rhyolite Quarry Site inAdams County. Rhyolite was mined there starting 11,500 years ago.[32] Tons of rhyolite were traded across theDelmarva Peninsula,[32] because the rhyolite kept a sharp point whenknapped and was used to make spear points and arrowheads.[33]
Obsidian is usually of rhyolitic composition, and it has been used for tools since prehistoric times.[34] Obsidian scalpels have been investigated for use in delicate surgery.[35] Pumice, also typically of rhyolitic composition, finds important uses as anabrasive, inconcrete,[36] and as asoil amendment.[37] Rhyolitic tuff was used extensively for construction in ancient Rome[38] and has been used in construction in modern Europe.[21]: 138
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