Rhyolitic tuff has been extensively used for construction. Obsidian, which is rhyolitic volcanic glass, has been used for tools from prehistoric times to the present day because it can be shaped to an extremely sharp edge. Rhyolitic pumice finds use as an abrasive, in concrete, and as a soil amendment.
QAPF diagram with rhyolite field highlighted
TAS diagram with rhyolite field highlighted
Rhyolite is an extrusive igneous rock, formed from magma rich in silica that is extruded from a vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content of mafic minerals, and it is typically very fine-grained (aphanitic) or glassy.
An extrusive igneous rock is classified as rhyolite when quartz constitutes 20% to 60% by volume of its total content of quartz, alkali feldspar, and plagioclase (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 the IUGS 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 and alkali metal oxides (K2O plus Na2O). Rhyolite is high in silica and total alkali metal oxides, placing it in the R field of the TAS diagram.:140–146
Due to their high content of silica and low iron and magnesium contents, rhyolitic magmas form highly viscous lavas.:23–26 As a result, many eruptions of rhyolite are highly explosive, and rhyolite occurs more frequently as pyroclastic rock than as lava flows.:22 Rhyolitic ash flow tuffs are the only volcanic product with volumes rivaling those of flood basalts.:77 Rhyolites also occur as breccias or in lava domes, volcanic plugs, and dikes.: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).:20
Eruptions of rhyolite 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 the St. Andrew Strait volcano in Papua New Guinea and Novarupta volcano in Alaska as well as at Chaitén and Cordón Caulle volcanoes in southern Chile. The eruption of Novarupta in 1912 was the largest volcanic eruption of the 20th century, and began with explosive volcanism that later transitioned to effusive volcanism and the formation of a rhyolite dome in the vent.
Rhyolite magmas can be produced by igneous differentiation of a more mafic (silica-poor) magma, through fractional crystallization or by assimilation of melted crustal rock (anatexis). Associations of andesites, dacites, and rhyolites in similar tectonic settings and with similar chemistry suggests that the rhyolite members were formed by differentiation of mantle-derived basaltic magmas at shallow depths. In other cases, the rhyolite appears to be a product of melting of crustal sedimentary rock.:21 Water vapor plays an important role in lowering the melting point of silicic rock,:43 and some rhyolitic magmas may have a water content as high as 7–8 weight percent.:44
High-silica rhyolite (HSR), with a silica content of 75 to 77·8% SiO 2, forms a distinctive subgroup within the rhyolites. HSRs are the most evolved of all igneous rocks, with a composition very close to the water-saturated granite eutectic and with extreme enrichment in most incompatible elements. However, they are highly depleted in strontium, barium, and europium. They are interpreted as products of repeated melting and freezing of granite in the subsurface. HSRs typically erupt in large caldera eruptions.
Rhyolite has been found on islands far from land, but such oceanic occurrences are rare.
Rhyolite in the Kaldaklofsfjöll, Landmannalaugar, Iceland
Obsidian is usually of rhyolitic composition, and it has been used for tools since prehistoric times. Obsidian scalpels have been investigated for use in delicate surgery. Pumice, also typically of rhyolitic composition, finds important uses as an abrasive, in concrete, and as a soil amendment. Rhyolitic tuff was used extensively for construction in ancient Rome and has been used in construction in modern Europe.:138
^Spell, Terry L.; Kyle, Philip R. (1989). "Petrogenesis of Valle Grande Member rhyolites, Valles Caldera, New Mexico: Implications for evolution of the Jemez Mountains Mgmatic System". Journal of Geophysical Research: Solid Earth. 94 (B8): 10379–10396. Bibcode:1989JGR....9410379S. doi:10.1029/JB094iB08p10379.
^Raymond, Loren A. (1997). Petrology : the study of igneous, sedimentary, metamorphic rocks (Complete customized version ed.). Dubuque, IA: McGraw-Hill Companies, Inc. p. 27. ISBN0697413403.
^ abcdFarndon, John (2007) The Illustrated Encyclopedia of Rocks of the World. Southwater. p. 54. ISBN1844762696
^Martí, J.; Aguirre-Díaz, G.J., and Geyer, A. (2010). "The Gréixer rhyolitic complex (Catalan Pyrenees): an example of Permian caldera". Workshop on Collapse Calderas – La Réunion. IAVCEI – Commission on Collapse Calderas.
^Jackson, M. D.; Marra, F.; Hay, R. L.; Cawood, C.; Winkler, E. M. (2005). "The Judicious Selection and Preservation of Tuff and Travertine Building Stone in Ancient Rome*". Archaeometry. 47 (3): 485–510. doi:10.1111/j.1475-4754.2005.00215.x.