Rhyolitic tuff has been used extensively 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.
Description
QAPF diagram with rhyolite field highlightedTAS diagram with rhyolite field highlighted
Rhyolite is an extrusive igneous rock, formed from magma rich in silica 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 of mafic minerals, and it is typically very fine-grained (aphanitic) or glassy.[5]
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.[6][7][8][9]: 140–146
Due to their high content of silica and low iron and magnesium contents, rhyolitic magmas form highly viscous lavas.[9]: 23–26 As a result, many eruptions of rhyolite are highly explosive, and rhyolite occurs more frequently as pyroclastic rock than as lava flows.[10]: 22 Rhyolitic ash flow tuffs are the only volcanic product with volumes rivaling those of flood basalts.[9]: 77 Rhyolites also occur as breccias or in lava domes, volcanic plugs, and dikes.[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
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 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.[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]
Petrogenesis
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.[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 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.[22]
Occurrence
Rhyolite is common along convergent plate boundaries, where a slab of oceanic lithosphere is being subducted into the Earth's mantle beneath overriding oceanic or continental lithosphere. It can sometimes be the predominant igneous rock type in these settings. Rhyolite is more common when the overriding lithosphere 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] The tholeiitic magmas erupted at volcanic ocean islands, such as Iceland, can sometimes differentiate all the way to rhyolite, and about 8% of the volcanic rock in Iceland is rhyolite. However, this is unusual, and the Hawaiian Islands (for example) have no known occurrences of rhyolite. The alkaline magmas of volcanic ocean islands will very occasionally differentiate all the way to peralkaline rhyolites, but differentiation usually ends with trachyte.[25]
Small volumes of rhyolite are sometimes erupted in association with flood basalts, late in their history and where central volcanic complexes develop.[26]
Name
The name rhyolite was introduced into geology in 1860 by the German traveler and geologist Ferdinand von Richthofen[27][28][29] from the Greek word rhýax ("a stream of lava")[30] and the rock name suffix "-lite".[31]
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 an abrasive, in concrete,[36] and as a soil amendment.[37] Rhyolitic tuff was used extensively for construction in ancient Rome[38] and has been used in construction in modern Europe.[21]: 138
^ abcBlatt, Harvey; Tracy, Robert J. (1996). Petrology : igneous, sedimentary, and metamorphic (2nd ed.). New York: W.H. Freeman. pp. 55, 74. ISBN0716724383.
^ abcdePhilpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. ISBN9780521880060.
^Hanson, Richard E.; Schweickert, Richard A. (1 November 1982). "Chilling and Brecciation of a Devonian Rhyolite Sill Intruded into Wet Sediments, Northern Sierra Nevada, California". The Journal of Geology. 90 (6): 717–724. Bibcode:1982JG.....90..717H. doi:10.1086/628726. S2CID128948336.
^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.
^Wolff, J. A.; Ramos, F. C. (1 February 2014). "Processes in Caldera-Forming High-Silica Rhyolite Magma: Rb-Sr and Pb Isotope Systematics of the Otowi Member of the Bandelier Tuff, Valles Caldera, New Mexico, USA". Journal of Petrology. 55 (2): 345–375. doi:10.1093/petrology/egt070.
^Richthofen, Ferdinand Freiherrn von (1860). "Studien aus den ungarisch-siebenbürgischen Trachytgebirgen" [Studies of the trachyte mountains of Hungarian Transylvania]. Jahrbuch der Kaiserlich-Königlichen Geologischen Reichsanstalt (Wein) [Annals of the Imperial-Royal Geological Institute of Vienna] (in German). 11: 153–273.
^Simpson, John A.; Weiner, Edmund S. C., eds. (1989). Oxford English Dictionary. Vol. 13 (2nd ed.). Oxford: Oxford University Press. p. 873.
^Grasser, Klaus (1990). Building with Pumice(PDF). Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ). p. 64. ISBN3-528-02055-5. Retrieved 23 March 2019.
