Rhyolite
Igneous rock
PinkRhyolite.tif
Composition
Felsic: igneous quartz and alkali feldspar (sanidine and sodic plagioclase), biotite and hornblende

Rhyolite (/ˈr.əˌlt/ RY-ə-lyte)[1][2][3][4] is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained rock. The mineral assemblage is predominantly quartz, sanidine and plagioclase. It is the extrusive equivalent to granite.

Magma with the composition of rhyolite is extremely viscous, due to its high silica content. This favors explosive eruptions over effusive eruptions, so rhyolitic magma is more often erupted as pyroclastic rock than as lava flows. Rhyolitic ash-flow tuffs are among the most voluminous of continental igneous rock formations.

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.

Description

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.[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

The alkali feldspar in rhyolites is sanidine or, less commonly, orthoclase. It is rarely anorthoclase. These feldspar minerals sometimes are present as phenocrysts. The plagioclase is usually sodium-rich (oligoclase or andesine). Cristobalite and trydimite are sometimes present along with the quartz. Biotite, augite, fayalite, and hornblende are common accessory minerals.[5]

Geology

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

Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian.[13] Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice.[5]

Peralkaline rhyolites include comendite and pantellerite.[14] Peralkalinity has significant effects on lava flow morphology and mineralogy, such that peralkaline rhyolites can be 10–30 times more fluid than typical calc-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 comprise bimodal shield volcanoes at hotspots and rifts (e.g. Rainbow Range, Ilgachuz Range and Level Mountain in British Columbia, Canada).[15]

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.[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% 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.[22]

Occurrence

Rhyolite has been found on islands far from land, but such oceanic occurrences are rare.[23]

Examples

Europe

Rhyolite in the Kaldaklofsfjöll, Landmannalaugar, Iceland
Germany
Rhyolite quarry, Löbejün, Saxony-Anhalt

North America

Mount Haynes, an eroded remnant of a rhyolite dome in Yellowstone National Park, USA

South America