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. Andrew Strait volcano in Papua New Guinea and Novarupta volcano in Alaska as well as at Chaitén and Cordón Caulle volcanoes in southern Chile. Only four eruptions of rhyolite have been recorded since the start of the 20th century: at the St. Įruptions of rhyolite lava are relatively rare compared to eruptions of less felsic lavas. Rainbow Range, Ilgachuz Range and Level Mountain in British Columbia, Canada). They comprise bimodal shield volcanoes at hotspots and rifts (e.g. Peralkaline rhyolites erupt at relatively high temperatures of more than 1,200 ☌ (2,190 ☏). As a result of their increased fluidity, they are able to form small-scale flow folds, lava tubes and thin dikes. 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. Peralkaline rhyolites (rhyolites unusually rich in alkali metals) include comendite and pantellerite. Some rhyolite is highly vesicular pumice. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian.
: 71–72 Rhyolitic lavas erupt at a relatively low temperature of 800 to 1,000 ☌ (1,470 to 1,830 ☏), significantly cooler than basaltic lavas, which typically erupt at temperatures of 1,100 to 1,200 ☌ (2,010 to 2,190 ☏). : 77 Rhyolites also occur as breccias or in lava domes, volcanic plugs, and dikes. : 22 Rhyolitic ash flow tuffs are the only volcanic product with volumes rivaling those of flood basalts. : 23–26 As a result, many eruptions of rhyolite are highly explosive, and rhyolite occurs more frequently as pyroclastic rock than as lava flows. Geology ĭue to their high content of silica and low iron and magnesium contents, rhyolitic magmas form highly viscous lavas. Biotite, augite, fayalite, and hornblende are common accessory minerals. Cristobalite and trydimite are sometimes present along with the quartz. The plagioclase is usually sodium-rich ( oligoclase or andesine). These feldspar minerals sometimes are present as phenocrysts. The alkali feldspar in rhyolites is sanidine or, less commonly, orthoclase. Rhyolite is high in silica and total alkali metal oxides, placing it in the R field of the TAS diagram.
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The rock must then be classified chemically based on its content of silica and alkali metal oxides ( K 2O plus Na 2O). 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. This makes rhyolite the extrusive equivalent of granite. Īn 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. It is generally light in color due to its low content of mafic minerals, and it is typically very fine-grained ( aphanitic) or glassy. 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. Rhyolitic pumice finds use as an abrasive, in concrete, and as a soil amendment.ĭescription QAPF diagram with rhyolite field highlighted TAS diagram with rhyolite field highlighted 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 tuff has been extensively used for construction. Rhyolitic ash-flow tuffs are among the most voluminous of continental igneous rock formations. This favors explosive eruptions over effusive eruptions, so this type of magma is more often erupted as pyroclastic rock than as lava flows. Rhyolitic magma is extremely viscous, due to its high silica content. It is the extrusive equivalent to granite.
The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is generally glassy or fine-grained ( aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. ə l aɪ t/ RY-ə-lyte) is the most silica-rich of volcanic rocks. Glassy or aphanitic, sometimes porphyritic
Quartz and alkali feldspar ( sanidine and sodic plagioclase) Igneous, volcanic rock, of felsic (silica-rich) composition Rhyolite Igneous rock
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