Rhyolite Rhyolite is an igneous , volcanic rock , of felsic composition. It may have any texture from glassy to aphanitic to porphyritic . The mineral assemblage is usually quartz , sanidine and plagioclase . Biotite and hornblende are common accessory minerals . It is the extrusive equivalent to granite .Geology Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolitic magmas form highly viscous lavas. They also occur as breccias or in volcanic plugs and dikes . Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian . 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 . Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites. Eruptions of rhyolite are relatively rare compared to eruptions of less felsic lavas. Only three eruptions of rhyolite have been recorded since the start of the 20th century: at the St. Andrew Strait volcano in Papua New Guinea , Novarupta volcano in Alaska , and Chaiten in southern Chile .Occurrence Rhyolite has been found on islands far from land, but such oceanic occurrences are rare. , Saxony-Anhalt Europe Etsch Valley Vulcanite Group near Bolzano and the surrounding area Gréixer rhyolitic complex at Moixeró range Vosges Iceland:, e.g. Torfajökull , Leirhnjúkur / Krafla , Breiddalur central volcano Papa Stour in Shetland Copper Coast Geopark in southeast Ireland Various locations around Snowdonia , Wales Massif de l'Esterel , FranceGermany The Thuringian Forest consists mainly of rhyolites, latites and pyroclastic rocks of the Rotliegendes Saxony , especially the north west Saxony-Anhalt north of Halle Saar-Nahe Basin e.g. the Königstuhl on the Donnersberg mountain Black Forest e.g. on the Karlsruher Grat Odenwald The Americas Andes Castle Rock, Colorado Cascade Range Cobalt, Ontario Sheep Creek, Idaho San Juan volcanic field Jemez Mountains Rhyolite, Nevada was named after a rhyolite deposit that characterises the area. Wichita Mountains within the Southern Oklahoma Aulacogen St. Francois Mountains Mount Jasper , Berlin, New Hampshire Yellowstone Crater Lake, Oregon Palisade Head , a formation found at Tettegouche State Park , Minnesota Mount Kineo , a volcanic plug ; an iconic mountain feature located in Moosehead Lake , Rockwood, Maine, United States Oceania The Taupo Volcanic Zone in New Zealand has a large concentration of young rhyolite volcanoes. Glass House Mountains National Park , Queensland, Australia The Gondwana Rainforests of Australia World Heritage Area contains rhyolite-restricted flora along the Great Dividing Range . The Flinders Peak Group and the Teviot Range in the Fassifern Valley is a rhyolite of varying colours.Asia The Malani Igneous Suite , Rajasthan , India . The Yandang Shan mountain chain , near the town of Wenzhou , Zhejiang province , China Tambora , Indonesia Africa Mount Kilimanjaro , Kenya/TanzaniaName The name rhyolite was introduced into geology in 1860 by the German traveler and geologist Ferdinand von Richthofen from the Greek word rhýax and the rock name suffix "-lite".Quarrying by Native Americans In North American pre-historic times , rhyolite was quarried extensively in eastern Pennsylvania in the United States. Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site in Adams County . Rhyolite was mined there starting 11,500 years ago . Tons of rhyolite were traded across the Delmarva Peninsula , because the rhyolite kept a sharp point when knapped and was used to make spear points and arrowheads.Characteristics
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