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Rhyolite

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

Rhyolite is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO₂ – see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1 – see the QAPF diagram). 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.^[1]

Rhyolite in the Kaldaklofsfjöll, Landmannalaugar, Iceland

Rhyolite quarry, Löbejün, Saxony-Anhalt

Europe

• Etsch Valley Vulcanite Group near Bolzano and the surrounding area


• Gréixer rhyolitic complex at Moixeró range (Catalonia, Spain)^[2]
  


• Vosges


• 
  Iceland: all active and extinct central volcanoes^{[citation needed]}, 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, France

Germany

• 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 (Pfalz) on the Donnersberg mountain


• 
  Black Forest e.g. on the Karlsruher Grat
  


• Odenwald

The Americas

• Andes


• 
  Cascade Range^[3]
  


• 
  Cobalt, Ontario^[4]
  


• Sheep Creek, Idaho


• Rocky Mountains


• Jemez Mountains


• 
  Rhyolite, Nevada was named after a rhyolite deposit that characterised the area.^[5]
  


• 
  Wichita Mountains within the Southern Oklahoma Aulacogen
  


• St. Francois Mountains


• 
  Mount Jasper, Berlin, New Hampshire
  


• Yellowstone


• 
  Crater Lake, Oregon^[1]
  


• 
  Palisade Head, a formation found at Tettegouche State Park, Minnesota.

Oceania

Mount Tibrogargan, a rhyolite volcanic plug in Queensland, Australia

• 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 in 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,^[6] near the town of Wenzhou, Zhejiang province, China
  


• 
  Tambora, Indonesia^[1]
  

Africa

• 
  Mount Kilimanjaro, Kenya/Tanzania^[1]
  

Name

The name rhyolite was introduced into geology in 1860 by the German traveler and geologist Ferdinand von Richthofen^[7][8][9] from the Greek word rhýax ("a stream of lava")^[10] and the rock name suffix "-lite".^[11]

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.^[12] Tons of rhyolite were traded across the Delmarva Peninsula,^[12] because the rhyolite kept a sharp point when knapped and was used to make spear points and arrowheads.^[13]

See also

• 
  Comendite – A hard, peralkaline igneous rock, a type of light blue grey rhyolite


• 
  List of rock types – A list of rock types recognized by geologists


• 
  Pantellerite – A peralkaline rhyolite type of volcanic rock


• 
  Thunderegg – A nodule-like rock, that is formed within rhyolitic volcanic ash layers

References

External links

Wikimedia Commons has media related to Rhyolite.

• University of North Dakota description of rhyolite


• Information from rocks-rock.com