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Rhyolite

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Rhyolite

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.

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, rhyolite melts are highly polymerized and 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.

History

Top: obsidian (vitrophyre), below: pumice, lower right: is rhyolite (light colour)

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, where as many as fifty small quarry pits are known.[1]

Eruptions of this advanced form of Igneous rock are rare, only three eruptions of rhyolite have been recorded since the 20th century—the eruptions were at the St. Andrew Strait Volcano in Papua New Guinea, Novarupta Volcano in Alaska, United States and Chaiten in Southern Chile.

Occurrence

Old rhyolite quarry in the Hohburg Hills
Rhyolite quarry, Löbejün, Saxony-Anhalt
Rhyolite in the Kaldaklofsfjöll, Landmannalaugar, Iceland

Europe

Germany

Americas

Oceania

  • the Taupo Volcanic Zone in New Zealand has a large concentration of young rhyolite-volcanoes
  • the Gondwana Rainforests of Australia World Heritage Area contains rhyolite-restricted flora along the Great Dividing Range

Asia

Name

The name rhyolite was introduced into science by the German traveler and geologist Ferdinand von Richthofen after his explorations in the Rocky Mountains in the 1860s.

See also

References

  1. ^ Beckerman, Ira. National Register of Historic Places Inventory/Nomination: Carbaugh Run Rhyolite Quarry Site (36AD30). National Park Service, 1981, 2.
  2. ^ J. Martí, G.J. Aguirre-Díaz, A. Geyer. "The Gréixer rhyolitic complex (Catalan Pyrenees): an example of Permian caldera". Workshop on Collapse Calderas - La Réunion 2010. IAVCEI - Commission on Collapse Calderas.
  3. ^ USGS Cascade Range Volcanoes and Volcanics
  4. ^ "Rhyolite Ghost Town" (in German). Retrieved 2009-12-22. 
  5. ^ [1]
  6. ^ "Yandang Shan" (in German). Retrieved 2011-12-22. 

External links

  • University of North Dakota description of rhyolite
  • Information from rocks-rock.com
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