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. 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, 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.

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

Europe

Germany

The 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
  • the Flinders Peak Group in the Teviot Range in the Fassifern Valley is a rhyolite of varying colours.

Asia

Africa

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

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.[12]

See also

References

  1. 1 2 3 4 The Illustrated Encyclopedia of Rocks of the World by John Farndon, page 54.
  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. ↑ Cascades Volcano Observatory. "Cascades Volcano Observatory". usgs.gov. Retrieved 19 January 2015.
  4. ↑ ROBERT CINITS. "The Proteus Property" (PDF).
  5. ↑ "Rhyolite Ghost Town". Retrieved 2009-12-22.
  6. ↑ "Yandang Shan". Archived from the original on 2016-02-17. Retrieved 2011-12-22.
  7. ↑ Richthofen, Ferdinand Freiherrn von (1860). "Studien aus den ungarisch-siebenbürgischen Trachytgebirgen" [Studies of the trachyte mountains of Hungarian Transylvania]. Jahrbuch der Kaiserlich-Königlichen Geologischen Reichsanstalt (Wein) [Annals of the Imperial-Royal Geological Institute of Vienna] (in German). 11: 153–273.
  8. ↑ Simpson, John A.; Weiner, Edmund S. C., eds. (1989). Oxford English Dictionary. 13 (2nd ed.). Oxford: Oxford University Press. p. 873.
  9. ↑ Young, Davis A. (2003). Mind Over Magma: The Story of Igneous Petrology. Princeton University Press. p. 117. ISBN 0-691-10279-1.
  10. ↑ "Definition of RHYOLITE". www.merriam-webster.com.
  11. ↑ "Definition of LITE". www.merriam-webster.com.
  12. ↑ Beckerman, Ira. National Register of Historic Places Inventory/Nomination: Carbaugh Run Rhyolite Quarry Site (36AD30). National Park Service, 1981, 2.
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