Rhyolite

This article is about the volcanic rock. For the ghost town, see Rhyolite, Nevada. For the satellite system formerly known as Rhyolite, see Aquacade (satellite).

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.

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.

Eruptions of this advanced form of Igneous rock are rare, only three eruptions of rhyolite have been recorded since the start of 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

Rhyolite quarry, Löbejün, Saxony-Anhalt

Rhyolite in the Kaldaklofsfjöll, Landmannalaugar, Iceland

Europe

• Etsch Valley Vulcanite Group near Bolzano and the surrounding area
• Gréixer rhyolitic complex at Moixeró range (Catalonia, Spain)
• Vosges
• Iceland: all active and extinct central volcanoes, e.g. Torfajökull, Leirhnjúkur / Krafla, Breiddalur central volcano
• Copper Coast Geopark in southeast Ireland
• various locations around Snowdonia, Wales

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

Americas

• Andes
• Cascade Range
• Rocky Mountains
• Jemez Mountains
• Rhyolite, Nevada was named after a rhyolite deposit that characterised the area.
• St. Francois Mountains
• Jasper Beach - Machiasport, Maine

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

• The Malani Igneous Suite, Rajasthan, India.
• The Yandang Shan mountain chain, near the town of Wenzhou, Zhejiang province, China

• 

  Rocks from the Bishop tuff, uncompressed with pumice on left; compressed with fiamme on right.

• 

  A sample of Rhyolite from the Conical Hill dome at the head Lyttelton Harbour, Banks Peninsula, New Zealand

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

• Comendite
• List of rock types
• Pantellerite
• Thunderegg

External links

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