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.
Video Rhyolite
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.
Maps Rhyolite
Occurrence
Rhyolite has been found on islands far from land, but such oceanic occurrences are rare.
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
- 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
America
- Andes
- Cascade Range
- Cobalt, Ontario
- Rocky Mountains
- Jemez Mountains
- Rhyolite, Nevada was named after a rhyolite deposit that characterised the area.
- Wichita Mountains within the Southern Oklahoma Aulacogen
- St. Francois Mountains
- Jasper Beach - Machiasport, Maine
- Yellowstone
- Crater Lake, Oregon
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
- 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/Tanzania
Name
The name rhyolite was introduced into geology in 1860 by the German traveler and geologist Ferdinand von Richthofen from the Greek word rhýax ("a stream of lava") and the rock name suffix "-lite".
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.
See also
- Comendite
- List of rock types
- Pantellerite
- Thunderegg
References
External links
- University of North Dakota description of rhyolite
- Information from rocks-rock.com
Source of the article : Wikipedia