"This page is about a volcanic rock. For the ghost town see
Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade."
Rhyolite is an igneous, volcanic (
extrusive) rock, of felsic( silicon-rich) composition (typically >69% SiO2 — see the TAS classification). It may have any texture from aphaniticto porphyritic. The mineralassemblage is usually quartz, alkali feldsparand plagioclase(in a ratio > 1:2 — see the QAPF diagram). Biotiteand hornblendeare common accessory minerals.
Rhyolite can be considered as the
extrusiveequivalent to the plutonic graniterock, and consequently, outcroppings of it often 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 can also occur as breccias or in volcanic plugs and dykes. 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 tephraor of ignimbrites.
List of rock types
* [http://volcano.und.edu/vwdocs/vwlessons/lessons/Slideshow/Igrocks/Igrock9.html University of North Dakota description of rhyolite]
* [http://www.rocks-rock.com/rhyolite.html Information from rocks-rock.com]
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