Rhyolite

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.

Rhyolite

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 aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2 — see the QAPF diagram). Biotite and hornblende are common accessory minerals.

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcroppings of it often bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolites polymerize quickly and form highly viscous lavas. They can also occur as breccias or in volcanic necks 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.

Top stone is obsidian, below that is pumice and in lower right corner is rhyolite (light color)

See also

• List of rock types
• Granite
• Penroseite

References

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