rhyolite

A silica-rich volcanic igneous rock with a composition approximately equivalent to granite. It is fine-grained or glassy, because of rapid cooling, and occurs in several forms.

This page is about a volcanic rock. For the satellite system, see Rhyolite/Aquacade.

Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (acidic) composition (typically >69% SiO2 -- see the TAS classification).

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, due to their high content of silica and low iron and magnesium contents, rhyolites polymerize quickly and form highly viscous lavas. 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.