Ryolitt

Ryolitt is a felsic, silica-rich volcanic igneous rock that forms from the rapid cooling of high-viscosity rhyolitic magma at or near the Earth’s surface. It is the volcanic, extrusive counterpart to granite and is classified within the granitic family of rocks.

Composition and texture are dominated by silica, with abundant alkali feldspar and quartz. Commonly present are plagioclase feldspar, and minor amounts of biotite or hornblende, along with accessory minerals such as zircon or apatite. The texture ranges from a fine-grained groundmass in rhyolitic lava flows to porphyritic varieties in which larger crystals, especially quartz and feldspar, are set in a finer matrix. In some instances, ryolitt appears as volcanic glass in obsidian.

Formation and occurrence: Ryolitt forms during explosive volcanic activity associated with silica-rich magmas. It occurs in continental crust settings, particularly in subduction-related volcanic arcs, and is found worldwide in formations ranging from lava domes to pyroclastic deposits and rhyolitic tuffs.

Features and uses: It is typically light-colored, ranging from pale pink to light gray, and often fractures conchoidally. Economically, ryolitt is not a major ore host, but can be used as a decorative or building stone when massive and defect-free. It also provides important information about crustal magmatic processes.

See also: Granite, Rhyolite, Obsidian, Porphyry, Igneous rocks.