Rhyoliit

Rhyoliit is a felsic, extrusive igneous rock that corresponds to the volcanic counterpart of granite. It forms from high-silica magmas that erupt at or near the surface and crystallize rapidly, producing a fine-grained to glassy rock. The mineral composition typically includes quartz and alkali feldspar (such as orthoclase or sanidine) as major constituents, with plagioclase, and minor amounts of biotite or amphibole. Accessory minerals can include zircon, magnetite, and apatite.

Texture ranges from aphanitic to porphyritic. In porphyritic varieties, larger crystals (phenocrysts) of quartz and/or feldspar stand out against a finer groundmass. Glassy varieties, such as obsidian, occur when cooling is extremely rapid. Rhyoliit often appears light-colored due to its felsic mineralogy, though variations can occur.

Geologically, rhyoliit forms in continental crust settings, commonly in volcanic arcs above subduction zones or in hotspots. It is linked to highly viscous magmas that trap volatiles, leading to explosive eruptions. Associated rock types include pumice, rhyolitic tuffs and ignimbrites, and rhyolitic lava flows, which can create caldera structures in large eruptions.

Relation to other rocks: rhyoliit is the extrusive equivalent of granite; the plutonic counterpart is granite, while dacite and rhyodacite represent rocks of somewhat lower silica and different mineral proportions. Economic significance is limited to minor ore deposits associated with hydrothermal activity in some rhyolitic systems; rhyiolit can also be used as decorative or dimension stone where it is well indurated and free of cracks.