Riolita

Riolita, also known as rhyolite in English, is a felsic extrusive igneous rock with a high silica content and a light-colored mineral assemblage. It is the volcanic (extrusive) counterpart to granite and typically forms from highly viscous magmas that erupt or crystallize at or near the surface. The silica content of riolita commonly ranges around 69–76 percent, and the rock is rich in alkali feldspar and quartz, with lesser plagioclase.

Mineralogy and texture vary. The groundmass is usually fine-grained (aphanitic), but riolita can be porphyritic, containing larger quartz or feldspar crystals (phenocrysts) set in a finer matrix. Accessory minerals may include biotite, hornblende, magnetite, and sphene. Some riolitas have a glassy texture (obsidian) or are highly vesicular, forming pumice or volcanic tuff when fragmented during explosive eruptions. The overall appearance is generally light-colored to pinkish due to its felsic composition.

Formation and settings. Riolita forms from silicic magmas generated in continental crust, often in subduction-related or post-collision tectonic environments. It occurs in volcanic arcs and in regions of significant crustal extension where rhyolitic magmas rise and erupt. Eruptions can produce dome-building lava flows, as well as explosive ash clouds and ignimbrites.

Occurrence and significance. Riolita is widespread in continental crust regions with silicic volcanism. It is used in geology as a key indicator of crustal processes and magmatic differentiation and has been quarried for decorative stone and construction in some regions.

Note: riolita is the term used in several languages for the rhyolite rock type.