ryolit

Ryolit, known in English as rhyolite, is a felsic extrusive igneous rock corresponding to granite at the surface. It forms from high-silica magma that erupts and cools rapidly, producing fine-grained textures; some samples are porphyritic with quartz and alkali feldspar phenocrysts, while others are volcanic glassy.

Mineralogy and texture: The rock predominantly consists of quartz and alkali feldspar. Plagioclase is common in some varieties. Accessory minerals may include biotite, hornblende, magnetite, and apatite. Textures range from a fine-grained aphanitic groundmass to porphyritic with visible phenocrysts; obsidian is the glassy endmember.

Chemical composition: Rhyolite typically has a high silica content, around 69–76 percent SiO2, and a high alkali feldspar content, placing it among felsic lava types. It represents the surface-extrusive counterpart to granitic plutons and is often discussed in the context of crustal differentiation processes in continental settings.

Formation and occurrence: Rhyolite forms from silica-rich magmas that originate in the crust, commonly in continental volcanic arcs and other regions where granitic magmas differentiate. It is the extrusive equivalent of granitic rocks and is frequently associated with lava domes, volcanic tuffs, and other pyroclastic deposits.

Volcanic features and eruptions: Rhyolitic magma is highly viscous and gas-rich, which tends to produce explosive eruptions. Typical eruptive products include pumice, ash flows (ignimbrite), tuff, lava domes, and obsidian. Large rhyolitic volcanic centers can form calderas and extensive pyroclastic sequences.