riolit

Riolit, or rhyolite, is a felsic volcanic rock defined by a high silica content and light color. It represents the extrusive (volcanic) counterpart to granite, formed from silica-rich magmas that crystallize rapidly near or at the surface. The mineral assemblage is typically quartz and alkali feldspar, with minor plagioclase and light-colored micas; accessory minerals such as zircon or iron oxides may occur.

Texturally, rhyolite is usually fine-grained (aphanitic) due to rapid cooling, but it can be porphyritic with larger quartz or feldspar phenocrysts. Glassy varieties exist as obsidian, and some rhyolites are vesicular, forming pumice when highly gas-rich.

Rhyolitic magmas are highly viscous, which tends to trap gases and drive explosive eruptions. Common eruptive products include thick rhyolitic lava flows and lava domes, as well as pyroclastic deposits such as pumice and ignimbrites. Large ignimbrite sheets and calderas are characteristic of substantial rhyolitic volcanism.

Geologically, rhyolite forms in continental crust settings, particularly in subduction-related arcs and zones of crustal melting or differentiation. It is widely distributed in silicic volcanic fields and volcanic complexes, often associated with caldera-forming events and long-lived silicic volcanism.

In summary, riolit is a light-colored, high-silica volcanic rock of felsic composition, typically fine-grained or glassy, with a tendency toward explosive eruptive styles and association with continental crustal volcanism.