Rhyolite

Rhyolite is a felsic volcanic igneous rock characterized by high silica content and a light color, formed from the rapid cooling of viscous magma at or near the Earth’s surface. Its silica content is typically around 69–72% SiO2, and it commonly contains quartz and alkali feldspar with plagioclase as a major ferromagnesian-free mineral. Accessory minerals may include biotite, fluorite, or hornblende. In hand specimen it is usually fine-grained (aphanitic), but it can be porphyritic if larger crystals (phenocrysts) are present. Glassy varieties yield obsidian, while vesicular forms can become pumice.

Texture and varieties: Rhyolite ranges from aphanitic to porphyritic textures. When the lava cools rapidly, crystals remain too small to see, producing a fine-grained rock; if some crystals form first, the groundmass is fine-grained with visible phenocrysts. Glassy rhyolite is represented by obsidian, and highly vesicular rhyolite can form pumice.

Formation and where it occurs: Rhyolite represents the volcanic equivalent of granitic compositions and forms from high-silica, high-viscosity magmas that crystallize near the surface. It is commonly produced in continental crust settings, especially in subduction-related volcanic arcs and caldera-producing eruptions. Eruptions can be highly explosive due to gas pressure, generating ash flows and ignimbrites. The intrusive equivalent of rhyolite is granite, with rhyolitic magma forming intrusive bodies such as dikes and small plutons.

Distribution and significance: Rhyolite is widespread in continental volcanic regions and is a key indicator of felsic, crustal-influenced magmatic processes. Its presence is associated with lava domes, calderas, and extensive ash-flow tuffs in various regions around the world.