Ryoliet

Ryoliet is a felsic extrusive igneous rock formed from silica-rich lava that erupts onto the surface and cools rapidly. It is the volcanic counterpart to granite and typically contains high amounts of silica, with SiO2 commonly around 69–75%. The mineralogy is dominated by alkali feldspar and quartz, with plagioclase and minor ferromagnesian minerals such as biotite or hornblende; accessory minerals may include zircon, apatite, and other trace minerals.

Texture and appearance vary from fine-grained (aphanitic) to porphyritic, in which larger mineral crystals (phenocrysts) are embedded in a fine-grained groundmass. Glassy varieties exist, such as obsidian, and vesicular forms can create pumice. The high viscosity of rhyolitic magma tends to trap gases, leading to explosive eruptions and the formation of pyroclastic deposits, lava domes, and rhyolitic ignimbrites.

Formation and occurrence: ryoliet forms from rapidly cooled rhyolitic lava at or near the surface. It occurs in continental crustal settings, especially in volcanic arcs, continental rifts, and caldera systems, where silica-rich magmas evolve in crustal reservoirs. Rhyolite is commonly associated with other felsic rocks in large, silicic volcanic complexes and chains of volcanic centers.

See also: rhyolite’s plutonic equivalent is granite; its glassy variety is obsidian, and its vesicular form is pumice.