rhyoliths

Rhyoliths are extrusive igneous rocks dominated by a rhyolitic composition, a highly silica-rich, felsic material. They crystallize from felsic magma and are the volcanic equivalents of granitic rocks. Rhyolites are typically light in color and can be fine-grained or porphyritic, with a groundmass that may be glassy in places. Because rhyolitic magmas are highly viscous, eruptions tend to be explosive, producing features such as ash clouds, pumice, and thick lava domes or obsidian-rich flows.

Texture and mineral content in rhyoliths vary, but common assemblages include quartz, potassium feldspar (sanidine or orthoclase), and plagioclase, with accessory minerals such as biotite, magnetite, and amphibole. Some rhyoliths solidify with a glassy or microcrystalline texture, forming obsidian or other volcanic glasses when cooling is rapid. The presence of phenocrysts can yield porphyritic rhyolites, whereas rapidly cooled eruptions may produce very fine-grained or glassy rocks.

Rhyolites are widespread in continental crust settings, especially in volcanic arcs and large silicic volcanic provinces where crustal melting and differentiation produce highly evolved magmas. The intrusive counterparts of rhyolite are granites and quartz monzonites, which crystallize underground from similar melts. In modern terminology, rhyolite is the standard term for the extrusive rock; “rhyolith” appears in some older or less formal usages, but is not widely adopted in current geology literature.