Rhyolite

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Reddish-brown, gray, or pink; Luster: Dull to vitreous; Crystal structure: Aphanitic (fine-grained) often with visible phenocrysts of quartz or feldspar; Specific gravity: 2.4-2.6.

Formation & Geological History

Formed through the rapid cooling of high-silica (felsic) magma that erupts from a volcano onto the Earth's surface. It is the extrusive equivalent of granite.

Uses & Applications

Used primarily in road construction, as aggregate in concrete, and occasionally as decorative stone or for stone tool production in prehistoric times.

Geological Facts

Rhyolite is so high in silica that the lava it forms from is extremely viscous, often resulting in explosive volcanic eruptions rather than fluid flows.

Field Identification & Locations

Identify by its light color (felsic) and fine-grained texture; it often contains small, glassy crystal inclusions. Frequently found in volcanic mountain ranges such as the Andes or the Cascades.

Identified on: 4/30/2026

Mode: Standard