Rhyolite

Physical Properties

Fine-grained (aphanitic) or porphyritic texture. Hardness: 6-7 on Mohs scale. Color: Light gray, white, or pinkish. Luster: Dull to earthy. Composition: High silica content (>69% SiO2). Features visible phenocrysts of quartz or feldspar in a fine-grained groundmass.

Formation & Geological History

Formed from the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. It is the volcanic equivalent of granite. Often associated with continental volcanic eruptions.

Uses & Applications

Used as a construction aggregate, road stone, and occasionally for decorative stone or abrasives. In ancient times, it was used for stone tools because of its hardness.

Geological Facts

Rhyolite lavas are extremely viscous, meaning they flow very slowly and can trap gases, leading to explosive volcanic eruptions. Pumice and obsidian are also varieties of volcanic glass with rhyolitic compositions.

Field Identification & Locations

Identify in the field by its light color, high hardness (cannot be scratched by steel), and the presence of small, glassy quartz or feldspar crystals in a very fine, matte matrix. Commonly found in volcanic regions like the Yellowstone area or the Andes Mountains.

Identified on: 4/13/2026

Mode: Standard