igneous
Rhyolite
Rhyolite (felsic volcanic rock)
Hardness: 6-7 on Mohs scale; Color: typically gray, reddish, or tan; Luster: dull to earthy; Crystal structure: fine-grained (aphanitic) scale with occasional phenocrysts; Cleavage: none; Specific gravity: 2.4-2.6
- Hardness
- 6-7 on Mohs scale
- Color
- typically gray, reddish, or tan
- Luster
- dull to earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically gray, reddish, or tan; Luster: dull to earthy; Crystal structure: fine-grained (aphanitic) scale with occasional phenocrysts; Cleavage: none; Specific gravity: 2.4-2.6
Formation & geological history
Formed by the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. It has the same chemical composition as granite but cooled much faster.
Uses & applications
Used in construction as crushed stone or road fill. Historically used by indigenous peoples for making stone tools like scrapers and projectile points due to its conchoidal fracture.
Geological facts
Rhyolite eruptions can be extremely explosive because of the high silica content which makes the lava very viscous, trapping gas bubbles until they burst.
Field identification & locations
Identify by its fine-grained texture, light color, and the presence of small visible crystals (phenocrysts) like quartz or feldspar. Often shows banding from lava flow.
