igneous
Rhyolite (Porphyritic)
Rhyolite (Felsic Extrusive Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: typically light gray, tan, or pinkish; Luster: dull to earthy groundmass with vitreous (glassy) phenocrysts; Structure: Aphanitic to porphyritic with visible crystals of quartz or feldspar; Specific Gravity: 2.4-2.6
- Hardness
- 6-7 on Mohs scale
- Color
- typically light gray, tan, or pinkish
- Luster
- dull to earthy groundmass with vitreous (glassy) phenocrysts
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically light gray, tan, or pinkish; Luster: dull to earthy groundmass with vitreous (glassy) phenocrysts; Structure: Aphanitic to porphyritic with visible crystals of quartz or feldspar; Specific Gravity: 2.4-2.6
Formation & geological history
Formed from the rapid cooling of high-silica (felsic) lava erupted from volcanoes. Can range from Precambrian to Holocene in age. Often associated with continental volcanic arcs and caldera-forming eruptions.
Uses & applications
Commonly used as crushed stone for road construction and concrete aggregate. Historically used for stone tools (knapping) due to its high silica content and conchoidal fracture. Some banded varieties are polished for lapidary use.
Geological facts
Rhyolite is the volcanic equivalent of granite; they share the same chemical composition but differ in grain size. Extremely explosive volcanic eruptions, like those at Yellowstone, are typically fueled by rhyolitic magma due to its high viscosity.
Field identification & locations
Identify in the field by its light color, fine-grained 'stony' texture, and the presence of small, glassy quartz or feldspar crystals (phenocrysts). Found in volcanic regions like the American Southwest, Iceland, and New Zealand. Collect specimens with unique banding or large crystals.
