igneous
Rhyolite
Rhyolite (felsic volcanic rock)
Hardness: 6-7 Mohs; Color: pink, reddish-brown, or gray; Luster: dull to sub-glassy; Texture: fine-grained (aphanitic) or porphyritic with small visible phenocrysts of quartz or feldspar; Specific Gravity: 2.4-2.6
- Hardness
- 6-7 Mohs
- Color
- pink, reddish-brown, or gray
- Luster
- dull to sub-glassy
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Physical properties
Hardness: 6-7 Mohs; Color: pink, reddish-brown, or gray; Luster: dull to sub-glassy; Texture: fine-grained (aphanitic) or porphyritic with small visible phenocrysts of quartz or feldspar; Specific Gravity: 2.4-2.6
Formation & geological history
Formed from silica-rich (felsic) lava that cools rapidly at or near the Earth's surface. It is the extrusive equivalent of granite, often associated with continental volcanic eruptions.
Uses & applications
Used primarily as crushed stone for road construction and concrete aggregate. Select varieties like 'Wonderstone' or porcelain rhyolite are used for lapidary work and beads.
Geological facts
Rhyolite lava is extremely viscous (thick), which often leads to explosive volcanic eruptions because gas bubbles cannot easily escape. It can cool so quickly that it forms obsidian (volcanic glass) instead of a crystalline rock.
Field identification & locations
Identify in the field by its light color (white, pink, light gray) compared to black basalt, and its fine-grained texture. It often shows 'flow banding' where different colored layers formed as the lava moved.
