igneous
Porphyritic Andesite
Andesite (Intermediate Volcanic Rock)
Porphyritic texture with large phenocrysts in a fine-grained groundmass. Hardness: 6-7 Mohs. Color: Gray to brown. Luster: Dull to sub-vitreous. Structure: Aphanitic to porphyritic.
- Hardness
- 6-7 Mohs
- Color
- Gray to brown
- Luster
- Dull to sub-vitreous
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Physical properties
Porphyritic texture with large phenocrysts in a fine-grained groundmass. Hardness: 6-7 Mohs. Color: Gray to brown. Luster: Dull to sub-vitreous. Structure: Aphanitic to porphyritic.
Formation & geological history
Formed via the rapid cooling of intermediate lava at or near the Earth's surface, typically at convergent plate boundaries (subduction zones). Likely Cenozoic age in many volcanic arcs.
Uses & applications
Used primarily as crushed stone for road construction, aggregate in concrete, and occasionally as decorative dimension stone or pavers.
Geological facts
The name 'Andesite' is derived from the Andes Mountains where this rock type is incredibly common. It is the volcanic equivalent of plutonic diorite.
Field identification & locations
Identified by the 'chocolate chip cookie' appearance of dark mineral crystals (phenocrysts) embedded in a lighter, fine-grained matrix. Found in volcanic mountain ranges worldwide.
