igneous
Porphyritic Andesite
Andesite
Hardness: 6-7 (Mohs scale); Color: Brownish-gray with white phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained) groundmass with porphyritic texture (larger visible crystals); Specific gravity: 2.4-2.8
- Hardness
- 6-7 (Mohs scale)
- Color
- Brownish-gray with white phenocrysts
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Brownish-gray with white phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained) groundmass with porphyritic texture (larger visible crystals); Specific gravity: 2.4-2.8
Formation & geological history
Formed from the rapid cooling of intermediate-composition lava at or near the Earth's surface, typically in volcanic arc settings above subduction zones. This specific specimen has been water-worn into a rounded cobble.
Uses & applications
Used in construction as crushed stone, road base, and occasionally as architectural stone or for garden landscaping.
Geological facts
Andesite is named after the Andes Mountains, where it is extremely common. The presence of 'phenocrysts' (white spots) indicates a two-stage cooling process: a slow phase deep underground followed by a rapid phase during eruption.
Field identification & locations
Identify in the field by its intermediate color (between light rhyolite and dark basalt) and porphyritic texture. Commonly found in mountainous volcanic regions and as river-rolled stones.
