igneous
Andesite Porphyry
Porphyritic Andesite
Hardness: 6-7 (Mohs scale); Color: Greenish-grey matrix with tan, brown, and black phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (large crystals in a fine-grained groundmass); Specific Gravity: 2.5-2.8
- Hardness
- 6-7 (Mohs scale)
- Color
- Greenish-grey matrix with tan, brown, and black phenocrysts
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Greenish-grey matrix with tan, brown, and black phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (large crystals in a fine-grained groundmass); Specific Gravity: 2.5-2.8
Formation & geological history
Formed through the multi-stage cooling of intermediate volcanic magma. Large crystals (phenocrysts) form slowly at depth, followed by rapid cooling of the remaining magma during eruption; often found in volcanic arc environments near subduction zones.
Uses & applications
Commonly used in construction as crushed stone for road base, concrete aggregate, and occasionally as decorative landscaping stone or for lapidary rough rock tumbling.
Geological facts
The name Andesite is derived from the Andes Mountains where these rocks are exceptionally common. The porphyritic texture provides a permanent record of two distinct stages of cooling in the magma chamber.
Field identification & locations
Identify in the field by the 'spotted' appearance of distinct crystals (phenocrysts) embedded in a much finer, uniform matrix. It is commonly found in the Pacific Northwest, the Andes, and Japan.
