igneous
Andesite Porphyry
Porphyritic Andesite (Plagioclase-rich)
Hardness: 6-7 Mohs; Color: Gray to brownish-gray matrix with white phenocrysts; Luster: Dull/Sub-vitreous; Crystal structure: Porphyritic (large crystals in fine-grained matrix); Specific gravity: 2.4-2.8
- Hardness
- 6-7 Mohs
- Color
- Gray to brownish-gray matrix with white phenocrysts
- Luster
- Dull/Sub-vitreous
Identified More igneous →
Explore Andesite Porphyry in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 Mohs; Color: Gray to brownish-gray matrix with white phenocrysts; Luster: Dull/Sub-vitreous; Crystal structure: Porphyritic (large crystals in fine-grained matrix); Specific gravity: 2.4-2.8
Formation & geological history
Formed through the cooling of intermediate-composition magma, typically in volcanic arc environments above subduction zones. The 'porphyritic' texture indicates a two-stage cooling process: slow cooling at depth (forming large crystals) followed by rapid cooling during eruption.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and as aggregate in concrete due to its durability and resistance to slip.
Geological facts
Andesite is the second most common volcanic rock in the Earth's crust after basalt. It is named after the Andes Mountains where it is exceptionally abundant.
Field identification & locations
Identify by looking for 'salt and pepper' or light-colored rectangular crystals (phenocrysts) embedded in a fine-grained, gray stony groundmass. Commonly found in mountainous volcanic regions like the Cascades or the Andes.
