igneous
Andesite
Andesite (Intermediate Volcanic Rock)
Hardness: 6-7 on Mohs scale. Color: Gray to grayish-brown with fine-grained (aphanitic) texture; features small visible phenocrysts of plagioclase or hornblende. Luster: Dull to sub-vitreous. Crystal Structure: Fine-grained matrix with larger crystals (porphyritic).
- Hardness
- 6-7 on Mohs scale
- Color
- Gray to grayish-brown with fine-grained (aphanitic) texture
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Gray to grayish-brown with fine-grained (aphanitic) texture; features small visible phenocrysts of plagioclase or hornblende. Luster: Dull to sub-vitreous. Crystal Structure: Fine-grained matrix with larger crystals (porphyritic).
Formation & geological history
Formed from the rapid cooling of intermediate-composition lava at or near the surface, typically near subduction zones or volcanic arcs. Ages vary by location, from recent Quaternary flows to ancient Paleozoic formations.
Uses & applications
Used primarily in construction for road base, aggregate, and building stone. Because of its durability, it is often used as crushed stone in paving and railroad ballast.
Geological facts
Andesite is named after the Andes Mountains where it is found in abundance. It is the volcanic equivalent of diorite and represents one of the most common rock types in volcanic island arcs.
Field identification & locations
Identify by its gray color and 'intermediate' weight (heavier than rhyolite, lighter than basalt). Common in the Pacific Northwest (USA), Japan, and South America. Collectors look for large porphyritic crystals.
