Andesite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Light to dark grey, green, or brown. Luster: Dull to sub-vitreous. Texture: Aphanitic (fine-grained) to porphyritic with visible phenocrysts. Crystal Structure: Orthorhombic/Monoclinic. Specific Gravity: 2.4 - 2.8.

Formation & Geological History

Formed through the rapid cooling of intermediate-composition lava. It is typically found in volcanic arcs at convergent plate boundaries (subduction zones). Geological age varies from Precambrian to modern-day eruptions.

Uses & Applications

Used primarily as crushed stone for road construction and railroad ballast, in some cases it is used for building tiles and sculpture, though it is less decorative than granite.

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 found in great abundance along the volcanic chain.

Field Identification & Locations

Identify by its intermediate color (lighter than basalt, darker than rhyolite) and the presence of small, light-colored plagioclase feldspar or dark hornblende crystals. Common in volcanic regions such as the Pacific Ring of Fire.

Identified on: 5/19/2026

Mode: Standard