Andesite

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Light to dark gray, sometimes bluish or greenish. Luster: Dull/earthy (fine-grained) with occasional sub-vitreous crystals. Crystal structure: Porphyritic (large crystals in a fine-grained matrix). Specific gravity: 2.5-2.8.

Formation & Geological History

Formed through the rapid cooling of intermediate-composition lava at or near the surface, typically near subduction zones (volcanic arcs). Most specimens are from the Cenozoic era, though it can be older.

Uses & Applications

Primarily used in construction as crushed stone for road base, aggregate in concrete, and as a durable building stone. Occasionally used for decorative items or monuments.

Geological Facts

Andesite is the second most common volcanic rock in the Earth's crust after basalt. It is named after the Andes Mountains in South America, where it is incredibly abundant.

Field Identification & Locations

Identify in the field by its characteristic gray color and porphyritic texture (notice the distinct white/light gray crystals of plagioclase feldspar embedded in a darker gray background). Found in mountainous volcanic regions like the Cascades or the Andes.

Identified on: 4/14/2026

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