Porphyritic Andesite

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Reddish-brown (oxidized) with white phenocrysts; Luster: Dull to earthy; Crystal Structure: Porphyritic (visible crystals in a fine-grained matrix); Cleavage: None; Specific Gravity: 2.4-2.8

Formation & Geological History

Formed through the rapid cooling of intermediate-composition lava at or near the Earth's surface, typically in volcanic arc environments. The large white crystals (phenocrysts, likely plagioclase feldspar) formed slowly underground before the final eruption.

Uses & Applications

Used primarily in construction as crushed stone for road base or aggregate. Highly durable and resistant to weathering.

Geological Facts

The name Andesite is derived from the Andes Mountains in South America, where this type of volcanic rock is extremely common. The reddish color of this specific specimen is due to the oxidation of iron-bearing minerals during or after cooling.

Field Identification & Locations

Identify by looking for a fine-grained 'groundmass' containing distinct, visible crystals (phenocrysts). Commonly found in the Pacific Northwest (Cascades) and other subduction zone volcanic ranges.

Identified on: 5/11/2026

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