Andesite

Physical Properties

Hardness: 6-7 on Mohs scale; Color: typically medium gray, grayish-blue, or salt-and-pepper appearance; Luster: dull to earthy; Crystal Structure: aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.4-2.8.

Formation & Geological History

Formed by the rapid cooling of intermediate-composition lava at or near the surface, typically associated with subduction zones and volcanic arcs like the Andes Mountains. Geological age varies from Precambrian to Cenozoic.

Uses & Applications

Used primarily in construction for crushed stone, road base, aggregate, and as a durable material for stone masonry or landscaping. Often used where granite is unavailable.

Geological Facts

Named after the Andes Mountains in South America. It is the volcanic equivalent of plutonic diorite and is one of the most common volcanic rocks on Earth after basalt.

Field Identification & Locations

Identify in the field by its fine-grained gray matrix, often containing small visible phenocrysts of plagioclase feldspar or hornblende. It is lighter in color than basalt but darker than rhyolite. Found throughout the Pacific Ring of Fire.

Identified on: 4/30/2026

Mode: Standard