igneous

Andesite Porphyry

Porphyritic Andesite

Hardness: 6-7 (Mohs scale); Color: Greenish-grey matrix with tan, brown, and black phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (large crystals in a fine-grained groundmass); Specific Gravity: 2.5-2.8

Hardness: 6-7 (Mohs scale)
Color: Greenish-grey matrix with tan, brown, and black phenocrysts
Luster: Dull to sub-vitreous

Identified May 8, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the multi-stage cooling of intermediate volcanic magma. Large crystals (phenocrysts) form slowly at depth, followed by rapid cooling of the remaining magma during eruption; often found in volcanic arc environments near subduction zones.

Uses & applications

Commonly used in construction as crushed stone for road base, concrete aggregate, and occasionally as decorative landscaping stone or for lapidary rough rock tumbling.

Geological facts

The name Andesite is derived from the Andes Mountains where these rocks are exceptionally common. The porphyritic texture provides a permanent record of two distinct stages of cooling in the magma chamber.

Field identification & locations

Identify in the field by the 'spotted' appearance of distinct crystals (phenocrysts) embedded in a much finer, uniform matrix. It is commonly found in the Pacific Northwest, the Andes, and Japan.

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mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous