igneous

Andesite Porphyry

Porphyritic Andesite (Intermediate volcanic rock)

Hardness: 6-7 on Mohs scale; Color: Gray to purplish-gray with darker blue-black phenocrysts; Luster: Dull/earthy matrix; Crystal structure: Aphanitic matrix with visible crystals; Specific gravity: 2.4-2.8.

Hardness: 6-7 on Mohs scale
Color: Gray to purplish-gray with darker blue-black phenocrysts
Luster: Dull/earthy matrix

Identified February 19, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the slow cooling of magma below the Earth's surface followed by a rapid volcanic eruption. This two-stage cooling creates large crystals in a fine-grained matrix. Found typically at subduction zones above convergent plate boundaries.

Uses & applications

Primarily used in construction for road aggregate, gravel, and dimension stone due to its durability. Occasionally used for decorative masonry or as a base for landscaping.

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 abundance. The large crystals (phenocrysts) in this specimen are likely plagioclase feldspar or hornblende.

Field identification & locations

Identify in the field by looking for a fine-grained 'salty' or gray matrix containing larger, darker rectangular or needle-like crystals. Common in volcanic ranges like the Andes or the Cascades in North America.

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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.

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