igneous
Porphyritic volcanic rock (likely andesite porphyry)
Porphyritic andesite; composition cannot be confirmed from the photograph
AI-generated identification · may be incorrectFine brown-gray groundmass containing conspicuous pale crystals and darker mineral grains. Likely feldspar phenocrysts; hardness and exact luster are not reliably determined from an image, and the rock has no visible cleavage.
Identified More igneous →
Explore Porphyritic volcanic rock (likely andesite porphyry) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Fine brown-gray groundmass containing conspicuous pale crystals and darker mineral grains. Likely feldspar phenocrysts; hardness and exact luster are not reliably determined from an image, and the rock has no visible cleavage.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew first, followed by rapid cooling of the finer groundmass. Andesitic rocks commonly form in volcanic arcs, often during the Cenozoic, though this specimen’s age and origin are unknown.
Uses & applications
Mainly collected as a geological specimen; andesite is also used locally as crushed aggregate and dimension stone.
Geological facts
Its contrasting large crystals and fine-grained matrix are called porphyritic texture, recording a change in cooling conditions.
Field identification & locations
Look for distinct crystals embedded in a much finer, continuous matrix; this hand-sized, rounded piece may be weathered. A thin section or mineral tests would be needed to distinguish andesite from other porphyritic rocks.
