igneous
Porphyritic volcanic rock, likely porphyritic andesite
Porphyritic andesite; chiefly plagioclase and pyroxene/amphibole phenocrysts in a fine volcanic groundmass
AI-generated identification · may be incorrectHardness about 6–7; brown to reddish-gray, fine-grained matrix with conspicuous pale crystals and possible weathered vesicles; generally dull to slightly vitreous, massive, with no obvious cleavage. Exact identification is limited by the photograph.
Identified More igneous →
Explore Porphyritic volcanic rock, likely porphyritic andesite in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness about 6–7; brown to reddish-gray, fine-grained matrix with conspicuous pale crystals and possible weathered vesicles; generally dull to slightly vitreous, massive, with no obvious cleavage. Exact identification is limited by the photograph.
Formation & geological history
An extrusive igneous rock formed when intermediate-composition magma erupted and cooled rapidly, preserving larger crystals formed underground; commonly Cenozoic to older volcanic arc material. The rounded shape is likely beach or stream abrasion.
Uses & applications
Used locally as aggregate, road base, riprap, landscaping stone, and occasionally as a decorative cobble; generally not a jewelry material.
Geological facts
The pale spots may be altered plagioclase phenocrysts, while some irregular white patches could be mineral infill or surface lichen rather than primary crystals.
Field identification & locations
Look for a compact volcanic texture, scattered light phenocrysts, and a lack of layering or sedimentary grains; a hand lens and streak, hardness, and thin-section tests would distinguish andesite from basalt, dacite, or altered volcanic breccia. Common in volcanic arcs worldwide.
