igneous
Weathered porphyritic volcanic rock, likely andesite
Porphyritic andesite; primarily plagioclase feldspar, pyroxene, and possibly amphibole in a fine-grained groundmass
AI-generated identification · may be incorrectTypically gray to brown, dull to slightly earthy luster, with visible feldspar crystals in a finer matrix; hardness about 6–7, no obvious cleavage as a rock, and specific gravity roughly 2.5–2.8. The specimen appears strongly weathered and may contain iron-oxide staining or alteration.
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Physical properties
Typically gray to brown, dull to slightly earthy luster, with visible feldspar crystals in a finer matrix; hardness about 6–7, no obvious cleavage as a rock, and specific gravity roughly 2.5–2.8. The specimen appears strongly weathered and may contain iron-oxide staining or alteration.
Formation & geological history
Forms when intermediate-composition magma cools relatively quickly, commonly in volcanic arcs and stratovolcanoes. Its exact age cannot be determined from the photograph; andesite ranges from ancient Precambrian examples to modern volcanic flows.
Uses & applications
Used locally as crushed aggregate, road stone, riprap, and occasionally building stone; attractive fresh pieces may be collected, but this weathered specimen has little jewelry value.
Geological facts
The lighter, blocky grains are likely phenocrysts formed before eruption, while the fine matrix cooled afterward. Weathering can obscure the original texture and make visual identification uncertain.
Field identification & locations
Common in volcanic mountain belts, including the Cascades, Andes, Japan, and Indonesia. Confirm with a fresh surface, hardness test, streak, and ideally thin-section or chemical analysis; it could also be altered basalt or dacite.
