igneous
Weathered porphyritic volcanic rock (likely basaltic andesite)
Porphyritic basaltic andesite; mineral assemblage likely plagioclase, pyroxene, and iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained groundmass with conspicuous pale tan phenocrysts and strongly weathered surfaces; typically Mohs 5–6, dull to slightly earthy luster, massive texture, and no obvious cleavage. Exact identification is uncertain from the photograph alone.
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Physical properties
Dark gray-brown, fine-grained groundmass with conspicuous pale tan phenocrysts and strongly weathered surfaces; typically Mohs 5–6, dull to slightly earthy luster, massive texture, and no obvious cleavage. Exact identification is uncertain from the photograph alone.
Formation & geological history
An extrusive igneous rock formed when lava cooled relatively rapidly at or near Earth’s surface, after earlier crystal growth in a magma chamber. Such rocks commonly range from Mesozoic to Cenozoic in age, depending on locality.
Uses & applications
Used locally as aggregate, riprap, roadstone, and decorative rough stone; attractive specimens may be collected but are generally not jewelry material.
Geological facts
The contrasting large crystals in a fine matrix produce a porphyritic texture, recording two-stage cooling. Brown and tan coloration likely reflects oxidation and surface alteration of iron-bearing minerals.
Field identification & locations
Look for a dense, heavy, non-layered rock with scattered feldspar crystals; a streak, hardness test, and fresh broken surface would distinguish it from weathered sedimentary rock. Common in volcanic provinces worldwide.
