igneous
Porphyritic volcanic rock, likely andesite
Porphyritic andesite; mainly plagioclase, pyroxene/amphibole, and volcanic groundmass
AI-generated identification · may be incorrectMedium- to fine-grained porphyritic texture with pale pinkish phenocrysts in a dark green-gray matrix; typically Mohs 6–7, dull to slightly vitreous luster, poor cleavage as a rock, and specific gravity about 2.5–2.8.
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Physical properties
Medium- to fine-grained porphyritic texture with pale pinkish phenocrysts in a dark green-gray matrix; typically Mohs 6–7, dull to slightly vitreous luster, poor cleavage as a rock, and specific gravity about 2.5–2.8.
Formation & geological history
Formed when intermediate-composition magma cooled in or near a volcano, producing large crystals in slower-cooling magma followed by rapid surface cooling. Most andesite is Cenozoic to older, but age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally dimension stone; attractive rounded specimens are collected but generally have low commercial value.
Geological facts
The contrasting large crystals are called phenocrysts, while the finer matrix is the groundmass. Green coloration may reflect chlorite or other alteration minerals, and the pale areas may be feldspar or weathered patches.
Field identification & locations
Identify it by its volcanic texture, scattered light feldspar crystals, dense fine-grained matrix, and lack of obvious layering or foliation; common in volcanic arcs and subduction-zone regions worldwide.
