igneous
Porphyritic andesite (weathered volcanic rock)
Andesite, commonly plagioclase-phyric; approximate composition 57–63% SiO₂
AI-generated identification · may be incorrectFine-grained gray groundmass with conspicuous pale plagioclase phenocrysts and scattered dark mafic minerals; typically hardness 6–7, dull to slightly vitreous luster, massive texture, and no prominent cleavage. Surface weathering and lichen obscure the original texture.
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Physical properties
Fine-grained gray groundmass with conspicuous pale plagioclase phenocrysts and scattered dark mafic minerals; typically hardness 6–7, dull to slightly vitreous luster, massive texture, and no prominent cleavage. Surface weathering and lichen obscure the original texture.
Formation & geological history
A volcanic igneous rock formed when intermediate-composition lava cooled relatively quickly at or near Earth’s surface, commonly in volcanic arcs; individual examples may range from recent to Precambrian, depending on locality. Larger crystals formed before eruption, followed by rapid cooling of the groundmass.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally building or paving stone; attractive polished material has limited decorative use. Ordinary weathered specimens generally have little commercial value.
Geological facts
The visible large crystals are phenocrysts, producing a porphyritic texture. Andesite is compositionally intermediate between basalt and rhyolite and is characteristic of subduction-zone volcanism.
Field identification & locations
Identify it by the gray intermediate groundmass, abundant white plagioclase crystals, and dark amphibole/pyroxene grains; distinguish it from granite by its finer matrix and from basalt by its lighter color. Common in volcanic arcs, including the Andes, Cascades, Japan, and Indonesia.
