igneous
Porphyritic volcanic rock, likely porphyritic andesite
Porphyritic andesite (feldspar phenocrysts in a fine-grained groundmass); visual identification is tentative
AI-generated identification · may be incorrectDark gray-green, fine-grained groundmass with pale, blocky mineral crystals; likely plagioclase phenocrysts. Hardness and crystal structure vary by minerals; specimen appears naturally worn or shaped.
Identified More igneous →
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Physical properties
Dark gray-green, fine-grained groundmass with pale, blocky mineral crystals; likely plagioclase phenocrysts. Hardness and crystal structure vary by minerals; specimen appears naturally worn or shaped.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew slowly, then the remaining melt cooled rapidly. Andesites commonly form in volcanic arcs, often during the Cenozoic, though the specimen’s age cannot be determined from the image.
Uses & applications
Andesite is used as crushed aggregate and dimension stone; this small piece is mainly suitable for collecting or display.
Geological facts
The contrasting large crystals and fine groundmass are a porphyritic texture, evidence of changing cooling conditions.
Field identification & locations
Look for pale feldspar crystals embedded in a darker volcanic matrix; comparable rocks occur around subduction-zone volcanoes worldwide. A definitive identification would require examining the specimen directly.
