igneous
Porphyritic volcanic rock, likely porphyritic andesite
Porphyritic andesite (tentative; exact identification requires testing)
AI-generated identification · may be incorrectRough, brownish-gray matrix with scattered pale and dark crystals; likely plagioclase and mafic minerals. Typical andesite hardness is about 6–7 Mohs, with interlocking crystals and no obvious cleavage at rock scale.
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Physical properties
Rough, brownish-gray matrix with scattered pale and dark crystals; likely plagioclase and mafic minerals. Typical andesite hardness is about 6–7 Mohs, with interlocking crystals and no obvious cleavage at rock scale.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew slowly underground, then the remaining melt cooled more quickly after eruption. Andesite commonly forms in volcanic arcs; its age cannot be determined from the photograph.
Uses & applications
Similar rocks are used as aggregate and road stone; this small specimen is mainly suitable for a collection.
Geological facts
The conspicuous larger crystals set in a finer groundmass are called phenocrysts, producing a porphyritic texture.
Field identification & locations
Look for visible crystals embedded in a finer volcanic matrix; this appearance is consistent with andesite but is not diagnostic from a photo alone. Andesite is widespread around subduction-zone volcanoes, including the Andes and Japan.
