igneous
Porphyritic volcanic rock, likely porphyritic andesite
Porphyritic andesite (tentative; exact composition cannot be confirmed from the photo)
AI-generated identification · may be incorrectPurple-brown, fine-grained groundmass with conspicuous pale crystals, likely feldspar phenocrysts. Hardness and luster are not reliably determined from the image; andesite is typically about 6 on the Mohs scale and has an aphanitic groundmass.
Identified More igneous →
Explore Porphyritic volcanic rock, likely porphyritic andesite in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Purple-brown, fine-grained groundmass with conspicuous pale crystals, likely feldspar phenocrysts. Hardness and luster are not reliably determined from the image; andesite is typically about 6 on the Mohs scale and has an aphanitic groundmass.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew slowly before the remaining melt cooled rapidly. Andesitic rocks commonly form in volcanic arcs, generally in geologically recent to ancient volcanic settings.
Uses & applications
Mostly used as crushed aggregate or building stone; attractive pieces may be kept as specimens.
Geological facts
The contrasting crystal sizes record different cooling rates, a texture called porphyritic.
Field identification & locations
Look for pale, visible crystals embedded in a much finer matrix; this appearance supports a porphyritic volcanic rock identification. Similar rocks occur near subduction-zone volcanoes worldwide.
