igneous
Porphyritic volcanic rock (likely porphyritic andesite)
Porphyritic andesite; exact identification requires examination of a fresh surface
AI-generated identification · may be incorrectDark gray, fine-grained groundmass with conspicuous pale feldspar crystals; crystals are typically plagioclase, Mohs hardness about 6, with a dull to slightly vitreous luster. The specimen appears rounded and weather-worn; crystal structure is too fine to see in the 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
Dark gray, fine-grained groundmass with conspicuous pale feldspar crystals; crystals are typically plagioclase, Mohs hardness about 6, with a dull to slightly vitreous luster. The specimen appears rounded and weather-worn; crystal structure is too fine to see in the groundmass.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew slowly underground, then the remaining melt cooled rapidly after eruption. Such rocks commonly formed in volcanic arcs over a wide range of geological ages.
Uses & applications
Used as crushed aggregate and construction stone; attractive pieces may be kept as natural curiosities.
Geological facts
The large crystals set in a finer matrix are called phenocrysts, giving the rock its porphyritic texture.
Field identification & locations
Look for pale, blocky feldspar crystals embedded in a much finer dark matrix; distinguish it from concrete by its natural, interlocking texture and lack of manufactured aggregate-binder structure. Andesite is common in volcanic belts such as the Andes, Japan, and the Cascades.
