igneous
Porphyritic volcanic rock (likely porphyritic andesite)
Porphyritic andesite; chiefly plagioclase feldspar with fine-grained volcanic groundmass
AI-generated identification · may be incorrectDark gray-purple, with conspicuous pale feldspar-like crystals or patches in a finer matrix; typically Mohs 6–7, dull to slightly vitreous, with no obvious cleavage in the whole rock. The rounded shape and lighting make exact identification uncertain.
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-purple, with conspicuous pale feldspar-like crystals or patches in a finer matrix; typically Mohs 6–7, dull to slightly vitreous, with no obvious cleavage in the whole rock. The rounded shape and lighting make exact identification uncertain.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew slowly before the remaining melt cooled rapidly during volcanic eruption. Andesites commonly formed in subduction-zone arcs, mainly during the Cenozoic, though this specimen’s age cannot be determined visually.
Uses & applications
Usually collected as a hand specimen; andesite is also used as crushed aggregate, paving stone, and building stone.
Geological facts
The contrasting crystal sizes give porphyritic rocks their distinctive texture and record changing cooling conditions.
Field identification & locations
Look for visible feldspar crystals embedded in a much finer volcanic matrix; compare a fresh, unweathered surface, as surface patterns can be misleading. Andesite occurs widely in volcanic arcs, including the Andes, Japan, and Cascades.
