igneous
Porphyritic volcanic rock (likely andesite)
Porphyritic andesite; exact identification requires closer examination
AI-generated identification · may be incorrectDark gray-brown, rough rock with conspicuous pale feldspar crystals in a fine-grained groundmass. Feldspar is Mohs 6; rock has no regular cleavage and typically a specific gravity around 2.5–2.8.
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Physical properties
Dark gray-brown, rough rock with conspicuous pale feldspar crystals in a fine-grained groundmass. Feldspar is Mohs 6; rock has no regular cleavage and typically a specific gravity around 2.5–2.8.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew slowly before the remaining lava cooled rapidly. Andesite commonly forms in volcanic arcs, generally from Cenozoic to recent eruptions.
Uses & applications
Used locally as aggregate, road stone, and building stone; attractive pieces may be kept as geological specimens.
Geological facts
The visible larger crystals are phenocrysts, a texture indicating a change in cooling rate during formation.
Field identification & locations
Look for pale, blocky feldspar crystals embedded in a darker, fine-grained matrix; this image is consistent with porphyritic andesite but does not establish it conclusively. Andesite is common around subduction-zone volcanoes, including the Andes and Pacific Ring of Fire.
