igneous
Porphyritic basalt (likely porphyritic andesite)
Plagioclase-phyric volcanic rock; plagioclase phenocrysts in a dark, fine-grained groundmass
AI-generated identification · may be incorrectDark gray to black, fine-grained matrix with conspicuous tan feldspar crystals. Plagioclase has Mohs hardness about 6; rock hardness varies, typically around 5–6, with no consistent cleavage at the whole-rock scale.
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Physical properties
Dark gray to black, fine-grained matrix with conspicuous tan feldspar crystals. Plagioclase has Mohs hardness about 6; rock hardness varies, typically around 5–6, with no consistent cleavage at the whole-rock scale.
Formation & geological history
Formed when feldspar crystals grew in magma before it cooled rapidly at or near the surface. Its age cannot be determined from the photograph; porphyritic lavas occur in volcanic rocks of many ages.
Uses & applications
Commonly used as crushed aggregate and dimension stone; attractive examples may be collected or polished for decorative use.
Geological facts
The larger crystals are phenocrysts, recording a slower early cooling stage followed by rapid solidification of the surrounding lava.
Field identification & locations
The dark fine-grained matrix and scattered pale, blocky crystals suggest a plagioclase-porphyritic volcanic rock; confirm with a fresh surface and mineral tests, as identification from a photo is uncertain. Similar rocks occur in volcanic regions worldwide.
