igneous
Porphyritic basalt (possibly basaltic andesite)
Plagioclase-phyric basalt; mainly plagioclase feldspar and pyroxene, with minor olivine or iron oxides
AI-generated identification · may be incorrectDark gray-black, fine-grained groundmass containing abundant pale, angular plagioclase phenocrysts; generally dull to subvitreous luster, Mohs hardness about 5–6, little visible cleavage, and specific gravity around 2.8–3.0.
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Physical properties
Dark gray-black, fine-grained groundmass containing abundant pale, angular plagioclase phenocrysts; generally dull to subvitreous luster, Mohs hardness about 5–6, little visible cleavage, and specific gravity around 2.8–3.0.
Formation & geological history
An extrusive volcanic rock formed when basaltic magma cooled rapidly at or near the surface, preserving larger crystals that formed earlier underground. Its exact geological age cannot be determined from the image; such rocks range from Precambrian to modern.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and occasionally building or decorative stone; attractive specimens are collected for their porphyritic texture.
Geological facts
The contrasting large feldspar crystals and dark matrix record two-stage cooling: slow crystallization followed by rapid eruption. Vesicles may occur, though none are obvious here.
Field identification & locations
Identify it by its black fine-grained matrix and conspicuous cream-white feldspar crystals; a hand lens, streak test, and thin section help distinguish it from porphyritic andesite. Common in volcanic belts, oceanic crust, and flood-basalt provinces.
