igneous
Porphyritic basalt with weathering or mineral-filled veins
Basalt, likely plagioclase-porphyritic; composition dominated by plagioclase and pyroxene with possible secondary quartz or calcite
AI-generated identification · may be incorrectDark gray, fine-grained volcanic rock containing conspicuous pale white to tan patches and veins; typically dull to slightly glassy, with Mohs hardness about 5–6 and no obvious cleavage. The light material may be plagioclase phenocrysts or later vein minerals.
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Physical properties
Dark gray, fine-grained volcanic rock containing conspicuous pale white to tan patches and veins; typically dull to slightly glassy, with Mohs hardness about 5–6 and no obvious cleavage. The light material may be plagioclase phenocrysts or later vein minerals.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, producing a fine matrix around larger crystals; white veins or patches may have formed later from hydrothermal fluids. Basalt ranges from Precambrian to Recent in age.
Uses & applications
Used as crushed aggregate, road base, riprap, building stone, and occasionally decorative stone; specimens have modest collecting value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. A streak test, fresh fracture, and acid test on the pale material can help distinguish calcite-filled veins from feldspar.
Field identification & locations
Common in volcanic regions, oceanic islands, flood-basalt provinces, and old lava flows. Identify it by its dark fine-grained groundmass, relatively high density, and pale feldspar or vein material; exact identification requires a fresh surface or microscopy.
