igneous
Porphyritic basalt or weathered volcanic rock
Basalt, likely plagioclase–pyroxene rock with possible olivine; exact identification requires testing
AI-generated identification · may be incorrectDark gray to brown, fine-grained groundmass with pale mineral grains and rusty weathering; typically dull to subtly glassy. Basalt hardness is about Mohs 6, with no obvious cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray to brown, fine-grained groundmass with pale mineral grains and rusty weathering; typically dull to subtly glassy. Basalt hardness is about Mohs 6, with no obvious cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; brown coloration likely reflects oxidation and surface weathering.
Uses & applications
Used as crushed aggregate, road metal, riprap, paving stone, and occasionally as a decorative specimen; this small weathered piece has minimal commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Pale spots may be plagioclase feldspar or altered vesicles rather than true large crystals.
Field identification & locations
Look for a dense dark fine-grained texture, high weight, and resistance to scratching; a steel nail usually will not scratch fresh basalt. Common worldwide in volcanic regions, including Hawaii, Iceland, the Pacific Northwest, and ocean-floor exposures.
