igneous
Dark igneous rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and dull to weakly vitreous, with interlocking crystals too small to see; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The specimen’s rough texture and dark color are consistent with basalt, though the image alone cannot confirm the diagnosis.
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Physical properties
Typically dark gray to black, fine-grained, and dull to weakly vitreous, with interlocking crystals too small to see; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The specimen’s rough texture and dark color are consistent with basalt, though the image alone cannot confirm the diagnosis.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly at volcanic vents, mid-ocean ridges, and hotspots. It ranges from modern lava to rocks formed throughout Earth’s geological history.
Uses & applications
Used extensively as crushed aggregate, road material, riprap, railroad ballast, concrete additive, and basalt fiber; attractive varieties may be polished for decorative objects.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Vesicles, tiny gas cavities, or visible olivine crystals can provide additional clues.
Field identification & locations
Look for a heavy, dark, fine-grained rock that is harder than glass and lacks obvious layering; a streak test is usually gray and a magnet may respond weakly. Common localities include Hawaii, Iceland, India’s Deccan Traps, and oceanic crust.
