igneous
Weathered basalt (possibly vesicular basalt)
Basalt; chiefly plagioclase, pyroxene, and iron-titanium oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to slightly earthy luster, with irregular vesicles and weathering surfaces; typically hardness 5–6, massive with no obvious cleavage, and specific gravity about 2.8–3.0. The rusty-brown coating likely reflects oxidation of iron-bearing minerals.
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Physical properties
Dark gray-brown, fine-grained, dull to slightly earthy luster, with irregular vesicles and weathering surfaces; typically hardness 5–6, massive with no obvious cleavage, and specific gravity about 2.8–3.0. The rusty-brown coating likely reflects oxidation of iron-bearing minerals.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. Its geological age cannot be determined from the photograph; the strongly weathered texture indicates prolonged surface alteration.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces may be used decoratively. Ordinary hand specimens have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Vesicles form when gas bubbles are trapped in cooling lava; later alteration can fill them with zeolites, calcite, or quartz.
Field identification & locations
Identify it by its dark color, fine texture, high density, and lack of visible large crystals; a streak test is usually gray, and it commonly weakly attracts a magnet if magnetite-rich. Common localities include Hawaii, Iceland, the Deccan Traps, and oceanic volcanic regions.
