igneous
Weathered basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically dark gray to brown, dull to slightly glassy, with a fine-grained crystalline texture; Mohs hardness about 6, no prominent cleavage, and specific gravity roughly 2.8–3.0. The rough, dark appearance and small possible vesicles fit weathered basalt.
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Physical properties
Typically dark gray to brown, dull to slightly glassy, with a fine-grained crystalline texture; Mohs hardness about 6, no prominent cleavage, and specific gravity roughly 2.8–3.0. The rough, dark appearance and small possible vesicles fit weathered basalt.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Basalt ranges from ancient Archean rocks to modern eruptions and is widespread on Earth and other terrestrial planets.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, building stone, and occasionally decorative tile or landscaping rock. Small ordinary fragments generally have negligible commercial value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Weathering can produce reddish-brown iron oxides and soften its otherwise dark color.
Field identification & locations
Identify it by its dark color, heavy feel, fine texture, and lack of visible large crystals; a streak, magnetism, or acid test may help distinguish it from slag or other dark rocks. Common in volcanic regions, oceanic islands, and flood-basalt provinces.
