igneous
Dark volcanic rock, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectTypically black to dark gray, fine-grained, and dull to weakly vitreous; Mohs hardness about 5–6, no visible cleavage, and density roughly 2.8–3.0 g/cm³. The specimen appears dense and massive, though its jagged surface may reflect weathering or vesicles.
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Physical properties
Typically black to dark gray, fine-grained, and dull to weakly vitreous; Mohs hardness about 5–6, no visible cleavage, and density roughly 2.8–3.0 g/cm³. The specimen appears dense and massive, though its jagged surface may reflect weathering or vesicles.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in lava flows, volcanic fields, oceanic crust, and flood basalts. It may range from recent volcanic material to rocks billions of years old.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, concrete additive, building stone, and basalt fiber; usually has little jewelry value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up most oceanic crust. Its dark color and fine texture distinguish it from coarse-grained gabbro, while vesicles may indicate trapped volcanic gas.
Field identification & locations
Identify it by its heavy feel, dark color, fine texture, and lack of obvious crystals; a streak test is usually gray to black. Common locations include Hawaii, Iceland, the Pacific Northwest, India’s Deccan Traps, and oceanic volcanic regions.
