igneous
Black basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with dull to faintly vitreous luster; hardness about 6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears massive and dense, though its small size and lighting limit certainty.
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Physical properties
Typically dark gray to black, fine-grained, with dull to faintly vitreous luster; hardness about 6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears massive and dense, though its small size and lighting limit certainty.
Formation & geological history
Forms when low-viscosity mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic plateaus, and shield volcanoes. Basalt ranges from modern eruptions to rocks billions of years old.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, concrete additive, building stone, and decorative landscaping rock; attractive pieces may be polished or collected.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its uniformly dark color, fine texture, high density, and lack of visible large crystals; a streak, hardness, and magnetism test can help. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic regions.
