igneous
Basalt
Mafic volcanic rock; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with a dull to weakly lustered surface; hardness about 6 on the Mohs scale, dense, and generally lacking visible cleavage. The specimen appears massive and fine-grained, with scattered lighter mineral grains and fractures.
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Physical properties
Typically dark gray to black, fine-grained, with a dull to weakly lustered surface; hardness about 6 on the Mohs scale, dense, and generally lacking visible cleavage. The specimen appears massive and fine-grained, with scattered lighter mineral grains and fractures.
Formation & geological history
Forms when basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and shield volcanoes. Its age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished basalt is also used decoratively. Ordinary hand specimens generally have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. It commonly occurs with vesicles, amygdules, columnar jointing, or magnetic iron minerals.
Field identification & locations
Identify it by its dark color, high density, fine texture, and absence of obvious quartz or large crystals; a streak, hardness, magnetism, and thin-section test can confirm it. Common localities include oceanic islands, mid-ocean ridges, and continental flood-basalt provinces.
