igneous
Dark volcanic rock fragment, likely basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase and pyroxene, commonly with minor olivine and magnetite
AI-generated identification · may be incorrectTypically dark gray to black, matte to subtly vitreous, and fine-grained; Mohs hardness about 5–6, with irregular fracture and no visible cleavage. The specimen appears dense and angular, though the image is insufficient for certainty.
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Physical properties
Typically dark gray to black, matte to subtly vitreous, and fine-grained; Mohs hardness about 5–6, with irregular fracture and no visible cleavage. The specimen appears dense and angular, though the image is insufficient for certainty.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and mid-ocean ridges. Most examples are geologically young relative to continental rocks, ranging from recent eruptions to Precambrian formations.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive varieties are used for landscaping and polished objects.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the oceanic crust. Vesicles, if present, indicate gas bubbles trapped during cooling.
Field identification & locations
Identify it by its black-gray color, fine texture, high density, and lack of visible large crystals; a streak, magnet test, and fresh broken surface would help distinguish it from slag or other dark rocks. Common worldwide, especially volcanic regions and oceanic environments.
