igneous
Basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly vitreous luster, generally massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The specimen’s dense, dark appearance and subtle granular texture are consistent with basalt, though image-only identification is approximate.
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Physical properties
Dark gray to black, fine-grained, dull to weakly vitreous luster, generally massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The specimen’s dense, dark appearance and subtle granular texture are consistent with basalt, though image-only identification is approximate.
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, shield volcanoes, and flood-basalt provinces. Basalt ranges from ancient Precambrian rocks to modern eruptions and is still forming today.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, paving stone, and building material; attractive polished pieces may be sold as decorative stone.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, glassy edges, or visible olivine crystals can help distinguish varieties such as vesicular basalt or olivine basalt.
Field identification & locations
Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, the Columbia River Plateau, and oceanic islands. Field clues include black color, high density, fine texture, and lack of visible quartz; a streak or thin-section test can confirm it.
