igneous
Basalt
Fine-grained mafic volcanic rock, chiefly plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, dense, dull to faintly vitreous luster, typically massive and fine-grained; Mohs hardness about 5–6, with no obvious cleavage. The surface appears weathered or slightly vesicular, though texture is difficult to confirm from the image.
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Physical properties
Dark gray to black, dense, dull to faintly vitreous luster, typically massive and fine-grained; Mohs hardness about 5–6, with no obvious cleavage. The surface appears weathered or slightly vesicular, though texture is difficult to confirm from the image.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic flows, and hotspots. Basalt ranges from Archean to modern in age and is still actively forming.
Uses & applications
Used as crushed aggregate, road metal, railroad ballast, building stone, and rock wool; attractive polished pieces are sold as decorative specimens. Ordinary hand specimens generally have modest value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. It commonly weathers to iron-rich soils and may contain vesicles filled later by zeolite, calcite, or quartz.
Field identification & locations
Identify it by its black, heavy, fine-grained appearance and inability to scratch easily with a steel knife; unlike meteorite material, it commonly shows tiny crystals or vesicles and is not strongly magnetic. Common localities include Hawaii, Iceland, India’s Deccan Traps, and oceanic volcanic regions.
