igneous
Basalt
Basalt; fine-grained mafic volcanic rock, mainly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically dark gray to black, dense, and fine-grained, with dull to subtly vitreous luster; Mohs hardness about 6, no prominent cleavage, and specific gravity roughly 2.7–3.3. The specimen appears weathered and may contain small pale mineral or alteration patches.
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Physical properties
Typically dark gray to black, dense, and fine-grained, with dull to subtly vitreous luster; Mohs hardness about 6, no prominent cleavage, and specific gravity roughly 2.7–3.3. The specimen appears weathered and may contain small pale mineral or alteration patches.
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 age; this hand specimen’s age cannot be determined visually.
Uses & applications
Used widely as crushed aggregate, road base, railroad ballast, riprap, building stone, and basalt fiber; attractive polished pieces are occasionally sold as décor or teaching specimens.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up most oceanic crust. Vesicles, if present, indicate trapped volcanic gas; none are clearly prominent here.
Field identification & locations
Identify by its black, heavy, fine-grained texture and lack of visible large crystals; distinguish it from gabbro, which is coarse-grained, and slag, which is often highly vesicular or glassy. Common localities include Hawaii, Iceland, India’s Deccan Traps, ocean floors, and volcanic regions worldwide.
