igneous
Weathered basalt column
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to brown, earthy weathered surface, dull to slightly crystalline luster, hardness about 5–6 Mohs, and massive texture; the apparent blocky form may reflect columnar jointing or human shaping.
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Physical properties
Typically dark gray to brown, earthy weathered surface, dull to slightly crystalline luster, hardness about 5–6 Mohs, and massive texture; the apparent blocky form may reflect columnar jointing or human shaping.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near the surface, commonly developing polygonal columns. Most examples are Cenozoic or younger, though basalt occurs throughout Earth’s geological record.
Uses & applications
Used extensively as crushed aggregate, road metal, riprap, paving stone, and building stone; attractive polished or column specimens have modest decorative and collecting value.
Geological facts
Its dark color reflects iron- and magnesium-rich minerals. Vertical joints commonly form as cooling contraction fractures, producing spectacular columnar landscapes.
Field identification & locations
Identify it by its dark, dense, fine-grained composition, lack of visible quartz, and possible vesicles or plagioclase specks; common worldwide in volcanic provinces, including Iceland, India, the western United States, and oceanic islands.
