igneous
Basalt
Fine-grained mafic volcanic rock, chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, dull to slightly glassy, with hardness about 5–6 Mohs; fine-grained texture obscures individual crystals, and cleavage is absent as a rock. The specimen appears dense and angular, possibly weathered or vesicle-poor.
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Physical properties
Typically dark gray to black, dull to slightly glassy, with hardness about 5–6 Mohs; fine-grained texture obscures individual crystals, and cleavage is absent as a rock. The specimen appears dense and angular, possibly weathered or vesicle-poor.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic plateaus, and lava flows. Basalt ranges from ancient Precambrian examples to actively forming modern lava.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, dimension stone, and basalt fiber; attractive polished pieces are used decoratively but have limited jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Rapid cooling can produce columnar jointing, while gas-rich flows may form vesicular basalt.
Field identification & locations
Identify it by its uniform dark color, high density, fine grain, and lack of visible large crystals; a streak, magnetism, and hardness test can help distinguish it from slag or dark sedimentary rocks. Common worldwide, especially volcanic regions and oceanic islands.
