igneous
Basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, here weathered brown-green, with a dull to faintly vitreous luster and fine-grained texture; hardness about 6, no visible cleavage as a rock, and specific gravity roughly 2.8–3.0.
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Physical properties
Typically dark gray to black, here weathered brown-green, with a dull to faintly vitreous luster and fine-grained texture; hardness about 6, no visible cleavage as a rock, and specific gravity roughly 2.8–3.0.
Formation & geological history
Forms when low-silica, iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, shield volcanoes, and flood-basalt provinces. Most examples are Cenozoic or younger, though basalt occurs throughout geologic time.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, concrete aggregate, building stone, and occasionally decorative or sculptural material; generally low-value as a specimen.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later fill with zeolite, calcite, or quartz.
Field identification & locations
Identify it by its dense, dark, fine-grained appearance, heavy feel, and lack of obvious large crystals; it commonly occurs with lava flows and volcanic landscapes worldwide. This specimen appears rounded and weathered, possibly waterworn.
