igneous
Basalt (likely vesicular basalt)
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine, with iron-titanium oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained, mostly dull to weakly glassy, with small vesicles and scattered reflective crystals. Hardness about 5–6 Mohs; dense, typically with no obvious cleavage as a whole rock; specific gravity about 2.8–3.1.
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Physical properties
Dark gray to black, fine-grained, mostly dull to weakly glassy, with small vesicles and scattered reflective crystals. Hardness about 5–6 Mohs; dense, typically with no obvious cleavage as a whole rock; specific gravity about 2.8–3.1.
Formation & geological history
An extrusive mafic igneous rock formed when low-silica, iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface. It may range from recent volcanic material to Precambrian examples, depending on locality.
Uses & applications
Widely used as crushed aggregate, road stone, riprap, railway ballast, and building stone; attractive polished pieces are used decoratively and by collectors.
Geological facts
Basalt makes up much of oceanic crust and many volcanic plateaus, including the Columbia River Basalt and Deccan Traps. Vesicles formed from gas bubbles trapped during lava cooling.
Field identification & locations
Identify it by its black fine-grained matrix, high density, hardness, and possible bubble holes; a streak test is usually gray, and it is generally nonmagnetic to weakly magnetic. Common worldwide in volcanic regions and oceanic crust.
