igneous
Basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly glassy luster; hardness about 6 on Mohs scale, high density (~2.8–3.0 g/cm³), and generally no visible cleavage. The broken surfaces and massive texture are consistent with basalt, though the photograph alone cannot exclude other dark volcanic rocks.
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Physical properties
Dark gray to black, fine-grained, dull to slightly glassy luster; hardness about 6 on Mohs scale, high density (~2.8–3.0 g/cm³), and generally no visible cleavage. The broken surfaces and massive texture are consistent with basalt, though the photograph alone cannot exclude other dark volcanic rocks.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, especially at mid-ocean ridges, hotspots, and volcanic fields. Specimen age depends on locality and may range from recent volcanic material to billions of years old.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and dimension stone; attractive polished basalt is also used decoratively. Ordinary hand specimens generally have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Vesicles, tiny crystals, or magnetic response from iron-bearing minerals are common identifying clues.
Field identification & locations
Look for a uniformly dark, dense, fine-grained rock with small interlocking crystals and possible magnetism; a streak test is usually gray-black. Common localities include Hawaii, Iceland, the Columbia River Basalt Group, India’s Deccan Traps, and ocean floors.
