igneous
Dark volcanic rock, likely basalt
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, generally dull to mildly glassy; hardness about 6, massive texture, with weak or no cleavage visible. The specimen is too out-of-focus for confident identification and may also be another mafic volcanic rock.
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Physical properties
Fine-grained, dark gray to black, generally dull to mildly glassy; hardness about 6, massive texture, with weak or no cleavage visible. The specimen is too out-of-focus for confident identification and may also be another mafic volcanic rock.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic fields. Basalt ranges from Precambrian to modern in age.
Uses & applications
Used extensively as crushed aggregate, road stone, railway ballast, concrete additive, and building stone; attractive dense pieces may be collected or polished.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. Vesicles, if present, indicate gas bubbles trapped during cooling.
Field identification & locations
In the field, check for a uniform dark color, fine grain, high density, and lack of visible quartz; a streak test is usually inconclusive. Common worldwide, especially volcanic islands, mid-ocean-ridge terrains, and flood-basalt provinces.
