igneous
Dark mafic rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectFine- to medium-grained, dark gray to black, with a dull to slightly vitreous luster; typically hard (about Mohs 6), dense, and lacking visible cleavage. The apparent granular texture may indicate basalt or a related mafic volcanic rock.
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Physical properties
Fine- to medium-grained, dark gray to black, with a dull to slightly vitreous luster; typically hard (about Mohs 6), dense, and lacking visible cleavage. The apparent granular texture may indicate basalt or a related mafic volcanic rock.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Most examples range from recent eruptions to Precambrian ages; age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive dense varieties are polished for architectural surfaces and specimens.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later fill with zeolites, calcite, or quartz.
Field identification & locations
Look for a dense black-gray rock with tiny interlocking crystals, possible small holes, and magnetic response from iron oxides. A hand lens, streak, magnet, and fresh fracture help distinguish it from slag, weathered gabbro, or meteorite material.
