igneous
Dark mafic rock, likely basalt
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectTypically fine-grained, dark gray to black, with a dull to weakly glassy luster; Mohs hardness about 6, generally massive with no visible cleavage. The uniformly fine texture indicates rapid cooling of lava.
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Physical properties
Typically fine-grained, dark gray to black, with a dull to weakly glassy luster; Mohs hardness about 6, generally massive with no visible cleavage. The uniformly fine texture indicates rapid cooling of lava.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled at or near Earth’s surface, commonly during volcanic eruptions from the Cenozoic to Precambrian. The image lacks enough detail for precise age or to exclude fine-grained gabbro.
Uses & applications
Widely used as crushed aggregate, road base, riprap, railroad ballast, asphalt component, and building stone; attractive polished pieces have modest decorative value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly forms immense lava plateaus and pillow lavas underwater.
Field identification & locations
Identify by its dark color, fine interlocking texture, high density, and lack of large crystals; a streak, fresh broken surface, and hand lens would help distinguish basalt from slate, slag, or weathered andesite. Common worldwide, especially oceanic crust and volcanic regions.
