igneous
Basalt
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine with minor iron-titanium oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained, dense, and typically dull to weakly vitreous; Mohs hardness about 5–6, with no obvious cleavage and a massive texture. The granular surface and lack of visible large crystals are consistent with basalt.
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Physical properties
Dark gray to black, fine-grained, dense, and typically dull to weakly vitreous; Mohs hardness about 5–6, with no obvious cleavage and a massive texture. The granular surface and lack of visible large crystals are consistent with basalt.
Formation & geological history
An extrusive mafic igneous rock formed when basaltic lava rapidly cooled at or near Earth’s surface, commonly from volcanic eruptions and seafloor spreading. Specimen age cannot be determined from the image; basalt ranges from Archean to modern.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, concrete aggregate, riprap, and building stone; attractive polished pieces are used decoratively. Common specimens generally have modest collecting value.
Geological facts
Basalt forms most of the oceanic crust and is the most common volcanic rock on Earth. Vesicles, if present, may later fill with zeolite, calcite, or quartz.
Field identification & locations
Common in volcanic regions, oceanic crust, flood-basalt provinces, and lunar mare samples. Identify it by its black fine-grained appearance, high density, and inability to scratch easily with a steel knife; confirm with thin-section or chemical testing.
