igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, very fine-grained, dull to weakly submetallic, with rounded weathered surfaces and possible vesicles or alteration seams. Typical hardness is about 6 Mohs; massive, with no obvious cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray-brown, very fine-grained, dull to weakly submetallic, with rounded weathered surfaces and possible vesicles or alteration seams. Typical hardness is about 6 Mohs; massive, with no obvious cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, likely in a volcanic flow; its exact geological age and locality cannot be determined from the photograph. Surface rounding and oxidation indicate transport or prolonged weathering.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, and occasionally building stone; this individual cobble has mainly educational or decorative value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms from mantle-derived magma. The apparent light-gray patches may be weathering, quartz/chalcedony, or a fracture coating.
Field identification & locations
Identify it by its dense, dark, fine-grained texture, lack of visible large crystals, and gray-black streak or powder; a glassy streak or strong magnetism may indicate iron-rich alteration. Common in volcanic terrains and stream gravels.
