igneous
Weathered basalt cobble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with minor olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, dull to weakly vitreous, very fine-grained, and dense; Mohs hardness about 6, with no visible cleavage and irregular conchoidal-to-uneven fracture. The surface appears strongly weathered, with iron-oxide staining and natural cracks or vesicle-like pits.
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Physical properties
Dark gray-brown, dull to weakly vitreous, very fine-grained, and dense; Mohs hardness about 6, with no visible cleavage and irregular conchoidal-to-uneven fracture. The surface appears strongly weathered, with iron-oxide staining and natural cracks or vesicle-like pits.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the exact age cannot be determined from the photograph. Weathering rounded it into a cobble.
Uses & applications
Used widely as crushed aggregate, road stone, riprap, railway ballast, and building stone; attractive polished pieces may serve as décor or inexpensive specimens.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and pillow lavas.
Field identification & locations
Identify by its dark color, fine texture, high density, and lack of large crystals; a streak, magnetism test, and fresh broken surface would help distinguish it from andesite or dark sedimentary rock. Common worldwide, especially volcanic regions and river gravels.
