igneous
Weathered basalt cobble
Basalt, a fine-grained mafic volcanic rock; commonly composed of plagioclase, pyroxene, and minor olivine or iron oxides
AI-generated identification · may be incorrectDark gray-brown, strongly weathered surface with dull to slightly earthy luster; typically very fine-grained, massive, and without visible cleavage. Hardness about 5–6 Mohs; specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray-brown, strongly weathered surface with dull to slightly earthy luster; typically very fine-grained, massive, and without visible cleavage. Hardness about 5–6 Mohs; specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when mafic lava rapidly cooled at or near Earth’s surface, generally during the Cenozoic or older volcanic episodes; its rounded shape and brown rind result from stream or beach abrasion and oxidation.
Uses & applications
Used as crushed aggregate, road stone, riprap, railroad ballast, and occasionally building stone; attractive dense cobbles may be collected or polished.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and seafloor pillow lavas.
Field identification & locations
Identify by its high density, dark fine-grained interior, lack of obvious quartz, and possible weak magnetism from iron oxides. A fresh broken surface would provide greater confidence; common worldwide near volcanic terrains and gravel deposits.
