igneous
Basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly glassy luster, typically massive with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded shape appears naturally weathered or tumbled.
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Physical properties
Dark gray to black, fine-grained, dull to slightly glassy luster, typically massive with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded shape appears naturally weathered or tumbled.
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; exact age cannot be determined from the photograph.
Uses & applications
Widely used as crushed aggregate, roadstone, railroad ballast, riprap, and occasionally building stone; attractive polished pieces are sold as display specimens. Common examples generally have little monetary value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Some varieties contain vesicles, amygdules, or magnetic iron minerals, but none are clearly visible here.
Field identification & locations
Identify it by its uniformly dark color, fine texture, great density, and lack of obvious layers or large crystals; it commonly occurs in lava flows, volcanic fields, river gravels, and glacial deposits. A magnet may show weak attraction if magnetite-rich.
