igneous
Likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly vitreous; typically hardness 6–7 Mohs, massive with no obvious cleavage, and specific gravity about 2.8–3.0. The specimen appears weathered and fractured, so exact identification is uncertain from the image alone.
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Physical properties
Dark gray to black, fine-grained, dull to slightly vitreous; typically hardness 6–7 Mohs, massive with no obvious cleavage, and specific gravity about 2.8–3.0. The specimen appears weathered and fractured, so exact identification is uncertain from the image alone.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in the Cenozoic or older volcanic terrains. It is widespread in oceanic crust, flood-basalt provinces, and volcanic fields.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished varieties are used decoratively. Ordinary hand specimens have minimal commercial value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Vesicles, olivine crystals, or a fine-grained groundmass can help distinguish it from gabbro and dark metamorphic rocks.
Field identification & locations
Common in Hawaii, Iceland, the Pacific Northwest, India’s Deccan Traps, and oceanic islands; identify by its heavy, dark, fine-grained appearance and lack of visible quartz. A streak, hardness, magnetism, and fresh broken surface would improve confidence.
