igneous
Likely weathered basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, often with olivine
AI-generated identification · may be incorrectDark gray-brown, dull to subtly granular, dense, and massive with irregular fracture; typically Mohs 5–6 and specific gravity about 2.8–3.0. The image’s weathered surface makes crystal size and exact composition uncertain.
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Physical properties
Dark gray-brown, dull to subtly granular, dense, and massive with irregular fracture; typically Mohs 5–6 and specific gravity about 2.8–3.0. The image’s weathered surface makes crystal size and exact composition uncertain.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age cannot be determined from the photograph; weathering has produced the brown coating.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished pieces may be collected, though this specimen appears ordinary.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It commonly weathers to brown or reddish iron-oxide surfaces.
Field identification & locations
Look for high density, dark color, fine grain, and possible tiny pale plagioclase specks; it should not fizz in dilute hydrochloric acid. Common worldwide, especially volcanic regions; a laboratory test or fresh broken surface is needed to distinguish it from andesite or weathered gabbro.
