igneous
Weathered basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray, dense, and fine-grained with a dull to faintly glassy luster; typically Mohs 6, no visible cleavage, and specific gravity about 2.8–3.0. The pale patches and surface crust may reflect weathering, alteration, or attached mineral veins.
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Physical properties
Dark gray, dense, and fine-grained with a dull to faintly glassy luster; typically Mohs 6, no visible cleavage, and specific gravity about 2.8–3.0. The pale patches and surface crust may reflect weathering, alteration, or attached mineral veins.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Basalt ranges from Archean to modern age; this specimen’s age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and construction stone; attractive polished pieces can be used decoratively. Common specimens generally have little commercial or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It commonly develops reddish-brown weathering from oxidation of iron-bearing minerals.
Field identification & locations
Commonly found in volcanic terrains, river gravels, beaches, and quarries; identify by its dark color, heavy feel, fine texture, and weak or absent reaction to dilute acid. A streak, density test, and magnification would help distinguish it from andesite or dark metamorphic rocks.
