igneous
Weathered basaltic cobble
Altered basalt, predominantly plagioclase and pyroxene
AI-generated identification · may be incorrectTypically dark gray to brown when fresh, but this specimen is heavily weathered to tan/orange with iron-oxide staining; fine-grained, massive, dull to weakly earthy luster, Mohs hardness about 5–6, and no obvious cleavage. The rounded shape indicates prolonged transport or abrasion.
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Physical properties
Typically dark gray to brown when fresh, but this specimen is heavily weathered to tan/orange with iron-oxide staining; fine-grained, massive, dull to weakly earthy luster, Mohs hardness about 5–6, and no obvious cleavage. The rounded shape indicates prolonged transport or abrasion.
Formation & geological history
Likely formed as volcanic lava that cooled rapidly at or near Earth’s surface, generally in the Cenozoic or older depending on locality. Surface oxidation and chemical weathering produced the ochre coating and reddish patches.
Uses & applications
Common basalt is used as aggregate, road base, riprap, and construction stone; attractive polished pieces may be sold as display specimens. This heavily weathered cobble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock in Earth’s oceanic crust and commonly forms lava flows, dikes, and pillow lavas.
Field identification & locations
Identify it by its heavy, fine-grained texture, lack of visible large crystals, and possible tiny feldspar or pyroxene grains; a streak, fresh broken surface, magnetism test, and locality are useful. Common worldwide, especially volcanic regions and river gravels.
