igneous
Weathered basalt boulder
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray-brown, massive, fine-grained, and weakly lustrous from weathering; typical hardness is about Mohs 6, with no visible cleavage or large crystals. The rusty-brown rind likely represents oxidation and surface weathering.
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Physical properties
Dark gray-brown, massive, fine-grained, and weakly lustrous from weathering; typical hardness is about Mohs 6, with no visible cleavage or large crystals. The rusty-brown rind likely represents oxidation and surface weathering.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions. Its exact age cannot be determined from the photograph; transport and prolonged weathering may have rounded the boulder.
Uses & applications
Used widely as crushed aggregate, road base, railway ballast, riprap, paving stone, and occasionally architectural facing; generally little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and ocean-floor pillow lavas.
Field identification & locations
Identify it by its heavy, dark, compact appearance, fine texture, and resistance to scratching; a steel file may scratch it only weakly, while it can scratch glass. Confirm with a fresh broken surface or thin section, since dark weathered rocks can also be andesite, diabase, or iron-rich sedimentary rock.
