igneous
Vesicular basalt
Basalt; predominantly plagioclase feldspar, pyroxene, and possible olivine
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly lustered, with rounded gas cavities (vesicles) and weathered pale surfaces. Hardness is typically about 6 on the Mohs scale; cleavage is not visible in the compact groundmass, and specific gravity is usually 2.8–3.0.
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Physical properties
Dark gray, fine-grained, dull to weakly lustered, with rounded gas cavities (vesicles) and weathered pale surfaces. Hardness is typically about 6 on the Mohs scale; cleavage is not visible in the compact groundmass, and specific gravity is usually 2.8–3.0.
Formation & geological history
An extrusive igneous rock formed when basaltic lava rapidly cooled at or near Earth’s surface, trapping volcanic gas bubbles. Most examples are geologically young relative to continental rocks, though basalt occurs from Archean formations to modern eruptions.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces may be collected but this specimen has little jewelry value.
Geological facts
Vesicles distinguish it from many dense dark rocks; cavities may later fill with zeolites, calcite, or quartz, producing amygdaloidal basalt.
Field identification & locations
Common in volcanic regions, oceanic crust, Hawaii, Iceland, the Columbia River Basalt Group, and many continental lava fields. Field identification relies on its dark fine-grained matrix, heavy weight, and gas holes; a streak test is usually gray and non-diagnostic.
