igneous
Weathered vesicular basalt (possible volcanic scoria)
Basalt; chiefly plagioclase, pyroxene, and commonly olivine, with secondary iron-oxide alteration
AI-generated identification · may be incorrectTypically dark gray to brown, but weathering produces the tan surface shown; dull to subtly vitreous, fine-grained, and highly vesicular. Hardness is about 5–6 Mohs, with no obvious cleavage and specific gravity commonly 2.7–3.1; the rounded form may reflect strong weathering or abrasion.
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Physical properties
Typically dark gray to brown, but weathering produces the tan surface shown; dull to subtly vitreous, fine-grained, and highly vesicular. Hardness is about 5–6 Mohs, with no obvious cleavage and specific gravity commonly 2.7–3.1; the rounded form may reflect strong weathering or abrasion.
Formation & geological history
Formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface, trapping gas bubbles; the cavities later weathered and may contain clay or iron oxides. Exact age cannot be determined from the photograph and may range from recent volcanic material to ancient flows.
Uses & applications
Used as aggregate, road base, riprap, railroad ballast, and building stone; vesicular varieties are also used as lightweight landscaping rock and abrasive material. This weathered specimen has modest decorative or teaching value.
Geological facts
Vesicles distinguish it from most dense basalt, while the tan coloration is likely secondary oxidation and mineral alteration rather than its original color. It may also be a weathered volcanic bomb if it was ejected during an eruption.
Field identification & locations
Look for a fine-grained matrix, abundant rounded holes, high density, and weak attraction to a magnet if iron-rich. Common sources include volcanic regions such as Hawaii, Iceland, the Pacific Northwest, and Mediterranean volcanic belts; a fresh broken surface and streak/magnet tests would improve identification.
