igneous
Vesicular basalt (scoriaceous volcanic rock)
Basalt, predominantly plagioclase + clinopyroxene ± olivine, with abundant vesicles
AI-generated identification · may be incorrectTypically dark gray to brown, porous, and dull to slightly glassy; Mohs hardness about 5–6, fine-grained crystalline groundmass, no obvious cleavage, and specific gravity commonly ~2.4–2.9 (lower when highly vesicular).
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Physical properties
Typically dark gray to brown, porous, and dull to slightly glassy; Mohs hardness about 5–6, fine-grained crystalline groundmass, no obvious cleavage, and specific gravity commonly ~2.4–2.9 (lower when highly vesicular).
Formation & geological history
Formed when basaltic lava cooled rapidly at or near the surface, trapping expanding gas bubbles as vesicles; generally Cenozoic or younger, though basalt occurs throughout geologic time. The rusty coloration likely reflects weathering of iron-bearing minerals.
Uses & applications
Used as lightweight aggregate, drainage and landscaping stone, abrasives, and occasionally decorative specimens; usually low commercial value unless unusually shaped or locality-documented.
Geological facts
The holes are vesicles, not fossils; their size and abundance record gas content and cooling conditions. If the rock is exceptionally lightweight, frothy, and glassier, it could instead be scoria or pumice.
Field identification & locations
Identify by its mafic gray-black color, irregular bubble cavities, and lack of layering or fossils; common in volcanic regions, lava flows, cinder cones, and oceanic crust. A fresh surface and magnet test can help distinguish basalt from slag, though industrial slag may have glassy textures or metal droplets.
