igneous
Porphyritic basalt (vesicular basalt)
Basalt; fine-grained mafic rock, commonly composed of plagioclase, clinopyroxene, olivine, and iron-titanium oxides
AI-generated identification · may be incorrectDark gray, fine-grained groundmass with conspicuous pale plagioclase phenocrysts and small vesicles; typically dull to slightly vitreous, hardness about 5–6, no obvious cleavage, specific gravity roughly 2.8–3.1. The rounded shape indicates water-worn transport.
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Physical properties
Dark gray, fine-grained groundmass with conspicuous pale plagioclase phenocrysts and small vesicles; typically dull to slightly vitreous, hardness about 5–6, no obvious cleavage, specific gravity roughly 2.8–3.1. The rounded shape indicates water-worn transport.
Formation & geological history
Extrusive volcanic rock formed when mafic lava cooled rapidly at or near Earth’s surface; vesicles formed from escaping gas and later cavities may be filled by secondary minerals. Basalt forms from the Precambrian to the present, commonly in oceanic crust and volcanic fields.
Uses & applications
Used as aggregate, road stone, riprap, paving, and occasionally decorative stone; attractive specimens are collected but not generally used as gemstones.
Geological facts
The pale spots are most likely plagioclase crystals or mineral-filled vesicles, producing a porphyritic texture. Exact identification would benefit from a close-up, streak, density, and thin-section or chemical analysis.
Field identification & locations
Common in volcanic regions, oceanic islands, flood-basalt provinces, and river or beach gravels. Look for dark fine grain, high density, blocky white plagioclase, and vesicles; it is harder and denser than many sedimentary rocks.
