igneous
Porphyritic basalt (vesicular basalt)
Basalt, commonly plagioclase-phyric; dark matrix of plagioclase, pyroxene, and possibly olivine
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; dark gray to black, fine-grained, with pale pink-white plagioclase phenocrysts and small vesicles. Typically has a dull to weakly vitreous luster and no prominent cleavage as a whole rock.
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Physical properties
Hardness about 5.5–6.5 Mohs; dark gray to black, fine-grained, with pale pink-white plagioclase phenocrysts and small vesicles. Typically has a dull to weakly vitreous luster and no prominent cleavage as a whole rock.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled rapidly at or near Earth’s surface; individual specimens may range from recent volcanic material to hundreds of millions of years old. The holes are gas vesicles, later partly filled or rimmed by secondary minerals.
Uses & applications
Used as aggregate, roadstone, riprap, and occasionally dimension stone; attractive pieces are collected but are generally low-value.
Geological facts
The pale crystals are likely plagioclase feldspar, while the rounded cavities record trapped volcanic gas. Its texture is called porphyritic because larger crystals formed before the remaining lava solidified.
Field identification & locations
Identify it by its dense black fine-grained groundmass, scattered light feldspar crystals, and vesicles; a hand lens and streak, hardness, or acid tests can help. Common in volcanic regions worldwide, including Iceland, Hawaii, Japan, and the Deccan Traps.
