igneous
Porphyritic Basalt
Extrusive igneous rock with plagioclase phenocrysts
Hardness of 6 on the Mohs scale. Dark gray to black groundmass with lighter greenish or white plagioclase phenocrysts (spots). Dull to sub-vitreous luster, fine-grained aphanitic matrix with visible larger crystals (porphyritic texture). Specific gravity is around 2.8 - 3.0.
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Physical properties
Hardness of 6 on the Mohs scale. Dark gray to black groundmass with lighter greenish or white plagioclase phenocrysts (spots). Dull to sub-vitreous luster, fine-grained aphanitic matrix with visible larger crystals (porphyritic texture). Specific gravity is around 2.8 - 3.0.
Formation & geological history
Formed by the rapid cooling of magnesium- and iron-rich lava at or very near the Earth's surface, often in volcanic environments.
Uses & applications
Used in construction, as crushed stone for roadbeds, aggregate in concrete, and sometimes polished as decorative landscape stones or lapidary material.
Geological facts
The spotted appearance is caused by a two-stage cooling process: initial slow cooling deep underground allowed large crystals (phenocrysts) to grow, followed by a rapid eruption and cooling at the surface that froze the finer-grained groundmass around them.
Field identification & locations
Commonly found in volcanic regions, beaches, and river beds where volcanic rocks have been weathered and rounded by water. Easily identified by its dark base contrasted with distinct lighter spots or crystals.
