igneous
Basalt with Plagioclase Phenocrysts
Porphyritic Basalt
Hardness: 6 (Mohs scale), Color: Dark grey to charcoal black (groundmass) with greyish-white laths (phenocrysts), Luster: Dull or earthy when weathered, sub-metallic when fresh, Structure: Aphanitic (fine-grained) groundmass with porphyritic texture, Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to charcoal black (groundmass) with greyish-white laths (phenocrysts), Luster: Dull or earthy when weathered, sub-metallic when fresh, Structure: Aphanitic (fine-grained) groundmass with porphyritic texture, Specific Gravity: 2.8-3.0
Formation & geological history
Formed through rapid cooling of mafic lava at or near the Earth's surface, where a two-stage cooling process allowed larger crystals (phenocrysts) to form before the rest of the magma solidified quickly. Most common in oceanic crust and volcanic hotspots.
Uses & applications
Used primarily as construction aggregate, road base material, and railroad ballast. Due to its durability, it is also used in high-quality asphalt and concrete. Polished porphyritic varieties are sometimes used in decorative stonework.
Geological facts
Basalt is the most common rock type in the Earth's crust and even makes up the majority of the surface of the Moon and Mars. It is also the main rock forming the dark 'seas' or maria seen on the Moon's surface.
Field identification & locations
Identified by its dark, dense, fine-grained matrix containing visible larger crystal inclusions. This specific specimen is river-tumbled or beach-worn, giving it a smooth, rounded shape. It is commonly found in volcanic regions such as the Pacific Northwest, Hawaii, or Iceland.
