igneous
Porphyritic Basalt
Basalt with plagioclase phenocrysts
Hardness: 6-7 (Mohs scale), Color: Dark grey to black groundmass with light-colored (white/cream) phenocrysts, Luster: Dull to sub-vitreous, Structure: Aphanitic matrix with visible crystals (porphyritic texture), Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark grey to black groundmass with light-colored (white/cream) phenocrysts, Luster: Dull to sub-vitreous, Structure: Aphanitic matrix with visible crystals (porphyritic texture), Specific Gravity: 2.8-3.0
Formation & geological history
Formed via rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface, where larger crystals (phenocrysts) began growing underground before the final eruption. Common in volcanic regions and ocean floor crust.
Uses & applications
Primarily used in construction as crushed stone, road base, or concrete aggregate. High-quality specimens are used in landscaping or as educational geological samples.
Geological facts
The light spots are called phenocrysts, usually consisting of plagioclase feldspar. This texture tells a story of two-stage cooling: slow cooling deep underground followed by rapid cooling during an eruption.
Field identification & locations
Identify by the 'salt and pepper' look where large distinct light crystals are embedded in a fine-grained dark background. Often found on beaches or in glacial till in regions with volcanic history.
