igneous
Porphyritic Basalt
Porphyritic Basalt (Mafic Volcanic Rock)
Hardness: 6 (Mohs). Color: Dark grey to black groundmass with light-colored phenocrysts. Luster: Dull to sub-vitreous. Structure: Aphanitic matrix with visible crystals (porphyritic texture). Cleavage: None (conchoidal fracture).
- Hardness
- 6 (Mohs)
- Color
- Dark grey to black groundmass with light-colored phenocrysts
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 (Mohs). Color: Dark grey to black groundmass with light-colored phenocrysts. Luster: Dull to sub-vitreous. Structure: Aphanitic matrix with visible crystals (porphyritic texture). Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the rapid cooling of magnesium and iron-rich lava on the surface, where larger crystals (phenocrysts) formed earlier in the magma chamber before the final eruption. Found in volcanic regions.
Uses & applications
Commonly used in construction as road aggregate, railroad ballast, and occasionally as decorative stone or in landscaping (river rocks).
Geological facts
The lighter spots are called phenocrysts, typically consisting of plagioclase feldspar. This texture tells a two-stage cooling story: slow cooling underground followed by a sudden eruption and rapid cooling.
Field identification & locations
Identify in the field by the 'salt and pepper' look on a dark, heavy base. Common near ancient volcanic sites or transported as glacial till or river stones.
