igneous
Porphyritic Basalt
Porphyritic Basalt (containing Plagioclase phenocrysts)
Hardness: 6-7 (Mohs); Color: Dark gray to black groundmass with white/tan rectangular crystals; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with large visible crystals); Specific gravity: 2.8-3.0.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark gray to black groundmass with white/tan rectangular crystals; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with large visible crystals); Specific gravity: 2.8-3.0.
Formation & geological history
Formed from volcanic activity where magma begins cooling slowly underground (creating the large crystals) and is then erupted, cooling rapidly on the surface. Common in oceanic crust and volcanic island arcs.
Uses & applications
Primarily used as crushed stone for construction, road base, and concrete aggregate. Attractive specimens are used in landscaping or as educational geological samples.
Geological facts
The large crystals transition from a liquid to a solid state first, which is why they are so much larger than the surrounding matrix. This texture tells a 'two-stage' cooling story of the volcano.
Field identification & locations
Identify by the 'salt and pepper' look of light rectangular crystals (phenocrysts) embedded in a dark, fine-grained rock. Look for it in areas with historical volcanic activity.
