igneous
Porphyritic Basalt
Porphyritic Basalt (containing Plagioclase phenocrysts)
Hardness: 6 (groundmass) to 6.5 (crystals); Color: dark brown/black matrix with white flecks; Luster: dull/earthy groundmass with vitreous crystals; Structure: Porphyritic texture; Cleavage: None in rock; Specific Gravity: 2.8-3.0.
- Hardness
- 6 (groundmass) to 6
- Color
- dark brown/black matrix with white flecks
- Luster
- dull/earthy groundmass with vitreous crystals
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Physical properties
Hardness: 6 (groundmass) to 6.5 (crystals); Color: dark brown/black matrix with white flecks; Luster: dull/earthy groundmass with vitreous crystals; Structure: Porphyritic texture; Cleavage: None in rock; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of basaltic lava at or near the surface, while the larger crystals (phenocrysts) formed deeper underground beforehand during a slower cooling phase. Usually associated with volcanic shield or rift zones.
Uses & applications
Primarily used as decorative stone, aquarium rocks, landscaping, or as crushed aggregate in road construction and concrete.
Geological facts
The 'polka dot' appearance is caused by two distinct stages of cooling, a process known as fractional crystallization. Most basalt is found on the ocean floor, but porphyritic varieties are common in continental volcanic fields.
Field identification & locations
Identify by looking for a fine-grained, dark-colored matrix containing distinct, visible light-colored crystals (phenocrysts). Commonly found in volcanic regions like the Pacific Northwest, Hawaii, or Iceland.
