igneous
Porphyritic Basalt
Porphyritic Basalt (with Plagioclase phenocrysts)
Hardness: 6-7 on Mohs scale. Color: Dark grey to black matrix with white or light grey phenocrysts. Luster: Dull to sub-vitreous. Texture: Porphyritic (fine-grained matrix with larger visible crystals). Cleavage: Generally none in the rock mass, though phenocrysts may show it.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to black matrix with white or light grey phenocrysts. Luster: Dull to sub-vitreous. Texture: Porphyritic (fine-grained matrix with larger visible crystals). Cleavage: Generally none in the rock mass, though phenocrysts may show it.
Formation & geological history
Formed from the rapid cooling of basaltic lava at or near the surface, allowing some crystals (phenocrysts) to grow larger underground before the final eruption and solidification of the fine-grained matrix.
Uses & applications
Used in construction as crushed stone or 'road metal', aggregate for concrete and asphalt, and occasionally in garden landscaping or as decorative stone.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple, as the first specimens described by Romans were a distinctive purple variety of this rock type. This specific specimen shows 'salt and pepper' aesthetics due to the plagioclase crystals.
Field identification & locations
Identify by looking for a dark, dense matrix containing larger, lighter-colored rectangular or lath-shaped crystals. It is commonly found in volcanic regions, such as the Pacific Northwest, Hawaii, or Iceland.
