igneous
Porphyritic Basalt
Basalt (porphyritic texture)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with darker rounded/sub-rounded phenocryst inclusions; Luster: Dull to earthy; Texture: Fine-grained aphanitic matrix containing distinct phenocrysts (porphyritic).
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with darker rounded/sub-rounded phenocryst inclusions; Luster: Dull to earthy; Texture: Fine-grained aphanitic matrix containing distinct phenocrysts (porphyritic).
Formation & geological history
Formed from the rapid cooling of mafic lava at or near Earth's surface, where larger crystals (phenocrysts like pyroxene or olivine) grew slowly underground before the remaining liquid lava erupted and cooled quickly.
Uses & applications
Used as crushed stone in construction, road base, asphalt aggregate, and as a decorative stone or mineral specimen.
Geological facts
Porphyritic textures record a two-stage cooling history of magma: slow cooling at depth allowed large phenocrysts to form, followed by rapid cooling during eruption which formed the fine matrix.
Field identification & locations
Identified by dark color, fine-grained matrix with embedded distinct darker crystal aggregates or phenocrysts. Commonly found in volcanic regions, oceanic crust exposures, and river beds carrying basaltic pebbles.
