igneous
Porphyritic Basalt (Porphyry)
Porphyritic Basalt
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with light-colored (white/grey) phenocrysts; Luster: Dull or earthy; Crystal structure: Aphanitic groundmass with subhedral to euhedral phenocrysts; Specific gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull or earthy
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with light-colored (white/grey) phenocrysts; Luster: Dull or earthy; Crystal structure: Aphanitic groundmass with subhedral to euhedral phenocrysts; Specific gravity: 2.8-3.0.
Formation & geological history
Formed from volcanic activity where magma cooled in two stages: initially cooling slowly underground to form large crystals (phenocrysts), then erupting and cooling rapidly at the surface to form the fine-grained matrix. Often found in continental volcanic arcs and tectonic rift zones.
Uses & applications
Used primarily as crushed stone for aggregate in construction and road building. Select attractive specimens are used as landscaping stones or by collectors of volcanic petrology.
Geological facts
This specific texture is called 'porphyritic.' The specimen shown has been naturally smoothed and rounded, likely by water action in a river or coastal environment, creating a 'river rock' appearance.
Field identification & locations
Identify by looking for distinct, larger visible crystals (white dots) embedded in a very fine-grained, dark-colored rock. Common in the Pacific Northwest, basaltic plateaus, and volcanic islands.
