igneous
Porphyritic Basalt
Porphyritic Basalt (with Plagioclase phenocrysts)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with light-colored (white/creamy) crystals; Luster: Dull/earthy matrix with vitreous crystals; Crystal Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: Distinct in phenocrysts; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull/earthy matrix with vitreous crystals
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with light-colored (white/creamy) crystals; Luster: Dull/earthy matrix with vitreous crystals; Crystal Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: Distinct in phenocrysts; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed through a two-stage cooling process of mafic lava: first, slow cooling deep underground allows large crystals (phenocrysts) to grow; second, a rapid volcanic eruption causes the remaining liquid to cool quickly into a fine-grained matrix. Found in volcanic regions of all geological ages, especially near mid-ocean ridges and hotspots.
Uses & applications
Used in construction as crushed stone or aggregate for roads and concrete. Often used as high-quality 'railroad ballast.' Higher-quality decorative specimens are used in landscaping and as educational geological samples for collectors.
Geological facts
The word 'basalt' is derived from a Latin word for 'very hard rock.' Basalt is the most common rock type on the Earth's surface and also makes up most of the surface of the Moon and Mars.
Field identification & locations
Identify in the field by looking for a 'salt and pepper' appearance where larger white crystals are embedded in a dark, dense, heavy rock. Common in Hawaii, Iceland, and the Pacific Northwest. Collectors should look for samples where the phenocrysts are sharp, well-defined, and show clear rectangular or tabular shapes.
