igneous
Porphyritic Basalt (with Weathered Jointing)
Porphyritic Basalt
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with white phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic groundmass with macroscopic crystals; SG: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal with white phenocrysts
- Luster
- Dull/Earthy
Identified More igneous →
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with white phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic groundmass with macroscopic crystals; SG: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface, where larger crystals (phenocrysts) grew before the main eruption.
Uses & applications
Commonly used as crushed stone for aggregate in construction, road base, and railroad ballast; sometimes used as decorative garden stones.
Geological facts
The unique 'cross' or 'face' pattern on this specimen is likely due to differential weathering along orthogonal joint planes (cracks) that formed as the lava cooled and contracted.
Field identification & locations
Identify in the field by its dark, dense matrix and visible 'spots' (phenocrysts). Look for it in volcanic fields, oceanic crust exposures, and riverbeds near volcanic activity.
