igneous
Porphyritic Basalt
Basalt (with plagioclase phenocrysts)
Hardness: 6 (Mohs); Color: Dark brown/grey matrix with white inclusions; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine grain) with visible phenocrysts; Cleavage: None visible.
- Hardness
- 6 (Mohs)
- Color
- Dark brown/grey matrix with white inclusions
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 (Mohs); Color: Dark brown/grey matrix with white inclusions; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine grain) with visible phenocrysts; Cleavage: None visible.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface. The white spots represent crystals (phenocrysts) that began cooling underground before the final eruption.
Uses & applications
Used primarily in construction (crushed stone), road base, and occasionally as pocket stones or river rocks for landscaping.
Geological facts
Basalt is common on Earth but also makes up most of the lunar mare on the Moon and the surface of Mars. It is the most common volcanic rock on Earth.
Field identification & locations
Identify by the contrast between a fine-grained dark matrix and larger crystalline 'spots'. Found commonly in volcanic regions, riverbeds, and glacial till.
