igneous
Basalt (Porphyritic)
Porphyritic Mafic Extrusive (Basalt)
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal grey; Luster: Dull/Earthy (matrix) to Sub-vitreous (phenocrysts); Crystal structure: Aphanitic (fine-grained) matrix with visible phenocrysts; Cleavage: Poorly developed; Specific Gravity: 2.8-3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal grey
- Luster
- Dull/Earthy (matrix) to Sub-vitreous (phenocrysts)
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal grey; Luster: Dull/Earthy (matrix) to Sub-vitreous (phenocrysts); Crystal structure: Aphanitic (fine-grained) matrix with visible phenocrysts; Cleavage: Poorly developed; Specific Gravity: 2.8-3.0.
Formation & geological history
Basalt is formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Porphyritic texture indicates a two-stage cooling process: a slow cooling phase deep underground forming larger crystals, followed by an eruptive phase that cools the remaining melt quickly.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Larger slabs are used for floor tiles, paving stones, and monuments.
Geological facts
Basalt is the most common rock in the Earth's crust and makes up most of the ocean floor. It is also found on other planetary bodies like Mars and the Moon (the dark 'seas' or Maria are basaltic).
Field identification & locations
Identify in the field by its high density, dark grey color, and the presence of small, dark, or greenish crystals (phenocrysts) embedded in a fine-grained background. It is ubiquitous in volcanic regions such as the Pacific Northwest of the USA, Iceland, and Hawaii.
