igneous
Porphyritic Basalt
Aphanitic-Porphyritic Basalt
Hardness: 6 (Mohs); Color: Dark grey to black matrix with white phenocrysts; Luster: Dull or earthy matrix, vitreous phenocrysts; Texture: Porphyritic (fine-grained matrix with larger crystals); Specific gravity: 2.8-3.0
- Hardness
- 6 (Mohs)
- Color
- Dark grey to black matrix with white phenocrysts
- Luster
- Dull or earthy matrix, vitreous phenocrysts
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to black matrix with white phenocrysts; Luster: Dull or earthy matrix, vitreous phenocrysts; Texture: Porphyritic (fine-grained matrix with larger crystals); Specific gravity: 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. The large white crystals (phenocrysts), likely plagioclase feldspar, formed slowly at depth before the final eruption.
Uses & applications
Commonly used as crushed stone for road construction, railroad ballast, and concrete aggregate. Select specimens are used for 'river rock' landscaping.
Geological facts
Basalt is the most common rock type on Earth's crust and also makes up most of the ocean floor. It is similar to rocks found on the Moon and Mars.
Field identification & locations
Identify in the field by its dark, dense matrix and visible 'speckled' crystals. Common in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland.
