igneous
Basalt with Plagioclase Phenocrysts
Porphyritic Basalt
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with light cream/white inclusions, Luster: Dull to sub-metallic on matrix, vitreous on phenocrysts, Structure: Aphanitic with porphyritic texture, Cleavage: None/Irregular
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with light cream/white inclusions, Luster: Dull to sub-metallic on matrix, vitreous on phenocrysts, Structure: Aphanitic with porphyritic texture, Cleavage: None/Irregular
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The larger crystals (phenocrysts) formed slowly at depth before the lava erupted.
Uses & applications
Commonly used as aggregate in construction, road base, and railroad ballast; smaller water-worn stones are used in landscaping and as decorative 'river rocks'.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Porphyritic textures tell a 'two-stage' cooling story of the magma's history.
Field identification & locations
Identify by its heavy weight (high density), dark fine-grained background, and visible larger mineral crystals. Often found in volcanic fields or as tumbled pebbles in riverbeds and beaches.
