igneous
Basalt with Feldspar phenocrysts
Aphanitic to Porphyritic Mafic Volcanic Rock
Hardness: 6 (Mohs scale), Color: Dark grey to black with white crystal inclusions, Luster: Dull/Earthy (matrix) to Vitreous (crystals), Crystal Structure: Fine-grained matrix with visible plagioclase crystals, Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black with white crystal inclusions, Luster: Dull/Earthy (matrix) to Vitreous (crystals), Crystal Structure: Fine-grained matrix with visible plagioclase crystals, Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. These rocks are common in oceanic crust and volcanic island arcs. This specimen shows small phenocrysts (likely plagioclase feldspar) that began cooling slowly underground before the final eruption.
Uses & applications
Commonly used as crushed stone for construction, road base, railroad ballast, and in the production of high-quality concrete. Dense varieties are used for shore protection (riprap).
Geological facts
Basalt is the most common rock type in the Earth's crust and also covers vast areas of the Moon, Mars, and Venus. It is the primary component of the oceanic floor.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. Found globally in volcanic regions like Iceland, Hawaii, and the Pacific Northwest. Localities with white inclusions (porphyritic texture) suggest a multi-stage cooling history.
