Basalt with Plagioclase Phenocrysts

Physical Properties

Hardness: 6-7 (Mohs scale), Color: Dark grey to black matrix with white or light-pink crystalline inclusions (phenocrysts), Luster: Dull to sub-vitreous, Crystal structure: Fine-grained aphanitic matrix with larger visible crystals, Cleavage: Generally poor or none, 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 larger crystals (phenocrysts) formed slowly underground before the lava was erupted. Most are from the Cenozoic era, though occurrences span all geological ages.

Uses & Applications

Used primarily in construction for crushed stone, aggregate in concrete and asphalt, and sometimes as a dimension stone for architectural accents.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also common on the Moon, Mars, and Venus.

Field Identification & Locations

Identify in the field by its dark, heavy nature and the presence of small, light-colored rectangular crystals within a dark mass. Found in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.

Identified on: 4/30/2026

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