igneous
Basalt
Basalt
Hardness: 6 (Mohs); Color: dark gray, black, or dark greenish; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic), sometimes vesicular; Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs)
- Color
- dark gray, black, or dark greenish
- Luster
- dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs); Color: dark gray, black, or dark greenish; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic), sometimes vesicular; 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. Common throughout Earth's history, especially in oceanic crust and volcanic islands.
Uses & applications
Extensively used in construction for road base, concrete aggregate, railroad ballast, and sometimes as a decorative stone or dimension stone in architecture.
Geological facts
Basalt is the most common rock type in Earth's crust and also makes up most of the surfaces of the Moon, Venus, and Mars. Many basalt flows exhibit columnar jointing like the Giant's Causeway.
Field identification & locations
Identify by its heavy weight (high density), dark color, and fine-grained texture with no visible large crystals. Commonly found in coastal areas, volcanic ranges, and ocean basins.
