igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None; 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 in oceanic crust and volcanic regions.
Uses & applications
Used primarily in construction for crushed stone, road base, concrete aggregate, and railroad ballast. Some high-density basalt is used for insulation (rock wool).
Geological facts
Basalt is the most common rock type in the Earth's crust and covers most of the ocean floor. It is also found on the Moon, Mars, and Venus.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture. It often shows vesicular (bubbly) textures if gases were trapped during cooling. Found globally in volcanic fields.
