igneous
Basalt
Mafic extrusive igneous rock
Hardness: 6.0; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0; Magnetism: Weakly magnetic due to magnetite content.
- Hardness
- 6
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6.0; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0; Magnetism: Weakly magnetic due to magnetite content.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Most common rock in Earth's crust.
Uses & applications
Used extensively in construction for aggregate, road base, concrete, and railroad ballast. Also used as stone wool insulation and occasionally in sculpture.
Geological facts
Basalt is the primary rock type on the seafloor and also makes up the lunar maria on the Moon. It creates iconic columnar joints like those found at Giant's Causeway.
Field identification & locations
Identify by its heavy weight (high density), dark color, and fine-grained texture. It often shows vesicules (small holes) if gas was trapped during cooling. Found worldwide in volcanic regions.
