igneous
Basalt
Extrusive Aphanitic Mafic Rock
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
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 on Earth's crust, primarily oceanic crust.
Uses & applications
Used in construction as crushed stone for road base, railroad ballast, and concrete aggregate; also used in stone wool insulation and occasionally for landscaping.
Geological facts
Basalt is the most common volcanic rock on Earth and also makes up most of the surface of the Moon, Mars, and Venus. On Earth, giant columnar structures can form during cooling, like the Giant's Causeway.
Field identification & locations
Identify by its dark color, heavy weight compared to other rocks, and fine-grained texture that lacks visible crystals to the naked eye. Commonly found in volcanic regions and ocean basin shorelines.
