igneous
Basalt
Extrusive mafic igneous rock
Hardness: 6.0 (Mohs); Color: Dark gray to black (weathering to tan/brown); Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
- Hardness
- 6
- Color
- Dark gray to black (weathering to tan/brown)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6.0 (Mohs); Color: Dark gray to black (weathering to tan/brown); 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 at the surface of a planet. Most common rock in Earth's crust, found in oceanic crust and continental flood basalts.
Uses & applications
Widely used as aggregate in construction, road base, concrete, and railroad ballast. Some varieties are used for stone sculpture and heat-retaining stones in spas.
Geological facts
Basalt is the most common rock type on the Moon's surface and the Martian surface. It often forms striking hexagonal columns during cooling, such as at the Giant's Causeway.
Field identification & locations
Identify by its heavy weight, dark interior, and fine-grained texture that lacks visible crystals. Commonly found in volcanic regions and along oceanic ridges.
