igneous
Basalt
Extrusive igneous rock
Hardness: 6 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Cleavage: None; Specific Gravity: 2.8 - 3.0
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Cleavage: None; Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a planet or moon. It is the most common rock type in the Earth's oceanic crust.
Uses & applications
Used extensively in construction (crushed stone for road base, concrete aggregate), railroad ballast, and in the manufacturing of rock wool insulation and basalt fiber.
Geological facts
Basalt is not only the most common rock type on Earth's crust, but it also makes up most of the surface of the Moon (forming the dark lunar maria) and Mars.
Field identification & locations
Identify by its dark, fine-grained appearance, heavy weight relative to size, and occasional vesicular textures (gas cavities). Found globally in volcanic provinces, oceanic islands, and flood basalt plateaus.
