igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic (if fresh); Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull to sub-metallic (if fresh)
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic (if fresh); Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Specific gravity: 2.8–3.0
Formation & geological history
Forms from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. It is the most common volcanic rock and forms oceanic crust.
Uses & applications
Primarily used as aggregate in construction, road base, concrete, and railroad ballast. Denser specimens are used for floor tiles and ornamental stone.
Geological facts
Basalt covers most of the ocean floor and is also found on the Moon and Mars. It forms unique hexagonal columns when cooling, known as columnar jointing.
Field identification & locations
Identify by its dark color, heavy weight compared to other common rocks, and very fine-grained texture. It is ubiquitous in volcanic regions like Hawaii, the Pacific Northwest, and Iceland.
