igneous
Basalt (Road Aggregate / Crushed Stone)
Aphanitic Mafic Igneous Rock
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal/black; Luster: Dull to sub-metallic when fresh; Crystal structure: Fine-grained/Aphanitic (not visible to naked eye); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal/black
- Luster
- Dull to sub-metallic when fresh
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal/black; Luster: Dull to sub-metallic when fresh; Crystal structure: Fine-grained/Aphanitic (not visible to naked eye); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Found in volcanic provinces and oceanic tectonic plates.
Uses & applications
Used primarily as road base, railroad ballast, and concrete aggregate due to its high compressive strength and durability. Also used for pavers and floor tiles.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It also exists on the Moon and Mars in the form of 'Maria' (seas).
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. Found abundantly in the Pacific Northwest, Hawaii, and Iceland.
