igneous
Basalt
Basalt
Hardness: 6 (Mohs); Color: Dark gray to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/conchoidal; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs)
- Color
- Dark gray to black
- Luster
- Dull to earthy
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Physical properties
Hardness: 6 (Mohs); Color: Dark gray to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/conchoidal; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. Most basalt is found on oceanic crust and around shield volcanoes. It can vary in age from modern to billions of years old.
Uses & applications
Extensively used in construction as an aggregate for roads, railroad ballast, and concrete. It is also used in high-quality floor tiles and as a raw material for stone wool insulation.
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. It often forms iconic hexagonal columns during cooling.
Field identification & locations
Identify in the field by its high density/weight, dark color, and fine texture with tiny or no visible crystals. Often has a weathered brown or tan exterior rind. Common in volcanic regions and ocean basins.
