igneous
Basalt
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6.0 (Mohs); Color: Dark gray to black (often weathers to brown); Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0.
- Hardness
- 6
- Color
- Dark gray to black (often weathers to brown)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6.0 (Mohs); Color: Dark gray to black (often weathers to brown); Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Most specimens found on the surface date from the Cenozoic era, though basaltic crust exists from various geological ages.
Uses & applications
Extensively used as aggregate in construction (road base, railroad ballast, concrete), as a raw material for stone wool insulation, and occasionally in ground-form as a soil supplement/fertilizer.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up the majority of the ocean floor. It is also found in abundance on the Moon (the dark 'Maria' plains) and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (high density), and lack of visible crystals to the naked eye. Commonly found in volcanic fields or as river cobbles in areas with volcanic history.
