igneous
Basalt
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark grey to black, sometimes greenish; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8-3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, sometimes greenish
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, sometimes greenish; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); 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. It is the most common rock type on Earth's crust, primarily produced at mid-ocean ridges.
Uses & applications
Used extensively in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. Also used as a raw material for stone wool insulation and occasionally in sculpture or outdoor landscaping.
Geological facts
Basalt is not only common on Earth but also covers vast areas of the Moon, Mars, and Venus. The 'Giant's Causeway' in Northern Ireland is a famous example of columnar basalt joints.
Field identification & locations
Identify by its heavy weight (high density), dark color, and fine-grained texture that lacks visible crystals to the naked eye. Found globally in volcanic regions and oceanic crust.
