igneous
Basalt
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown or rust; Luster: Dull to sub-metallic when fresh; Structure: Aphanitic (fine-grained), often vesicular or amygdaloidal; Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to brown or rust
- Luster
- Dull to sub-metallic when fresh
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown or rust; Luster: Dull to sub-metallic when fresh; Structure: Aphanitic (fine-grained), often vesicular or amygdaloidal; Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava at or very near the Earth's surface. Found in volcanic provinces, oceanic crust, and plate boundaries.
Uses & applications
Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Also used as a raw material for stone wool insulation and occasionally for carving.
Geological facts
Basalt is the most common rock on Earth's surface, making up most of the ocean floor. It is also found on the Moon, Mars, and Venus. On Earth, it can form unique hexagonal columns during cooling, such as at the Giant's Causeway.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and very fine-grained texture. It often shows small holes (vesicles) from gas bubbles. Common in volcanic regions and near seismic rifts.
