igneous
Basalt
Aphanitic basaltic rock
Hardness: 6 (Mohs scale); Color: dark gray to black; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- dark gray to black
- Luster
- dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: dark gray to black; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; 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. Basalt is the most common rock type in the Earth's crust, primarily making up the ocean floors.
Uses & applications
Widely used as an aggregate in construction (crushed stone), road base, concrete, and as dimensional stone for paving and flooring. It is also used as a material for heat-retaining sauna stones.
Geological facts
Basalt is common on other planetary bodies, covering vast areas of the Moon (the Lunar Maria), Mars, and Venus. It is the extrusive equivalent of gabbro.
Field identification & locations
Identifiable by its dark color, heavy weight compared to light-colored rocks, and fine-grained texture where individual crystals are too small to see with the naked eye. Often found in volcanic regions.
