igneous
Basalt
Mafic Igneous Basalt
Hardness: 6 on Mohs scale; Color: dark grey to black, often with rusty oxidation; Luster: dull to earthy; Crystal structure: fine-grained (aphanitic) due to rapid cooling; Specific gravity: 2.8-3.0.
- Hardness
- 6 on Mohs scale
- Color
- dark grey to black, often with rusty oxidation
- Luster
- dull to earthy
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Physical properties
Hardness: 6 on Mohs scale; Color: dark grey to black, often with rusty oxidation; Luster: dull to earthy; Crystal structure: fine-grained (aphanitic) due to rapid cooling; 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 and forms the majority of the ocean floor.
Uses & applications
Used widely in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality specimens can be used for stone carvings or as thermal insulation (stone wool).
Geological facts
Basalt covers approximately 70% of Earth's surface. It is also common on other terrestrial planets like Mars and Venus, and makes up the dark 'maria' plains on the Moon.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows 'conchoidal' fracture or columnar jointing. Found commonly in volcanic regions and ocean basins.
