igneous
Basalt
Extrusive mafic igneous rock
Hardness: 5-6 (Mohs scale); Color: dark grey to black; Luster: dull to sub-metallic; Structure: fine-grained (aphanitic) sometimes with visible phenocrysts; Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- dark grey to black
- Luster
- dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs scale); Color: dark grey to black; Luster: dull to sub-metallic; Structure: fine-grained (aphanitic) sometimes with visible phenocrysts; 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 surface of a terrestrial planet or moon. Most basalt on Earth formed during the Cenozoic era, though it exists across all geological ages.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, railroad ballast, and for high-durability floor tiles.
Geological facts
Basalt is the most common rock type in the Earth’s crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows a weathered brown exterior but is dark grey inside. Common in volcanic regions such as the Pacific Northwest, Hawaii, and Iceland.
