igneous
Basalt
Basalt (extrusive volcanic rock)
Hardness: 6 on Mohs scale; Color: dark grey to black; Luster: dull to earthy; Crystal structure: aphanitic (fine-grained); Specific gravity: 2.8-3.0; often contains small phenocrysts of olivine or pyroxene.
- Hardness
- 6 on Mohs scale
- Color
- dark grey to black
- Luster
- dull to earthy
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Physical properties
Hardness: 6 on Mohs scale; Color: dark grey to black; Luster: dull to earthy; Crystal structure: aphanitic (fine-grained); Specific gravity: 2.8-3.0; often contains small phenocrysts of olivine or pyroxene.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava (mafic lava) at or very near the Earth's surface. Found in oceanic crust, volcanic islands, and continental flood basalts.
Uses & applications
Extensively used in construction as an aggregate for roads, railroad ballast, and concrete. It is also used as a dimension stone for floor tiles and monuments.
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 common on the Moon, Mars, and Venus.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and very fine-grained texture. It usually doesn't show visible crystals to the naked eye. Look for 'vesicles' (small holes) or a solid 'massive' structure.
