igneous
Basalt
Basalt (extrusive volcanic rock)
Hardness: 5-6 (Mohs); Color: Dark grey to black, weathering to brown or red; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); Specific gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black, weathering to brown or red
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black, weathering to brown or red; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); Specific gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava exposed at or very near the Earth's surface. It comprises over 90% of all volcanic rock on Earth. Occurs in various geological ages from Precambrian to recent eruptions.
Uses & applications
Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality basalt is used in the manufacturing of stone wool insulation and as polished dimension stone for tiles and monuments.
Geological facts
Basalt is the most common rock in the Earth's crust and also makes up most of the ocean floors. It is a major component of the lunar maria on the Moon and is found on Mars and Venus. Giant's Causeway in Northern Ireland is a famous example of columnar basalt.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture where individual crystals are too small to see with the naked eye. Often found in volcanic regions, oceanic islands (like Hawaii), and large 'flood basalt' provinces.
