igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 5-6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Most common rock type on Earth's surface, particularly on the ocean floor.
Uses & applications
Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Also used for building stone, pavers, and as decorative landscaping rock.
Geological facts
Basalt makes up most of the ocean floors and is also found on the Moon, Mars, and Venus. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt joints formed by cooling contraction.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and very fine-grained texture that lacks visible crystals to the naked eye. Found worldwide in volcanic regions like Hawaii, the Pacific Northwest (Columbia River Basalts), and Iceland.
