Igneous
Basalt
Mafic Volcanic Rock (primarily Plagioclase and Pyroxene)
Hardness: 6.0 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0
- Hardness
- 6
- Color
- Dark gray to black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6.0 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron. They occur at divergent boundaries, oceanic hotspots, and mantle plumes. Most basalt currently on Earth is from the Cenozoic era, though it has formed throughout geological history.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used for building blocks, floor tiles, and in industrial 'cast basalt' for high-abrasion applications. Fine specimens are used in landscaping.
Geological facts
Basalt is the most common rock type in Earth's crust, covering most of the ocean floors. It is also found on the Moon (the dark lunar maria are basaltic) and Mars. It can form spectacular columnar jointing as it cools and shrinks.
Field identification & locations
Identify in the field by its heavy weight (dense), dark color, and fine-grained texture. It often shows small circular vesicles (air bubbles) or tiny white/green crystals. Common globally, especially in volcanic regions like Iceland, Hawaii, and the Pacific Northwest.
