igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6.0 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0. This specimen appears weathered and water-worn.
- Hardness
- 6
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6.0 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0. This specimen appears weathered and water-worn.
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 basalt flows are from the Cenozoic era or younger, though they occur throughout geological history.
Uses & applications
Used primarily in construction (crushed stone for road base, concrete aggregate, and railroad ballast), as thermal insulation in the form of stone wool, and occasionally for high-durability floor tiles or decorative landscaping.
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. The Giant's Causeway in Northern Ireland is a famous example of columnar basalt joints.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows small vesicles (gas bubbles) or olivine crystals. Commonly found in volcanic regions, near oceanic ridges, and as river-worn cobbles.
