igneous
Basalt
Basalt (Extrusive Igneous Rock)
Hardness: 5-6 (Mohs); Color: Dark grey to black, often weathering to brown/greenish; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black, often weathering to brown/greenish
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black, often weathering to brown/greenish; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); 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 basalt flows are from the Cenozoic era, but can date back billions of years.
Uses & applications
Extensively used in construction (crushed stone for road base and concrete aggregate), high-quality railroad ballast, and sometimes in industrial casting and stone wool insulation.
Geological facts
Basalt is the most common rock on Earth's surface and makes up most of the ocean floor. It is also found extensively on the Moon and Mars (the dark plains of the Moon are basaltic).
Field identification & locations
Identify by its heavy weight (high density), dark color, and fine-grained texture. It often shows vesicular (bubbly) textures or columnar jointing in large outcrops. Common in volcanic regions like Iceland, Hawaii, and the Pacific Northwest.
