igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6-7 on Mohs scale. Color: Dark gray to black. Luster: Dull to sub-metallic. Crystal structure: Aphanitic (fine-grained), sometimes containing phenocrysts. Specific gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark gray to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark gray to black. Luster: Dull to sub-metallic. Crystal structure: Aphanitic (fine-grained), sometimes containing phenocrysts. Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic lava) at or near the Earth's surface. It is the most common volcanic rock on Earth and covers much of the ocean floor.
Uses & applications
Extensively used in construction for aggregate in concrete and asphalt, road base, railroad ballast, and sometimes as a dimension stone for flooring or monuments.
Geological facts
Basalt also composes much of the crust of the Moon, Mars, and Venus. On Earth, famous formations like the Giant's Causeway in Ireland consist of columnar basalt joints.
Field identification & locations
Identify in the field by its heavy weight for its size, dark color, and very fine grains that are often invisible to the naked eye. It commonly forms in volcanic regions and ocean ridges.
