igneous
Basalt
Basalt (Mafic Extrusive)
Hardness: 5-6 (Mohs scale); Color: Dark gray, black, or dark brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often contains microcrystals of plagioclase and pyroxene; Cleavage: None (conchoidal fracture); Specific gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark gray, black, or dark brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark gray, black, or dark brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often contains microcrystals of plagioclase and pyroxene; 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. It is the most common volcanic rock type on Earth, found at mid-ocean ridges and hotspots.
Uses & applications
Extensively used in construction as aggregate for roads and railways, in concrete and asphalt, and for stone wool insulation. High-quality specimens with olivine inclusions may be used as ornamental stones.
Geological facts
Basalt is the primary rock of the Earth's oceanic crust. It also makes up the 'maria' (dark plains) on the moon and is the most common rock on the surface of Mars and Venus.
Field identification & locations
Identify in the field by its high density, dark color, and fine-grained texture with no visible crystals to the naked eye. Often exhibits columnar jointing in large formations. Found globally in volcanic regions like Iceland, Hawaii, and the Deccan Traps.
