igneous
Basalt
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of olivine or pyroxene; Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull to earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of olivine or pyroxene; Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Basalt is the most common rock in Earth's crust, forming the bulk of the ocean floor and volcanic islands.
Uses & applications
Extensively used in construction for crushed stone, road base, concrete aggregate, and railroad ballast. Some high-density varieties are used in 'stone wool' insulation or as polished floor tiles.
Geological facts
Basalt covers more than 70% of Earth's surface and also makes up the vast majority of the lunar maria on the Moon. It is the primary rock type produced at mid-ocean ridges.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and fine-grained texture. It often breaks into angular fragments. Commonly found in volcanic regions, oceanic basin edges, and as rounded river rocks in glacial deposits.
