igneous
Basalt
Mafic volcanic rock (formula mostly SiO2, MgO, FeO)
Hardness: 6 (Mohs); Color: Dark grey to black; Luster: Dull to submetallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
- Hardness
- 6 (Mohs)
- Color
- Dark grey to black
- Luster
- Dull to submetallic
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to black; Luster: Dull to submetallic; 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 Earth's surface. Found in oceanic crust and volcanic islands.
Uses & applications
Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Also used for fiber insulation and stone carvings.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large-scale cooling often creates hexagonal columns.
Field identification & locations
Look for a dark, heavy, fine-grained rock with occasional small vesicles (holes) or green olivine crystals. Found globally in tectonic rift zones.
