igneous
Basalt
Basalt (Mafic volcanic rock)
Color: Dark gray to black; Hardness: 6 on Mohs scale; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.9-3.1; Cleavage: None (conchoidal fracture)
- Hardness
- 6 on Mohs scale
- Color
- Dark gray to black
- Luster
- Dull to sub-metallic
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Physical properties
Color: Dark gray to black; Hardness: 6 on Mohs scale; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.9-3.1; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Common in oceanic crust and large volcanic provinces. Can range from recent formations to billions of years old.
Uses & applications
Extensively used in construction for aggregate, road base, concrete, and railroad ballast. Also used in high-temp insulation (rock wool) and occasionally for pavers or sculptures.
Geological facts
Basalt is the most common rock type in the Earth's crust and covers most of the ocean floor. It is also found on the Moon, Mars, and Venus. Unique hexagonal column formations can occur during cooling, such as the Giant's Causeway.
Field identification & locations
Identify by its dark color, heavy weight (density), and lack of visible crystals to the naked eye. Look for tiny holes (vesicles) or a uniform matte dark gray surface. Found globally in volcanic regions.
