igneous
Basalt
Extrusive Mafic Igneous Rock (composed primarily of plagioclase, pyroxene, and olivine)
Hardness: 6 on the Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0.
- Hardness
- 6 on the Mohs scale
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on the Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; 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. It is the most common volcanic rock and found across all geological ages, especially in oceanic crust and large volcanic provinces.
Uses & applications
Extensively used in construction as aggregate for roads, railroad ballast, and concrete; used in industrial manufacturing for rock wool insulation; occasionally used for decorative stonework and massage stones.
Geological facts
Basalt comprises more than 90% of all volcanic rock on Earth. It is also found on the Moon, Mars, and Venus, with the lunar 'maria' or dark plains being comprised of ancient basaltic lava flows.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and lack of visible large crystal grains. It often exhibits columnar jointing in large formations. Found globally in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.
