igneous
Basalt
Mafic Volcanic Rock (composed mainly of Plagioclase, Pyroxene, and Olivine)
Hardness: 6 (Mohs scale); Color: Dark gray to black (mafic); Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (aphanitic) or porphyritic; Cleavage: None (conchoidal fracture common); Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black (mafic)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black (mafic); Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (aphanitic) or porphyritic; Cleavage: None (conchoidal fracture common); Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface of a terrestrial planet. It covers most of the Earth's ocean floor and is commonly associated with shield volcanoes and oceanic hotspots.
Uses & applications
Extensively used in construction as aggregate for roads and concrete, railway ballast, and in the production of stone wool insulation. It is occasionally used in landscaping and as a base stone for masonry.
Geological facts
Basalt is the most common volcanic rock in the solar system, making up the vast majority of 'Mars' crust and the 'Maria' (dark plains) on the moon. It can form spectacular hexagonal columns during cooling, such as at the Giant's Causeway.
Field identification & locations
Identify by its dark, heavy, and fine-grained texture. It often features small vesicles (gas bubbles) or larger crystals (phenocrysts) embedded in a dark matrix. Common in the Pacific Northwest (Columbia River Basalt Group) and volcanic islands like Hawaii.
