igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6-7 on Mohs scale; Color: Black, dark gray, or dark green; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0
- Hardness
- 6-7 on Mohs scale
- Color
- Black, dark gray, or dark green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Black, dark gray, or dark green; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None (conchoidal fracture); 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 surface of a terrestrial planet or a moon. It is the most common rock type in the Earth's crust, primarily making up the ocean floors.
Uses & applications
Used extensively in construction as aggregate for concrete and asphalt, as road base, railroad ballast, and sometimes in high-quality floor tiles or as dimension stone for buildings and monuments.
Geological facts
Basalt is equivalent in composition to gabbro, but gabbro is intrusive (plutonic) while basalt is extrusive (volcanic). It also covers massive areas on Mars, Venus, and the Moon (the lunar maria).
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often weathers to a reddish-brown color due to iron oxidation. Found in volcanic regions and oceanic crust.
