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; Cleavage: None (conchoidal fracture); 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; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Basalt is found across all geological ages and makes up most of the ocean floor.
Uses & applications
Extensively used in construction as aggregate for roads, concrete, and railroad ballast. In industry, it is melted to create stone wool insulation. High-quality dense specimens are used for pavers and sculpture.
Geological facts
Basalt is the most common rock type in the Earth's crust. It is also prevalent on other planetary bodies, including the Moon (forming the dark 'maria' or seas), Mars, and Venus.
Field identification & locations
Identify by its dark, heavy feel and fine-grained texture. To identify in the field, look for a dark, uniform surface that doesn't reveal individual crystals to the naked eye; it may show tiny vesicles (gas bubble holes) or a weathered brown outer crust.
