igneous
Basalt
Basalt (extrusive volcanic mafic rock)
Hardness: 6 (Mohs scale); Color: Dark grey, black, or dark green; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of plagioclase, pyroxene, and olivine; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey, black, or dark green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey, black, or dark green; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of plagioclase, pyroxene, and olivine; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. It is the primary component of oceanic crust and can be found in volcanic archipelagos and large igneous provinces.
Uses & applications
Extensively used in construction as aggregate for roads, railway ballast, and in the production of high-strength concrete. Fine specimens are used in stone masonry or as garden accents. It can also be melted into basalt fiber for insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust and it also makes up most of the surface of the Moon (the lunar maria) and Mars.
Field identification & locations
Identify in the field by its dark weight, fine-grained texture, and lack of visible quartz. It is commonly found in areas of past or present volcanic activity, such as the Pacific Northwest or Hawaii. Collectors should look for vesicles (gas bubbles) or prismatic jointing.
