igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 5-6 Mohs. Color: Dark gray to black. Luster: Dull to vitreous when wet or polished. Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of olivine or pyroxene. Specific Gravity: 2.8–3.0.
- Hardness
- 5-6 Mohs
- Color
- Dark gray to black
- Luster
- Dull to vitreous when wet or polished
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Physical properties
Hardness: 5-6 Mohs. Color: Dark gray to black. Luster: Dull to vitreous when wet or polished. Crystal structure: Aphanitic (fine-grained), often containing phenocrysts of olivine or pyroxene. Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of a terrestrial planet or moon. Occurs frequently at mid-ocean ridges and volcanic islands.
Uses & applications
Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Also used as dimension stone for flagstones and occasionally in stone massage therapy or lapidary work.
Geological facts
Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It is also found on the Moon, Mars, and Venus, where it forms vast volcanic plains.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture. Samples found on beaches or in rivers are often rounded and smooth like the one pictured. It is very common in the Pacific Northwest and volcanic regions.
