igneous
Basalt
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Earthy; Crystal structure: Microcrystalline (aphanitic), too fine to see with the naked eye; Cleavage: Generally none; Specific gravity: 2.8 - 3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Earthy; Crystal structure: Microcrystalline (aphanitic), too fine to see with the naked eye; Cleavage: Generally none; Specific gravity: 2.8 - 3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava at or very near the Earth's surface. It can be found in any geological age, but most oceanic crust is relatively young (less than 200 million years).
Uses & applications
Primary use is in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. It is also used in high-temp industrial applications and occasionally as a architectural decorative stone.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also extremely common on other planetary bodies like the Moon (lunar maria) and Mars.
Field identification & locations
Identify by its heavy weight (density), dark color, and fine-grained texture. It often shows a 'frosted' or dull appearance when weathered. Common in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.
