igneous
Basalt with Weathering Crust
Extrusive mafic volcanic rock
Hardness: 5-6 Mohs. Color: Dark grey to black interior with a brownish-tan weathered exterior (oxidized). Luster: Dull/Earthy. Structure: Fine-grained (aphanitic). Specific gravity: 2.8-3.0.
- Hardness
- 5-6 Mohs
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 Mohs. Color: Dark grey to black interior with a brownish-tan weathered exterior (oxidized). Luster: Dull/Earthy. Structure: Fine-grained (aphanitic). Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The brownish coating is caused by chemical weathering (oxidation) of iron-bearing minerals like olivine and pyroxene.
Uses & applications
Used primarily in construction (crushed stone, road base), as a dimension stone, and in the production of stone wool insulation.
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 found in abundance on the Moon and Mars.
Field identification & locations
Identify in the field by its high density and dark interior visible when chipped. It is commonly found in volcanic regions or ancient rift zones. Small, weathered hand samples are ubiquitous in many geological settings.
