igneous
Basalt with Weathering Rind
Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Black to dark gray interior with a light brown/tan weathered exterior; Luster: Dull to sub-metallic; Structure: Fine-grained aphanitic; Cleavage: None; Specific Gravity: 2.8-3.0
- Hardness
- 6 (Mohs scale)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Black to dark gray interior with a light brown/tan weathered exterior; Luster: Dull to sub-metallic; Structure: Fine-grained aphanitic; Cleavage: None; 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 Earth's surface. The outer tan layer is a 'weathering rind' caused by chemical decomposition of minerals like olivine and pyroxene over time.
Uses & applications
Primarily used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as a dimension stone for floor tiles and monuments.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The lighter colored rim on this piece shows the 'front' of chemical weathering as water and oxygen penetrate the rock.
Field identification & locations
Look for a dark, heavy, fine-grained rock. If broken, it should show a distinct color difference between the outer weathered 'cortex' and the fresh interior. Commonly found in volcanic regions or till deposits.
