igneous
Basalt with Iron Oxidation
Mafic Igneous Rock (Basalt)
Hardness: 6 (Mohs scale); Color: Dark grey to black interior with reddish-brown exterior weathering; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); 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: Dark grey to black interior with reddish-brown exterior weathering; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); 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. The outer reddish hue is caused by chemical weathering (oxidation) of iron-bearing minerals over decades to centuries.
Uses & applications
Commonly used in construction as an aggregate for road base, concrete, and railroad ballast. Larger slabs are used for architectural masonry. Occasional use in stone massage therapy.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large-scale eruptions of basalt are known as 'Flood Basalts' and can cover thousands of square miles.
Field identification & locations
Identify by its heavy weight (high density), dark fine-grained interior, and characteristic rusty-orange weathering crust. Look for conchoidal to irregular fracture patterns. Common in volcanic regions or glacial till.
