igneous
Basalt with Iron Oxide staining
Basalt (Mafic volcanic rock)
Hardness: 5-6 (Mohs); Color: Dark grey to black with orange-brown weathering; Luster: Dull to sub-metallic; Structure: Fine-grained aphanitic; Cleavage: None
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black with orange-brown weathering
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black with orange-brown weathering; Luster: Dull to sub-metallic; Structure: Fine-grained aphanitic; Cleavage: None
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The orange-brown patch is a result of surface oxidation (rusting) of iron-bearing minerals like olivine or pyroxene.
Uses & applications
Commonly used as aggregate in construction, road base, and railroad ballast. Weathered specimens have little industrial use but are educational for geology students.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The orange staining is a geological process called chemical weathering.
Field identification & locations
Identify by its heavy weight (high density), dark color, and fine-grained texture. Found globally in volcanic regions and glacial till. Look for 'rusty' patches on dark stones to find similar weathering.
