igneous
Basalt with Weathered Crust
Aphanitic Mafic Igneous Rock (Basalt)
Hardness: 5-6 (Mohs); Color: Dark grey to black interior, reddish-brown weathered exterior; Luster: Dull to sub-metallic; Crystal structure: Fine-grained (aphanitic); Cleavage: None; Specific gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black interior, reddish-brown weathered exterior; Luster: Dull to sub-metallic; Crystal 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 exposed at or very near the surface of a terrestrial planet or moon. The reddish exterior is due to oxidation (rusting) of iron-bearing minerals.
Uses & applications
Commonly used as aggregate in construction, road base, concrete, and railroad ballast. Exceptionally durable for high-traffic paving.
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 extensively on the Moon and Mars.
Field identification & locations
Identify by its heavy weight (high density), dark interior fine-grain texture, and often a brown weathered 'rind'. Found globally in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.
