igneous
Basalt with Weathering Rind
Mafic Volcanic Rock (Primarily Plagioclase and Pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black interior, tan/brown exterior rind; Luster: Dull to sub-metallic; Structure: Fine-grained/aphanitic; Specific Gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black interior, tan/brown exterior rind
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black interior, tan/brown exterior rind; Luster: Dull to sub-metallic; 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 very near the Earth's surface. The outer tan layer is a 'weathering rind' caused by chemical oxidation of iron minerals when exposed to air and water over geological time.
Uses & applications
Commonly used in construction as aggregate for roads and railways, in concrete, and for floor tiles. High-quality specimens are sometimes used as thermal insulators or for basalt fiber.
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 heavy weight (density), fine grain, and the contrast between the dark unweathered interior and the lighter weathered exterior. Commonly found in volcanic fields, oceanic islands, and mid-ocean ridges.
