igneous
Basalt
Mafic Igneous Rock (composed primarily of pyroxene and calcium-rich plagioclase)
Hardness: 6 (Mohs scale); Color: Dark grey to black, often with brownish oxidation; Luster: Dull to sub-metallic; Structure: Fine-grained (aphanitic), sometimes vesicular or containing small visible phenocrysts; Specific Gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, often with brownish oxidation
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, often with brownish oxidation; Luster: Dull to sub-metallic; Structure: Fine-grained (aphanitic), sometimes vesicular or containing small visible phenocrysts; 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. These are common in oceanic crust, volcanic islands, and continental flood basalts.
Uses & applications
Extensively used in construction as an aggregate for road base, concrete, and railroad ballast. It is also used in high-performance insulation (rock wool) and as a building stone.
Geological facts
Basalt is the most common rock type on Earth's surface and covers most of the ocean floor. It is also the primary rock found on the Moon (the dark lunar maria) and Mars.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows a weathered brownish 'rind' on the outside. Common in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.
