igneous
Basalt
Basalt (Mafic volcanic rock, primarily Plagioclase and Pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black, often weathering to brown or green; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained, crystals not visible to the naked eye); Specific Gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, often weathering to brown or green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, often weathering to brown or green; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained, crystals not visible to the naked eye); 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. Most basalt is Cenozoic in age, though it has formed throughout Earth's history in oceanic ridges and volcanic hotspots.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used in the manufacturing of rock wool insulation and as high-quality 'basalt fiber' in industrial applications.
Geological facts
Basalt is the most common rock on Earth's surface, making up the majority of the ocean floor. It is also common on other terrestrial planets like Mars and Venus, and even makes up the 'maria' (dark plains) of the Moon.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often exhibits columnar jointing in large formations. Commonly found in volcanic regions like the Pacific Northwest, Iceland, and Hawaii.
