igneous
Basalt
Basalt (Aphanitic-to-porphyritic mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black; Luster: Dull to sub-metallic (when fresh); Structure: Fine-grained/Aphanitic (crystals not visible to the naked eye); Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal black
- Luster
- Dull to sub-metallic (when fresh)
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black; Luster: Dull to sub-metallic (when fresh); Structure: Fine-grained/Aphanitic (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 (mafic) at or very near the Earth's surface. It is the most common rock in Earth's crust, primarily making up the ocean floors.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality varieties are used for floor tiles, monuments, and stone carvings.
Geological facts
Basalt is not only the most common rock on Earth but also dominates the lunar maria (the dark patches on the moon) and much of the surface of Mars and Venus.
Field identification & locations
Identify by its heavy weight (relative to size), dark color, and fine-grained texture. It often shows 'vesicular' holes (trapped gas bubbles). Common in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.
