igneous
Basalt
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to tan/brown; Luster: Dull to sub-metallic; Structure: Fine-grained (aphanitic), often vesicular; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to tan/brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to tan/brown; Luster: Dull to sub-metallic; Structure: Fine-grained (aphanitic), often vesicular; Cleavage: None (conchoidal fracture); 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. Found in tectonic plates, hotspots (like Hawaii), and mid-ocean ridges. Formed throughout Earth's history.
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 the most common rock type in Earth's crust, covering most of the ocean floor. It is also found on the Moon, Mars, and Venus, making it a primary component of terrestrial planetary surfaces.
Field identification & locations
Identify by its heavy weight (high density), dark color, and lack of visible crystals to the naked eye. Look for tiny holes (vesicles) or a 'salt and pepper' appearance under a hand lens. Commonly found in volcanic fields.
