igneous
Basalt
Extrusive mafic igneous rock
Hardness (Mohs): 6. Color: dark grey to black. Luster: dull to earthy. Crystal structure: fine-grained (aphanitic) or porphyritic. Specific gravity: 2.8–3.0.
- Color
- dark grey to black
- Luster
- dull to earthy
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Physical properties
Hardness (Mohs): 6. Color: dark grey to black. Luster: dull to earthy. Crystal structure: fine-grained (aphanitic) or porphyritic. 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. Basalt underlies more of the Earth's surface than any other rock type due to its presence in oceanic crust.
Uses & applications
Widely used in construction for road base, concrete aggregate, railroad ballast, and dimension stone for pavers or countertops. High-quality pieces are used for sculpture.
Geological facts
Basalt is the most common volcanic rock on Earth and also makes up most of the surface of the Moon, Venus, and Mars (including Olympus Mons).
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and very fine grain where individual crystals are too small to see with the naked eye. Commonly found in volcanic fields and oceanic islands.
