igneous
Basalt
Basalt (extrusive volcanic rock)
Hardness: 6 on Mohs scale; Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8–3.0.
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Most common rock type in the Earth's crust, forming the ocean floor.
Uses & applications
Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. Can be used in stone sculptures and as a source for cast basalt fibers.
Geological facts
Basalt is equivalent in composition to gabbro, but gabbro is plutonic (coarse-grained). It is the most common volcanic rock on Earth and also makes up the lunar maria on the Moon.
Field identification & locations
Identify by its heavy weight, dark color, and fine-grained texture lacking visible crystals to the naked eye. Commonly found in lava flows, oceanic crust, and volcanic islands like Hawaii or Iceland.
