igneous
Basalt
Mafic volcanic rock (consisting mostly of plagioclase, pyroxene, and olivine)
Hardness: 6 on Mohs scale; Color: Dark grey to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black, often weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); 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. It covers about 70% of Earth's surface through oceanic crust.
Uses & applications
Extensively used in construction for aggregate, road base, concrete, and railroad ballast. High-quality specimens are used in stone carving and as semi-precious 'lava stones' in jewelry.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up the giant lunar maria on the Moon. It can form unique hexagonal columns when cooling slowly, such as the Giant's Causeway.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture with no visible large crystals. Commonly found in volcanic regions, oceanic islands (Hawaii, Iceland), and tectonic rift zones.
