igneous
Basalt
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing microcrystals of plagioclase feldspar and pyroxene; Cleavage: None; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing microcrystals of plagioclase feldspar and pyroxene; Cleavage: None; 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. It is the most common rock in the Earth's crust, making up most of the ocean floor and originating from volcanic eruptions throughout geological history.
Uses & applications
Extensively used in construction as aggregate for roads and railways, in concrete, and for stone masonry. It is also used as raw material for stone wool insulation and occasionally for decorative tiling or garden landscaping.
Geological facts
Basalt is not only the most common rock on Earth but also predominant on other terrestrial bodies like the Moon, Mars, and Venus. The 'Giant's Causeway' in Northern Ireland is a famous example of basalt forming hexagonal columns during cooling.
Field identification & locations
Identify in the field by its dark, dense appearance and heavy weight relative to its size. It often lacks visible crystals to the naked eye. Found worldwide in volcanic regions, such as the Pacific Northwest (USA), Iceland, and Hawaii.
