Igneous
Basalt
Mafic volcanic rock (primarily plagioclase, pyroxene, and olivine)
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to olive green or rusty brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None; Specific Gravity: 2.9–3.1
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to olive green or rusty brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to olive green or rusty brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None; Specific Gravity: 2.9–3.1
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava. Most often found at mid-ocean ridges and hotspots. Geological age varies from modern volcanic flows to over 1 billion years old.
Uses & applications
Extensively used in construction as crushed aggregate for roads, concrete, and railroad ballast. Also used as dimension stone for tiles and historical monuments, and melted down to create rock wool insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also an extremely common rock on other planetary bodies like the Moon (lunar maria) and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and fine-grained texture. It often weathers into a reddish or greenish exterior. Common in the Pacific Northwest (USA), Hawaii, and Iceland.
