igneous
Basalt
Afanitic Mafic Basalt
Hardness: 6 (Mohs scale); Color: Dark grey to black, sometimes greenish or brownish; Luster: Dull/Sub-metallic; Crystal Structure: Very fine-grained (aphanitic) though can have visible olivine phenocrysts; Density: 2.8–3.0 g/cm³.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, sometimes greenish or brownish
- Luster
- Dull/Sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, sometimes greenish or brownish; Luster: Dull/Sub-metallic; Crystal Structure: Very fine-grained (aphanitic) though can have visible olivine phenocrysts; Density: 2.8–3.0 g/cm³.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (magma) exposed at or very near the surface of a terrestrial planet or a moon. Most basalt seafloor crust is less than 200 million years old, though continental flood basalts can be billions of years old.
Uses & applications
Used extensively in construction as an aggregate (crushed stone) for roads, asphalt, and concrete. Also used for high-tensile basalt fiber, 'lava rock' for grills/landscaping, and polished for tiles or countertops.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up almost the entire ocean floor. It is also found extensively on the Moon (forming the dark 'maria') and is the primary rock type on Mars.
Field identification & locations
Identify by its dark, heavy, and fine-grained texture. It often shows a 'frosted' or weathered outer tan skin. In the field, look for columnar jointing or vesicular (pitted) surfaces. Found worldwide in volcanic regions like Iceland, Hawaii, and the Pacific Northwest.
