igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6 on Mohs scale; Color: Dark grey to black, sometimes greenish; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0.
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black, sometimes greenish
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black, sometimes greenish; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; 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 basalt on Earth was formed during the Cenozoic era, though occurrences date back billions of years.
Uses & applications
Extensively used in construction for road base, concrete aggregate, asphalt pavement aggregate, railroad ballast, and filter stone in drain fields. High-quality specimens are used in stone sculpture.
Geological facts
Basalt is the most common rock type in Earth's crust and makes up the majority of the ocean floor. It is also found on the Moon and Mars. It can form unique 'columnar jointing' structures like at the Giant's Causeway.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture with occasional small holes (vesicles). Commonly found in volcanic regions, oceanic crust, and riverbeds near volcanic outcrops.
