igneous
Basalt (with possible dendritic inclusions)
Extrusive mafic igneous rock
Hardness: 6-7 Mohs; Color: Dark grey to greenish-black; Luster: Dull to sub-metallic (when wet); Texture: Aphanitic (fine-grained); Specific Gravity: 2.9-3.1.
- Hardness
- 6-7 Mohs
- Color
- Dark grey to greenish-black
- Luster
- Dull to sub-metallic (when wet)
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to greenish-black; Luster: Dull to sub-metallic (when wet); Texture: Aphanitic (fine-grained); Specific Gravity: 2.9-3.1.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the surface of a terrestrial planet or moon. Most Basalt on Earth formed during the Cenozoic era, particularly in oceanic crust and large igneous provinces.
Uses & applications
Commonly used as crushed stone for road base, railroad ballast, and concrete aggregate. Also used in high-temperature textiles (basalt fiber) and as a building stone (pavers/statues).
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 found in abundance on the Moon and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. These rounded river-worn specimens often show dendritic patterns when wet, which are manganese oxide deposits that look like fossilized plants.
