igneous
Basalt
Mafic Volcanic Basalt (Major minerals: Plagioclase feldspar, Pyroxene; Minor: Olivine, Magnetite)
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull/earthy; Crystal Structure: Aphanitic (fine-grained, crystals not visible to eye); Cleavage: None (conchoidal/irregular fracture); Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull/earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull/earthy; Crystal Structure: Aphanitic (fine-grained, crystals not visible to eye); Cleavage: None (conchoidal/irregular fracture); 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 moon. Most basalt on Earth is less than 150 million years old due to tectonic recycling.
Uses & applications
Used extensively in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. Denser varieties are used for high-end countertops or in 'stone therapy' massages due to heat retention.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up almost most of the ocean floor. It is also common on the Moon, Mars, and Venus, where it forms the dark 'seas' known as lunar maria.
Field identification & locations
Identify in the field by its distinctive dark color, heavy weight (density), and fine-grained texture. It often weathers to a reddish or brownish crust. Commonly found in volcanic fields, oceanic islands (like Hawaii), and mid-ocean ridges.
