igneous
Basalt
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark grey to black, though it can weather to brown or red; Luster: Dull to earthy; Crystal Structure: Aphanitic (fine-grained) with possible porphyritic textures; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, though it can weather to brown or red
- Luster
- Dull to earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, though it can weather to brown or red; Luster: Dull to earthy; Crystal Structure: Aphanitic (fine-grained) with possible porphyritic textures; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. It can be found in volcanic fields, oceanic plates, and hot spots. Age varies by location from modern (active volcanoes) to billions of years old.
Uses & applications
Extensively used in construction for aggregate, road base, railroad ballast, and high-quality concrete. It is also used as a dimension stone for flooring, monuments, and stone carvings. Polished basalt is sometimes used in jewelry or as 'black granite' counter tops.
Geological facts
Basalt is the most common rock on Earth's surface and covers most of the ocean floor. It also exists on other planetary bodies; the lunar maria (dark plains on the Moon) are composed of basaltic lava flows.
Field identification & locations
Identify in the field by its heavy weight (high density), dark color, and fine-grained texture where individual crystals are too small to see with the naked eye. It often forms hexagonal 'columnar joints' which are striking in large outcrops. Commonly found in Hawaii, Iceland, and the Pacific Northwest's Columbia River Plateau.
