igneous
Basalt
Mafic Igneous Rock (primarily Plagioclase and Pyroxene)
Hardness: 6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
- Hardness
- 6 (Mohs)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); 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. Most specimens are from the Cenozoic era, though it has formed throughout Earth's history.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as a raw material for stone wool insulation.
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 on the Moon and Mars.
Field identification & locations
Look for a dark, heavy, fine-grained rock often with small holes (vesicles) or visible tiny shiny crystals. Commonly found in volcanic regions and oceanic crustal exposures.
