igneous
Basalt
Mafic Volcanic Rock (composed of Plagioclase Feldspar and Pyroxene)
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained, crystals not visible to naked eye); Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained, crystals not visible to naked eye); 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. Found on every continent and making up much of the ocean floor; ages range from modern eruptions to billions of years old.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used in high-quality floor tiles and as a raw material for stone wool insulation.
Geological facts
Basalt is the most common rock type in Earth's crust and also covers large portions of the surfaces of the Moon, Mars, and Venus. Famous formations include the Giant's Causeway in Northern Ireland and the Devils Postpile in California.
Field identification & locations
Identify by its dark color, heavy weight (density), and lack of visible crystals. It typically won't scratch with a steel knife but can be scratched by quartz. Commonly found in volcanic regions or as weathered river stones.
