igneous
Basalt
Extrusive mafic igneous rock
Hardness: 5-6 (Mohs); Color: dark grey to black; Luster: dull to sub-metallic (fine-grained); Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs)
- Color
- dark grey to black
- Luster
- dull to sub-metallic (fine-grained)
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Physical properties
Hardness: 5-6 (Mohs); Color: dark grey to black; Luster: dull to sub-metallic (fine-grained); Crystal structure: Aphanitic (fine-grained) to porphyritic; 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 is the most common rock in Earth's crust, making up most of the ocean floor.
Uses & applications
Used extensively in construction as aggregate for roads, railroad ballast, and concrete. It is also used as a dimension stone for floor tiles and monuments.
Geological facts
Basalt is the primary rock type on the Moon's maria and is the most common volcanic rock on Mars and Venus. Columnar jointing in basalt creates famous landmarks like the Giant's Causeway.
Field identification & locations
Identify by its heavy weight (high density), dark color, and very fine grains that are often too small to see with the naked eye. Commonly found in volcanic regions and oceanic crustal outcrops.
