igneous
Basalt
Extrusive Mafic Igneous Rock (composed primarily of Plagioclase and Pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown or green; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to brown or green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown or green; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava (magma) exposed at or very near the Earth's surface. Basalt is the most common volcanic rock on Earth and can date from billions of years ago to modern volcanic eruptions.
Uses & applications
Extensively used in construction as aggregate for roads and railways, in concrete and asphalt, and for high-durability floor tiles. It is also used as a raw material for stone wool insulation.
Geological facts
Basalt covers more than 90% of all volcanic rock on Earth and constitutes the vast majority of the ocean floor. It is also found on the Moon (the dark lunar maria) and Mars.
Field identification & locations
Identify by its dark color, heavy weight (density), and lack of visible large crystal grains (fine-grained texture). In this specific image, the specimen appears worked into an archaeological artifact shape (projectile point/arrowhead), which was a common use by ancient cultures for making durable tools.
