igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 on Mohs scale; Color: Dark grey to black or brownish-red; Luster: Dull to sub-metallic; Structure: Aphanitic (fine-grained) to vesicular; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black or brownish-red
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black or brownish-red; Luster: Dull to sub-metallic; Structure: Aphanitic (fine-grained) to vesicular; 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. Common in oceanic crust and mid-ocean ridges. Its age can range from modern eruptions to hundreds of millions of years old.
Uses & applications
Used extensively in construction as crushed stone for road bases, concrete aggregate, and railroad ballast. It is also used in the manufacturing of rock wool insulation and as a dimension stone for flooring or monuments.
Geological facts
Basalt is the most common volcanic rock in the solar system, covering most of the ocean floors on Earth and forming the 'seas' (maria) on the Moon. This specific specimen appears to be worked into an anthropogenic tool shape, such as a projectile point or scraper.
Field identification & locations
Identify in the field by its dark color, fine-grained texture, and high density. Look for small holes (vesicles) or a 'scaly' conchoidal fracture where it has been chipped. Commonly found in volcanic regions such as the Pacific Northwest, Iceland, and Hawaii.
