igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained, crystals too small to see with the naked eye); Specific Gravity: 2.8–3.0.
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained, crystals too small to see with the naked eye); Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Most common rock in the Earth's crust.
Uses & applications
Used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Also used for high-temperature insulation (stone wool) and as a building veneer.
Geological facts
Basalt underlies more of Earth's surface than any other rock type. The Giant's Causeway in Ireland is a famous example of columnar basalt jointing.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and lack of visible crystals. Commonly found in oceanic crust, volcanic islands (like Hawaii), and flood basalt plateaus.
