igneous
Basalt
Extrusive mafic igneous rock
Hardness: 6.0; Color: dark gray to black, often weathering to brown; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or porphyritic; Specific gravity: 2.8 - 3.0
- Hardness
- 6
- Color
- dark gray to black, often weathering to brown
- Luster
- dull to sub-metallic
Identified More igneous →
Explore Basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6.0; Color: dark gray to black, often weathering to brown; Luster: dull to sub-metallic; Crystal structure: fine-grained (aphanitic) or 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. Basalt is the most common volcanic rock on Earth and forms much of the ocean floor and large continental provinces.
Uses & applications
Extensively used in construction for road base, concrete aggregate, railroad ballast, and high-quality asphalt. It is also used in decorative landscaping and as 'basalt fiber' for thermal insulation.
Geological facts
Basalt is also found on other planetary bodies, including the Moon, Mars, and Venus. On Earth, the Giant's Causeway in Northern Ireland is a famous example of columnar basalt jointing.
Field identification & locations
Identify by its heavy weight (high density), dark color, and lack of visible large crystal grains without a lens. Look for a 'vesicular' texture (small holes from gas bubbles) or a smooth, dense fracture surface.
