Igneous
Basalt
Mafic Igneous Rock (composed primarily of plagioclase feldspar and pyroxene)
Hardness: 6 (Mohs scale); Color: Dark gray, black, or dark greenish-gray; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Cleavage: Generally lacks cleavage; Specific Gravity: 2.8-3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray, black, or dark greenish-gray
- 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 (Mohs scale); Color: Dark gray, black, or dark greenish-gray; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Cleavage: Generally lacks cleavage; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Basalt is the most common rock in the Earth's crust and underlies most ocean basins.
Uses & applications
Extensively used as an aggregate in construction for road base, railroad ballast, and concrete. It is also used as dimension stone for tiles and monuments, and finely ground basalt is used as a soil amendment.
Geological facts
Basalt covers approximately 70% of the Earth's surface. It is also the primary rock type on the Moon's maria, Mars, and Venus, making it a truly 'interplanetary' rock.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows columnar jointing or vesicular (pitted) surfaces when it contains trapped gas bubbles. Found in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.
