igneous
Basalt (Vesicular)
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: dark grey to black but can weather to reddish-brown; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- dark grey to black but can weather to reddish-brown
- Luster
- Dull/Matte
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Physical properties
Hardness: 6-7 on Mohs scale; Color: dark grey to black but can weather to reddish-brown; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained); Cleavage: None; 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. Most basalt specimens date from the Cenozoic to the Precambrian, found frequently at mid-ocean ridges and volcanic hotspots.
Uses & applications
Extensively used in construction as aggregate for roads, railroad ballast, and concrete; used in high-quality floor tiles and monuments; also processed into 'rock wool' for thermal insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floors. It is also extremely common on the Moon and Mars, forming the dark 'seas' or maria seen on the lunar surface.
Field identification & locations
Identify in the field by its weight (dense), dark color, and fine-grained texture. To distinguish it from shale, look for a lack of layering and a higher density. Often found in volcanic fields, riverbeds draining volcanic areas, or coastal cliffs.
