igneous
Basalt
Basalt (Mafic igneous rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8-3.0; Cleavage: None, often demonstrates concoidal fracture.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8-3.0; Cleavage: None, often demonstrates concoidal fracture.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface. It is the most common rock in Earth's crust, primarily forming oceanic plates and large volcanic flows.
Uses & applications
Extensively used in construction as aggregate for roads and concrete, railroad ballast, and sometimes as a dimension stone for floor tiles or monument bases.
Geological facts
Basalt is the primary rock type on the Moon's lunar maria and also composes the majority of the Martian surface. It can form unique towering hexagonal structures known as columnar jointing.
Field identification & locations
Identify in the field by its high density, dark color, and very fine-grained texture which makes individual crystals difficult to see without a lens. Often found in volcanic regions or coastal cliffs.
