igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 5-6 Mohs. Color: Dark gray to black (weathering to brown). Luster: Dull to sub-metallic. Crystal structure: Aphanitic (fine-grained), often containing micro-crystals of plagioclase, pyroxene, and olivine. Specific gravity: 2.8–3.0.
- Hardness
- 5-6 Mohs
- Color
- Dark gray to black (weathering to brown)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 Mohs. Color: Dark gray to black (weathering to brown). Luster: Dull to sub-metallic. Crystal structure: Aphanitic (fine-grained), often containing micro-crystals of plagioclase, pyroxene, and olivine. 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. It is the most common volcanic rock on Earth, comprising most of the ocean floor and large continental flood basalt provinces.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Select specimens are used for 'hot stone' massage or carved into decorative items.
Geological facts
Basalt is not only found on Earth; it is a major component of the Moon's 'maria' (dark plains) and covers much of the surface of Mars and Venus.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and lack of visible large crystals. It often forms hexagonal columns in large outcrops. Common in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.
