igneous
Basalt
Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown or green; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to brown or green
- Luster
- Dull or earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown or green; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (mafic lava) at or very near the Earth's surface. Found in oceanic crust, mid-ocean ridges, and continental flood basalts across all geological ages.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used in high-temp insulation (rock wool) and occasionally as 'hot stones' in spa treatments.
Geological facts
Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor. It is also common on other planetary bodies like the Moon (lunar maria), Mars, and Venus.
Field identification & locations
Identify in the field by its high density, dark color, and very fine-grained texture that lacks visible crystals to the naked eye. Commonly found in volcanic regions, cliffs, and riverbeds.
