igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Dark gray to black, weathering to brown or red; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black, weathering to brown or red
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black, weathering to brown or red; Luster: Dull to sub-metallic; 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 at or very near the Earth's surface. Found in oceanic crust, volcanic island chains, and continental flood basalts. Ages vary across geological history, from Archean to modern day.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Also used for high-temperature insulation (stone wool) and as a decorative stone in landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and also covers large areas of the Moon (the Lunar Maria), Mars, and Venus. It can form unique geometric 'columnar joints' when cooling slowly in thick layers.
Field identification & locations
Identify by its heavy weight (density), dark color, and fine grain size where individual crystals are too small to see with the naked eye. To distinguish from slate or shale, check for its extreme hardness and lack of splitting layers. Found globally near tectonic plate boundaries or volcanic hotspots.
