igneous
Basalt
Extrusive Mafic Igneous Rock
Hardness: 6 (Mohs scale); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black, often weathering to brown
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-vitreous; 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 exposed at or very near the surface of a terrestrial planet or a moon. Common in oceanic crust and shield volcanoes.
Uses & applications
Used extensively in construction as road base, railroad ballast, concrete aggregate, and as a decorative landscaping stone. Also used in the manufacturing of stone wool insulation.
Geological facts
Basalt is the most common rock type in Earth's crust and covers most of the ocean floor. It is also common on the Moon and Mars, where it forms the dark 'maria'.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture. It often shows vesiculation (small holes) or columnar jointing. Found globally in volcanic regions like Hawaii, the Pacific Northwest (USA), and Iceland.
