igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6\u20137 Mohs; Color: Grey, dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8\u20133.0.
- Hardness
- 6\u20137 Mohs
- Color
- Grey, dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6\u20137 Mohs; Color: Grey, dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8\u20133.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor.
Uses & applications
Extensively used in construction for aggregate in concrete and asphalt, road base, railroad ballast, and occasionally as dimension stone for tiles or monuments. High-quality specimens can be used for stone carvings.
Geological facts
Basalt also exists on other planetary bodies; the lunar maria (dark plains) on the Moon and Olympus Mons on Mars are primarily composed of basaltic lava flows.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows columnar jointing or vesicular (pitted) surfaces when freshly erupted. This specimen appears weathered and rounded by water or glacial transport.
