Igneous
Basalt
Mafic Volcanic Rock (Primarily Plagioclase and Pyroxene)
Hardness: 5-6 (Mohs scale); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark gray to black, often weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None (conchoidal fracture); 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. Most specimens are from the Cenozoic era, though it has formed throughout Earth's history via volcanic activity and seafloor spreading.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. In industry, it is melted to create stone wool insulation or cast into floor tiles. Fine-grained pieces with high density are also used in shoreline protection (riprap).
Geological facts
Basalt is the most common rock type in Earth's crust; it makes up most of the ocean floors. It is also found on other planetary bodies like the Moon (lunar maria), Mars, and Venus. Large formations, like the Giant's Causeway, showcase its ability to form hexagonal columns during cooling.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and very fine grains that are difficult to see without a lens. Look for small vesicles (gas bubbles) or tiny green olivine crystals. Common in volcanic regions like Hawaii, the Pacific Northwest, and Iceland.
