igneous
Basalt
Mafic volcanic rock (primarily plagioclase feldspar and pyroxene)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Sub-metallic; Structure: Fine-grained (aphanitic) or porphyritic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull/Sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Sub-metallic; Structure: Fine-grained (aphanitic) or porphyritic; 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 exposed at or very near the surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth, often found in oceanic crust and trap rock formations.
Uses & applications
Used primarily in construction for road base, concrete aggregate, and railroad ballast. It is also used in the manufacturing of rock wool insulation and as polished dimension stone for floor tiles and monuments.
Geological facts
Basalt is the primary rock type found on the Moon (the Lunar Maria are basaltic) and on Mars. It forms unique hexagonal columns when cooling, such as the Giant's Causeway in Northern Ireland.
Field identification & locations
Identify in the field by its very dark color, heavy weight (density), and lack of visible large crystals. It often feels slightly gritty and exhibits sharp edges when broken. Common globally, especially in volcanic regions like Hawaii or the Pacific Northwest.
