igneous
Basalt
Basalt (Phaneritic/Aphanitic Mafic Igneous Rock)
Hardness: 5–6 (Mohs scale); Color: Dark gray to black (often weathering to brown or rust-red); Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8–3.0
- Hardness
- 5–6 (Mohs scale)
- Color
- Dark gray to black (often weathering to brown or rust-red)
- 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 or rust-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. It is the most common rock type in the Earth's crust, primarily forming the ocean floors and large continental plateaus through volcanic eruptions.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, railroad ballast, and asphalt. High-quality dense basalt is sometimes used for floor tiles, monuments, and stone carvings.
Geological facts
Basalt covers more than 70% of the Earth's surface (mostly underwater). It is also found on other planetary bodies like the Moon (lunar maria), Mars, and Venus. Famous basalt structures include the Giant's Causeway in Northern Ireland and the Devils Postpile in California.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size due to iron content), and lack of visible large crystals. It often breaks with a conchoidal or uneven fracture. Commonly found in volcanic regions, oceanic island chains (like Hawaii), and basaltic plateaus.
