Igneous
Basalt
Basalt (dominated by plagioclase feldspar and pyroxene minerals)
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to olive-green or brown; Luster: Dull/earthy to sub-metallic; Crystal Structure: Aphanitic (fine-grained) or porphyritic; Specific Gravity: 2.8–3.0
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to olive-green or brown
- Luster
- Dull/earthy to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to olive-green or brown; Luster: Dull/earthy to sub-metallic; Crystal Structure: Aphanitic (fine-grained) or porphyritic; Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of low-viscosity lava rich in magnesium and iron at or very near the Earth's surface. Basalt is the most common rock type in the Earth's crust (mostly oceanic crust) and can range from modern to billions of years old.
Uses & applications
Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used for paving stones, sculptures, and occasionally as 'lava rock' for landscaping or grill beds.
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 basalt. It often forms striking geometric patterns known as columnar jointing when it cools.
Field identification & locations
Identify in the field by its high density (feels heavy for its size), dark color, and fine-grained texture with no visible large crystals. It often has small 'vesicles' (holes from air bubbles). Common in volcanic regions like Hawaii, the Pacific Northwest, and Iceland.
