igneous
Basalt
Basalt (dominated by plagioclase feldspar, pyroxene, and olivine)
Hardness: 6 (Mohs scale); Color: Dark gray, black, or dark brown; Luster: Dull to earthy; Crystal Structure: Aphanitic (fine-grained crystals generally not visible to the naked eye); Specific Gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray, black, or dark brown
- Luster
- Dull to earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray, black, or dark brown; Luster: Dull to earthy; Crystal Structure: Aphanitic (fine-grained crystals generally not visible to the naked eye); 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, covering much of the ocean floor.
Uses & applications
Used widely in construction as crushed stone for road base, railroad ballast, and concrete aggregate. It is also used in the manufacturing of stone wool insulation and occasionally as a decorative landscape stone.
Geological facts
Basalt underlies more of Earth's surface than any other rock type. Most of the ocean basins are basalt, and it is also found on the Moon, Mars, and Venus.
Field identification & locations
Identify in the field by its dark color, fine-grained texture, and weight (high density). It is commonly found in volcanic regions, such as the Pacific Northwest of the US, Iceland, and Hawaii. Small water-worn pebbles like this one are common on volcanic beaches and in riverbeds.
