igneous
Basalt (River Pebble)
Extrusive mafic igneous rock
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, occasionally greenish; Luster: Dull/Matte; Structure: Fine-grained (aphanitic); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black, occasionally greenish
- Luster
- Dull/Matte
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black, occasionally greenish; Luster: Dull/Matte; Structure: Fine-grained (aphanitic); Cleavage: None; 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. This specific specimen has been rounded by fluvial (river) or glacial erosion.
Uses & applications
Used widely in construction as aggregate, for road base, railroad ballast, and in high-strength concrete. As a pebble, used for decorative landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight for its size, and the absence of visible large crystals. Commonly found in volcanic regions or in riverbeds downstream from volcanic sources.
