igneous
Basalt River Cobble
Basalt (Mafic volcanic rock)
Hardness: 6 on Mohs scale; Color: Dark grey to dull brown; Luster: Dull/Earthy (weathered surface); Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); Specific gravity: 2.8-3.0.
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to dull brown
- Luster
- Dull/Earthy (weathered surface)
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to dull brown; Luster: Dull/Earthy (weathered surface); Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); 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 into a 'cobble' shape by fluvial (river) or tumble erosion over thousands of years.
Uses & applications
Crushed basalt is used in construction for road base, railroad ballast, and concrete aggregate. Smooth river cobbles like this are used for landscaping and decorative masonry.
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 the primary rock found on the Moon and Mars.
Field identification & locations
Identify by its heavy weight (high density), dark interior color (if chipped), and lack of visible crystals. Commonly found in volcanic regions or riverbeds downstream from volcanic plateaus.
