igneous
Pumice
Vesicular volcanic glass (primarily SiO2)
Hardness: 5-6 Mohs (though fragile); Color: light gray, cream, or pale tan; Luster: dull to pearly; Structure: highly vesicular (full of gas bubbles); Specific Gravity: < 1.0 (floats in water).
- Hardness
- 5-6 Mohs (though fragile)
- Color
- light gray, cream, or pale tan
- Luster
- dull to pearly
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Physical properties
Hardness: 5-6 Mohs (though fragile); Color: light gray, cream, or pale tan; Luster: dull to pearly; Structure: highly vesicular (full of gas bubbles); Specific Gravity: < 1.0 (floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when magma is rapidly depressurized and quenched, trapping gas bubbles. Associated with silicic lavas (rhyolite, dacite). Geological age varies by location (Recent to Cenozoic).
Uses & applications
Abrasives (pumice stones), lightweight concrete aggregate, horticultural soil amendments, and stone-washing textiles (distressing denim).
Geological facts
Pumice is the only rock that consistently floats until it becomes waterlogged. Large eruptions can create 'pumice rafts' several kilometers long that drift across the ocean for years.
Field identification & locations
Identify in the field by its extreme lightness, frothy texture, and ability to float in water. Commonly found in volcanic regions like the Cascades (USA), Italy, and New Zealand.
