igneous
Pumice
Pumice (a highly vesicular volcanic rock)
Hardness: 5-6 (Mohs scale); Color: Light gray to cream; Luster: Dull or silky; Structure: Vesicular (full of tiny air holes); Gravity: Extremely low, often floats on water.
- Hardness
- 5-6 (Mohs scale)
- Color
- Light gray to cream
- Luster
- Dull or silky
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Light gray to cream; Luster: Dull or silky; Structure: Vesicular (full of tiny air holes); Gravity: Extremely low, often floats on water.
Formation & geological history
Formed during explosive volcanic eruptions where gas-rich, frothy magma cools very rapidly, trapping bubbles of steam and volcanic gases. Usually associated with rhyolitic or andesitic volcanism.
Uses & applications
Used as an abrasive in soaps, foot care products, and industrial polishing. Also used in construction (lightweight concrete), horticulture (soil drainage), and stone-washing blue jeans.
Geological facts
Pumice is the only rock that can float on water because of its high porosity. It can sometimes float in the ocean for years, forming 'pumice rafts' that travel thousands of miles.
Field identification & locations
Identify in the field by its sponge-like appearance and surprisingly light weight; if it floats in water, it's almost certainly pumice. Commonly found near volcanic vent sites or on beaches near active volcanic zones.
