igneous
Pumice
Pumice (a highly vesicular volcanic rock)
Hardness: 5-6 (but brittle); Color: white, cream, or light grey; Luster: dull to pearly; Structure: highly vesicular (full of gas bubbles); Specific Gravity: 0.2-1.0 (often floats on water).
- Hardness
- 5-6 (but brittle)
- Color
- white, cream, or light grey
- Luster
- dull to pearly
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Physical properties
Hardness: 5-6 (but brittle); Color: white, cream, or light grey; Luster: dull to pearly; Structure: highly vesicular (full of gas bubbles); Specific Gravity: 0.2-1.0 (often floats on water).
Formation & geological history
Pumice is formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected and rapidly cools. The sudden drop in pressure causes dissolved gases to expand into bubbles, which are frozen into place as the rock solidifies.
Uses & applications
Used as an abrasive in polishing, exfoliating stones (lava rocks), lightweight concrete aggregate, pencil erasers, and horticulture as a soil amendment for drainage.
Geological facts
Pumice is the only rock that can float on water due to its incredible porosity. In large volcanic eruptions, floating 'pumice rafts' can form in the ocean, lasting for years and traveling thousands of miles.
Field identification & locations
Identified in the field by its sponge-like appearance, extremely light weight, and ability to float in water. Commonly found in volcanic regions such as Iceland, Italy (Lipari), and the Cascades in the USA.
