igneous
Pumice
Pumice (a highly vesicular felsic volcanic rock)
Hardness: 5-6 (Mohs), but brittle. Color: White, light gray, or cream. Luster: Dull to pearly. Structure: Highly vesicular/porous (frothy). Cleavage: None. Specific Gravity: 0.25-0.9 (floats on water).
- Hardness
- 5-6 (Mohs), but brittle
- Color
- White, light gray, or cream
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (Mohs), but brittle. Color: White, light gray, or cream. Luster: Dull to pearly. Structure: Highly vesicular/porous (frothy). Cleavage: None. Specific Gravity: 0.25-0.9 (floats on water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected. The rapid cooling and depressurization trap gas bubbles as the rock solidifies, creating a 'frothy' glass.
Uses & applications
Used as an abrasive in cleaning products (pumice stones), cosmetics (exfoliants), construction (lightweight concrete), and horticulture (soil aeration).
Geological facts
Pumice is the only rock that can float on water due to its extreme porosity. Massive 'pumice rafts' have been known to float in the ocean for years following major volcanic eruptions.
Field identification & locations
Identify in the field by its extreme lightness and pitted, sponge-like appearance; it will feel significantly lighter than average rocks of its size. Common in volcanic regions like the Andes, Cascades, and Italy.
