igneous
Pumice
Pumice (a highly vesicular, silicic volcanic rock)
Hardness: 5-6 Mohs; Color: light grey to cream; Luster: dull to pearly; Structure: highly vesicular (frothy glass); Specific Gravity: 0.25-0.9 (often floats on water); Composition: primarily amorphous volcanic glass (SiO2).
- Hardness
- 5-6 Mohs
- Color
- light grey to cream
- Luster
- dull to pearly
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Physical properties
Hardness: 5-6 Mohs; Color: light grey to cream; Luster: dull to pearly; Structure: highly vesicular (frothy glass); Specific Gravity: 0.25-0.9 (often floats on water); Composition: primarily amorphous volcanic glass (SiO2).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected. The rapid cooling and depressurization cause gas bubbles to expand quickly, creating a frothy structure that solidifies into glass.
Uses & applications
Used as an abrasive in polishing and cleaning, in lightweight concrete blocks, as a decorative garden stone, and in skincare as an exfoliant for removing dead skin.
Geological facts
Pumice is the only rock that consistently floats on water due to its trapped air pockets. In 1883, the eruption of Krakatoa created pumice rafts that floated on the ocean for years, some traveling across entire oceans.
Field identification & locations
Identify in the field by its extreme lightness (low density) and vesicular, sponge-like surface. It is commonly found near volcanic arcs and subduction zones like the Andes, Japan, or the Cascades.
