igneous
Pumice
Pumice (a vesicular volcanic glass)
Hardness: 5-6 (but very brittle); Color: Cream, light gray, or tan; Luster: Dull to pearly; Structure: Highly vesicular (honeycombed with air bubbles); Specific Gravity: 0.25-1.2 (can float in water).
- Hardness
- 5-6 (but very brittle)
- Color
- Cream, light gray, or tan
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 (but very brittle); Color: Cream, light gray, or tan; Luster: Dull to pearly; Structure: Highly vesicular (honeycombed with air bubbles); Specific Gravity: 0.25-1.2 (can float in water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected. The rapid depressurization and cooling trap gas bubbles in the glass, creating a frothy texture. Found in areas of recent volcanic activity.
Uses & applications
Used as an abrasive in cleaning products, cosmetic exfoliants, masonry (lightweight concrete), and horticultural soil amendments to improve aeration.
Geological facts
Pumice is the only rock that can float on water due to its extreme porosity. Massive 'pumice rafts' can sometimes be found floating on the ocean for years after an undersea eruption.
Field identification & locations
Identify in the field by its extreme lightness (it feels like styrofoam) and the presence of numerous tiny pits or bubbles. It is commonly found near volcanic arcs and subduction zones.
