igneous
Pumice
Pumice (Vesicular Rhyolite/Andesite)
Hardness: 5-6 Mohs; Color: Creamy white to light gray; Luster: Dull to pearly; Structure: Highly vesicular (honeycombed with air bubbles) and amorphous glass; Specific Gravity: 0.25-0.91 (floats in water).
- Hardness
- 5-6 Mohs
- Color
- Creamy white to light gray
- Luster
- Dull to pearly
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Physical properties
Hardness: 5-6 Mohs; Color: Creamy white to light gray; Luster: Dull to pearly; Structure: Highly vesicular (honeycombed with air bubbles) and amorphous glass; Specific Gravity: 0.25-0.91 (floats in water).
Formation & geological history
Formed during explosive volcanic eruptions when highly pressurized, gas-rich magma is ejected and cools rapidly, trapping gas bubbles. Occurs in Cenozoic and younger volcanic arcs.
Uses & applications
Used as an abrasive in soaps (Lava soap), foot stones, and denim stone-washing; as a lightweight aggregate in concrete/cinder blocks; and as a soil amendment in horticulture for drainage.
Geological facts
Pumice is the only rock that consistently floats on water initially; massive 'pumice rafts' from underwater volcanoes can float across oceans for thousands of miles.
Field identification & locations
Identify in the field by its extreme lightness (low density) and frothy, sponge-like texture. Common in volcanic regions such as the Cascades (USA), Italy, Greece, and New Zealand.
