igneous
Pumice (Vesicular Rhyolite/Andesite)
Pumice (a highly vesicular volcanic glass/extrusive igneous rock)
Hardness: 5-6 (but very brittle); Color: Grey, white, or reddish-pink; Luster: Dull or pearly; Structure: Amorphous (glassy with holes); Cleavage: None; Specific Gravity: <1.0 (it often floats in water)
- Hardness
- 5-6 (but very brittle)
- Color
- Grey, white, or reddish-pink
- Luster
- Dull or pearly
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Physical properties
Hardness: 5-6 (but very brittle); Color: Grey, white, or reddish-pink; Luster: Dull or pearly; Structure: Amorphous (glassy with holes); Cleavage: None; Specific Gravity: <1.0 (it often floats in water)
Formation & geological history
Formed during explosive volcanic eruptions when lava is ejected into the air. Gases trapped in the rapidly cooling lava expand to create the vesicular (holey) texture. Usually found in Cenozoic volcanic regions.
Uses & applications
Used as an abrasive in cleaning (pumice stones), in horticulture to aerate soil, in light-weight concrete production, and in stone-washing textiles like jeans.
Geological facts
Pumice is the only rock that can float on water until it eventually becomes waterlogged and sinks. Large 'pumice rafts' have been known to float in the ocean for years after major eruptions.
Field identification & locations
Identify in the field by its extremely light weight and porous 'sponge-like' appearance. Commonly found in volcanic areas like the Cascades, the Mediterranean, or New Zealand. Feel for a rough, sand-papery texture.
