igneous
Pumice
Pumice (Volcanic Rock / Glass Foam)
Hardness: 6 (on Mohs scale when scratched as a powder, though the bulk rock is fragile); Color: Light gray, white, yellowish, or cream; Luster: Dull to earthy; Crystal Structure: Amorphous (volcanic glass foam); Cleavage: None; Specific Gravity: Less than 1.0 (it floats on water due to high porosity).
- Color
- Light gray, white, yellowish, or cream
- Luster
- Dull to earthy
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Physical properties
Hardness: 6 (on Mohs scale when scratched as a powder, though the bulk rock is fragile); Color: Light gray, white, yellowish, or cream; Luster: Dull to earthy; Crystal Structure: Amorphous (volcanic glass foam); Cleavage: None; Specific Gravity: Less than 1.0 (it floats on water due to high porosity).
Formation & geological history
Formed through explosive volcanic eruptions involving felsic to intermediate lavas rich in dissolved gases. As the magma erupts and pressure rapidly drops, gases expand violently, creating a bubbly froth that cools instantly into volcanic glass before bubbles can escape.
Uses & applications
Used in lightweight aggregate for concrete, landscaping, abrasive materials (pumice stones for skin care, scouring powders, and stone-washing denim), horticulture, and filtration.
Geological facts
Pumice is one of the very few rocks that can float on water due to its trapped gas bubbles. It is extensively associated with violent Plinian eruptions such as the eruption of Krakatoa or Mount Vesuvius.
Field identification & locations
Easily identified in the field by its extreme lightness, vesicular (bubbly/spongy) texture, light color, and the fact that it floats. Found worldwide in regions with past or present explosive volcanic activity.
