igneous
Pumice (or Volcanic Tuff / Ash)
Pumice / Pyroclastic rock
Hardness 5-6 (Mohs scale), light gray to off-white color, dull/earthy luster, highly vesicular or fine-grained ash structure, extremely light weight and porous, specific gravity < 1.0 (often floats on water).
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Physical properties
Hardness 5-6 (Mohs scale), light gray to off-white color, dull/earthy luster, highly vesicular or fine-grained ash structure, extremely light weight and porous, specific gravity < 1.0 (often floats on water).
Formation & geological history
Formed during explosive volcanic eruptions when highly gas-charged, viscous magma cools rapidly, trapping gas bubbles as it solidifies into porous volcanic glass and ash fragments.
Uses & applications
Used as an abrasive in soaps and stonewashing textiles, lightweight aggregate in concrete, soil conditioner in horticulture, and cosmetic exfoliant.
Geological facts
Pumice is one of the only rocks that can float on water due to its low density and enclosed bubble structures. Floating pumice rafts created by undersea eruptions can travel thousands of miles across oceans.
Field identification & locations
Identified by its rough, highly porous, sponge-like texture, light gray color, and surprisingly lightweight nature. Commonly found in volcanic regions near active or extinct continental margin volcanoes.
