Pumice

Pumice is a highly vesicular volcanic glass, essentially a solidified foam of magma. It forms when gas-rich, silica-rich lava is ejected so violently that dissolved gases expand and the melt quenches before the bubbles can escape, trapping countless tiny voids.

The result is a rock so porous and lightweight that many pieces float on water, a property almost unique among rocks. It is usually pale, white to grey, and feels rough and abrasive.

Pumice is technically a glass rather than a crystalline rock, since it cools too fast for minerals to form. It is closely related to scoria, which is darker, denser, and has larger, less connected bubbles.

Pumice forms during explosive eruptions of viscous, silica-rich (felsic) magmas such as rhyolite and dacite. As this gas-charged magma decompresses on eruption, dissolved water and gases come out of solution and expand rapidly, frothing the melt.

The foam is then chilled almost instantly to glass as it is blasted into the air or onto the surface, freezing in the bubble structure. Large eruptions can blanket regions in pumice and pumice ash (pumicite) and produce floating rafts of pumice on the sea.

Major sources include the volcanoes of Italy (Lipari), Greece (Santorini), Turkey, Iceland, New Zealand, Japan, and the western United States. Rafts of floating pumice from undersea eruptions can drift across entire oceans.

Look for a very lightweight, pale (white, grey, cream) rock riddled with tiny holes and a frothy, spongy texture. The signature test is that genuine pumice often floats on water because of its trapped air.

It feels abrasive and gritty and crumbles easily. Under a lens you see a glassy, foam-like network of thin walls between bubbles.

Look-alikes include scoria (darker red-brown to black, denser, larger bubbles, usually sinks), and vesicular basalt (heavier, dark, holes but solid rock between them). Pumice's pale color, extreme lightness, and ability to float are the clearest identifiers.

Pumice is widely used as an abrasive and lightweight aggregate. Ground pumice goes into cleaning products, polishes, exfoliating soaps, and the classic pumice stone for removing calluses and dead skin. Stonewashed denim is famously softened with pumice.

As a lightweight aggregate it is added to concrete, building blocks, and insulation, and it is used horticulturally to improve soil drainage and aeration. It also serves as a filtration and absorbent medium.

Geologically, pumice and pumice ash layers are valuable markers for dating and correlating volcanic eruptions and for reconstructing the explosive history of volcanoes.