Pumice: Formation & Identification

01 / Origin story

How does pumice form?

Pumice forms when gas-rich, silica-rich lava erupts explosively and depressurizes so fast that the gas escapes violently, leaving the cooling rock riddled with bubble holes — essentially frozen lava foam.

02 / Read the clues

How can you identify pumice?

Use several clues together.
One photograph shows one specimen, not every variation.

Enlarged crop of the Pumice specimen — Texture study · enlarged crop of this specimen

01Extremely light, full of visible air pockets/holes (vesicles)
02Light-colored: white, gray, or tan
03Light enough to float on water — one of the only rocks that does

03 / Commonly confused with

Similar at first glance.

Compare the materials side by side.
Photographs are not shown to the same scale.

Pumice specimen for comparison — Pumice ↗
Extremely light, full of visible air pockets/holes (vesicles)

Scoria specimen for comparison — Scoria ↗
Full of visible holes (vesicles), similar to pumice

Scoria — a similar bubbly volcanic rock, but scoria is darker (basaltic composition) and denser, with larger, less-tightly-packed holes, and doesn't float.

04 / Out in the world

Where it’s found.

Explosive volcanic eruptions, especially rhyolitic/andesitic ones — common around the Pacific Ring of Fire, and often washes up on beaches after eruptions since it floats.

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