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Pumice cools so quickly that atoms in the melt are unable to arrange themselves into a crystalline structure, resulting in an amorphous volcanic glass known as a "mineraloid."
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At rare times, pumice can erupt from gas-charged magmas of basaltic and andesitic composition.
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Following the enormous rush of high-pressure gas from the vent of a volcano, the magma shreds and is blown out as a molten froth, it then solidifies as it flies through the air falling to the Earth as pumice.
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The size of pumice can range from tiny dust particles to pumice pieces the size of a house. Many cubic kilometers of pumice can be ejected during large volcanic eruptions.
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The volcanic eruption at Mount Pinatubo in 1991 resulted in a cubic mile of ash and pumice lapilli being released into the atmosphere and falling to the surface blanketing the countryside.
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The eruption of Mount Mazama over 7,500 years ago started as a single vent as a towering column of pumice and ash that reached 30 miles into the air. The area's valleys around the mountain filled with up to 300 feet of pumice and ash.
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There have been instances of large amounts of pumice being produced by some island and subsea eruptions that will float on the surface and then pushed by the winds.
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Pumice can float for very long periods of time, some floating for years before becoming waterlogged and sinking.
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Large masses of floating pumice are able to be tracked by satellites and are referred to as "pumice rafts." They can be a hazard to ships that sail through them.
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Pumice use in the United States is most often used for the production of lightweight concrete blocks and other concrete products. The concrete is mixed and the vesicles remain partially filled with air.
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Lighter building blocks of pumice reduce the structural steel requirements of buildings, as well as the trapped air also giving the blocks a greater insulating value.
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Pumice is also used in landscaping and horticulture used as a decorative ground cover in landscaping and planters.
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Pumice is also used as drainage rock and soil conditioner in plantings, and as a popular rock, along with scoria, for use as substrates in hydroponic gardening.
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Other minor uses of pumice are included abrasives in conditioning in stone-washed denim, in some bar and liquid soaps, in pencil erasers, in skin exfoliating products, and in polishing substances.
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Pumice is also used as a traction material on snow-covered roads, traction enhancer in tire rubber, absorbent in cat litter, and many other minor uses.
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In 2011, there was about 500,000 metric tons of pumice and pumicite mined in the United States, mostly in Oregon, Nevada, and Idaho, and other states west of the Mississippi River.
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