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Most rocks sink when placed in water, but pumice is an exception. It forms during explosive volcanic eruptions when gas bubbles become trapped inside cooling lava. These tiny air pockets make pumice so lightweight that it can float on water for months or even years.
Large floating "pumice rafts" have been observed drifting across oceans after underwater volcanic eruptions. These rafts can even transport small marine organisms to new locations.
© United States Geological Survey