Pumice's a textural term for a volcanic rock that's a solidified frothy lava typically created when super-heated, highly pressurized rock's violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation's due to the simultaneous actions of rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the boiling point of the lava (akin to the bubble-creation when a carbonated drink's opened). The simultaneous cooling then fr… (More on Pumice)