Pumice is the volcanic rock that can float in water because it is packed with gas bubbles.
Pumice forms when gas-rich magma is rapidly decompressed and cooled during an eruption. As pressure falls, dissolved gases such as water vapour and carbon dioxide come out of solution, creating countless bubbles. The surrounding melt cools into volcanic glass before those bubbles can escape, leaving a lightweight, sponge-like rock called pumice.
Its buoyancy depends on its unusually high porosity. Many pumice samples contain roughly 64–85 percent pore space by volume, so their overall density can be lower than water. Eventually, water may enter the pores and the rock can become waterlogged and sink. Underwater eruptions can produce large pumice rafts that drift across oceans.
Pumice is often confused with scoria, another bubbly volcanic rock. Scoria is generally darker, denser and has thicker bubble walls because it usually forms from less silica-rich magma. Pumice is commonly pale and glassy, and its rough texture has made it useful as an abrasive and in lightweight construction materials.