Pumice is the volcanic rock famous for being light enough to float on water.
Pumice forms when gas-rich magma erupts and depressurizes rapidly. Bubbles expand through the molten material, and the lava cools before those cavities can collapse. The result is a highly vesicular rock whose connected or enclosed pores can make its average density lower than water.
Most pumice is associated with explosive eruptions of silica-rich magma, although the exact composition varies. Floating pumice rafts can travel across oceans, sometimes carrying marine organisms with them. Over time, water may fill the pores, causing the rock to sink.
Pumice differs from scoria, another vesicular volcanic rock. Scoria generally has larger, less connected vesicles and is commonly darker and denser. Pumice’s pale color and abundance of tiny bubbles are its familiar identifying features.