English: Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Pumice is an extrusive igneous rock. It has a felsic chemistry (the same as granite and rhyolite). Felsic igneous rocks are generally light-colored, have >65% silica (“silica” = SiO2 chemistry) (felsic has also been defined as >70% silica), are rich in potassium (K) & sodium (Na), and are dominated by the minerals quartz and K-feldspar. Some pumice samples are intermediate in chemistry (52 to 65% silica; also defined as 55 to 65% silica).

Unlike granite and rhyolite, pumice has a frothy texture - it has large numbers of tiny holes formed by the presence of numerous gas bubbles in the original volcanic lava. If cooling & solidification was completed before all the gas bubbles escaped to the lava surface, the result is a rock with lots of empty space (porosity). Typical pumice is actually a volcanic glass having a frothy texture - the solid portions are not crystalline. So, pumice has no minerals - it has the same elemental chemistry as granite and rhyolite, but it doesn’t have any minerals.

Pumice is typically whitish to light gray to very light brown in color. Most pumice samples are so lightweight that they float on water. After the August 1883 eruption of Krakatoa in Indonesia, sailors observed pumice floating in the Indian Ocean for many months.

Pumice is typically encountered in the form of small chunks littering the land surface around eruptive centers. They often occur mixed with ash in air-fall pyroclastic deposits or pyroclastic flow deposits.