pumice

A very light and porous igneous rock, usually granitic in chemical composition, formed by solidifying the froth caused by vigorous degassing of volatile substances from a lava during eruption. It is used as an abrasive.

Pumice is formed as pyroclastic material is ejected into the air as a froth containing masses of gas bubbles or vessicles, the lava solidifies quickly and the vessicles are contained in the rock. The basaltic version of pumice is known as scoria and has many differences due to mineralogy. Pumice varies in density according to the thickness of the solid material between the bubbles; After the explosion of Krakatoa rafts of pumice up to 25m long floated through the Pacific Ocean for up to two years, some with palm trees and other vegetation growing on them. In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts, some as large as 30 km that floated hundreds of miles to Fiji. Some pumice contains tubular and subparallel ones which give the pumice a silky fibrous texture due to extrusion and flowage of the pumice as it is formed.

Pumice is widely used to make lightweight concrete and as an abrasive, especially in polishes and cosmetics exfoliants. Pumice stones are often used in salons during the pedicure process to remove dry and excess skin from the bottom of the foot and also calluses. When used as an additive for cement, a fine-grained version of pumice called pozzolan is mixed with lime to form a light-weight, smooth, plaster-like concrete.