Pumice is a highly vesicular pyroclastic igneous rock of intermediate to siliceous magmas including rhyolite, trachyte and phonolite. Pumice is usually light in colour ranging from white, yellowish, grey, grey brown, and a dull red. Pumice has an average porosity of 90%. Pumice is formed as pyroclastic material is ejected into the air as a froth containing masses of gas bubbles or vessicles, the lava solidifies and the vessicles are contained in the rock. The basaltic version of pumice is known as scoria and has many differences due to mineralogy. When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite. Pumice is considered a glass because it has no crystal structure. Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water. During 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.

There are two main forms of vesicles. 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. The other form of vesicles are subspherical to spherical and result from high vapour pressure during eruption.

Pumice is widely used to make lightweight concrete and as an abrasive, especially in polishes and cosmetics exfoliants. 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. This form of concrete was used as far back as Roman times.

Igneous rocks | Volcanology

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