Pumice is a type of igneous, extrusive rock. This means it forms above the earth's surface when volcanoes erupt, and the lava solidifies. Pumice rock is made of lava abundant in dissolved gas and volatile substances. Immediate depressurization at the time of eruption causes these substances to exolve, producing gas bubbles in the lava. The rapid cooling of this frothy lava gives the pumice stone its porous texture and ability to float in water.
Texture
The word "pumice" comes from the Latin word "pumex," which means "foam." Pumice does not get its name from its composition, but from its texture. Pumice is considered a volcanic glass because the quick cooling of the viscous lava from which it is formed does not allow for crystallization. When molecules do not have enough time to crystallize, the result is glass.
Classification
Pumice is classified as a rhyolite. Rhyolite is a volcanic rock with high concentrations of silica. Silica is quartz; it has the chemical formula SiO2. The high amount of silica gives pumice its light, usually gray color. Other colors of pumice include white, yellow, brown and dull red.The crystals of rhyolites like pumice are microscopic because the lava they come from cools so quickly; the faster lava cools, the smaller the crystals are. Pumice and all other rhyolites can be described as aphanitic, meaning their crystals cannot be seen by the unaided eye.
Composition
Rhyolite has the same composition as granite, another igneous rock. Since granite and rhyolite are chemically similar, the lava that produces pumice includes the minerals feldspar, quartz, biotite and horneblend. More specifically, pumice is made of the following oxides, according to Indian Granites: 70 to 77 percent silica, 11 to 14 percent alumina, 3 to 5 percent potassium oxide, 3 to 5 percent soda, 1 to 3 percent ferrous oxide, 1 to 2 percent ferric oxide, .5 to 1 percent magnesia, less than .38 percent titina and .03 percent water.
Varieties
The amount of gas in the lava and the activity of the lava at the time of solidification determines how the pumice turns out. The vesicles, or cavities left behind by gas bubbles, in pumice, can either be spherical or tubular. The spherical shape comes from the rapid evolving of the gases during eruption, and spherical bubbles are stretched into tubes as the lava flows. Lava with a higher gas content produces a finer-grained type of pumice called pumicite. When the lava does not contain gas, pumice is not produced; obsidian, another type of volcanic glass, forms upon cooling.
Function
The lightweight and porous composition of pumice makes it popular for many uses. The majority of pumice consumed is used in construction to make concrete and cement. Pumice is also commonly used as an exfoliant; these rocks can remove dead cells and calluses from the skin. According to Buzzle, ground pumice is often added to "abrasive cleaners, polishes, hand soaps and scrubs, thereby reducing the amount of extra chemicals in these products in order to help scrub off stains, dirt and grease." Textile manufacturers use pumice to stonewash denim, wearing out stiff new pairs of jeans to give them a soft, flexible feel.
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