Pumice is a type of extrusive volcanic rock, produced when lava with a very high content of water and gases (together these are called volatiles) is extruded (or thrown out of) a volcano. As the gas bubbles escape from the lava, it becomes frothy. When this lava cools and hardens, the result is a very light rock material filled with tiny bubbles of gas. Pumice is the only rock that floats on water, although it will eventually become waterlogged and sink. It is usually light-colored, indicating that it is a volcanic rock high in silica content and low in iron and magnesium, a type usually classed as rhyolite. If the lava hardens quickly with few volatiles, the resulting rock is volcanic glass, or obsidian. Pumice and obsidian are often found together.
In commerce, pumice is the term applied to larger pumice stones, while pumicite consists of fine grains or ash. Pozzolan is a fine-grained pumicious material (both natural and man-made), which combines with lime to make a smooth, plaster-like cement. These three similar materials may be found and mined together, but they have different characteristics and different uses.
The name pumice is derived from the Latin word pumex, meaning foam. Pozzolan (or pozzolana) is an Italian word, named from Pozzuoli, the place near Naples where pozzolan was first mined and used as cement, during Roman times.
Since pumice is a volcanic rock, and retains its useful properties only when it is young and unaltered, pumice deposits are found in areas with young volcanic fields. Worldwide, over 50 countries produce pumice products. The largest producer is Italy, which dominates pozzolan production and also produces some pumice. Other major pumice producers are Greece, Chile, Spain, Turkey, and the United States.
In the United States, Arizona, California, New Mexico and Oregon are the major producers of pumice, accounting for the majority of the nation’s pumice and pumicite production.
Pumice and pumicite are used to make lightweight construction materials such as concrete block and concrete. About three-quarters of pumice and pumicite is consumed annually for this purpose.
The remainder of the pumice mined is used in abrasives, horticulture, landscaping, and for washing blue jeans.
Pozzolan is used to make fine-grained, lightweight cement for finishing floors and building interiors.
Substitutes and Alternative Sources
Expandable shale can be substituted for pumice and pumicite in the building block and concrete applications. There is no lack of pumice and pumicite, as world resources are extensive. However, the costs related to mining and trucking the material from the mine to processing plants and the market will determine whether pumice from a particular mine is cheap enough to use. In other words, it is economics, not the abundance of pumice, which determines whether or not substitutes for pumice are necessary.
- Common Minerals and Their Uses, Mineral Information Institute.
- More than 170 Mineral Photographs, Mineral Information Institute.
Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Mineral Information Institute. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Mineral Information Institute should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.