Pumice (rock)

Where Found

Pumice is found where volcanic rock and pyroclastic flows have occurred. Major U.S. regions include California, Arizona, New Mexico, Idaho, Oregon, Nevada, and Kansas. Top world producers include Italy, Greece, Iran, Chile, and the United States.

Primary Uses

More than half of the pumice used is in construction building block. The remainder is used in horticulture, concrete, abrasives, absorbents, filtration media, landscaping materials, laundry stones, and other miscellaneous uses such as beauty aids.

Technical Definition

When molten volcanic rock, or magma, under high pressure and charged with dissolved gases and vapors rises to the surface of the Earth (terminologically becoming “lava”), the gases escape rapidly as the lava is quickly cooling to a solid state. If the rates of cooling and gas escape are properly matched, the lava hardens to a glass foam, pumice. The gas bubbles that form within the volcanic glass are called vesicles. Vesicle size and vesicle wall thickness vary greatly.

Description, Distribution, and Forms

Pumice is a naturally occurring glass foam associated with volcanism. Block or fragmental pumice is porous and lightweight, commonly lighter than water, very abrasive because of the sharp edges of the thin vesicle (gas bubble) walls, and of low compressive strength.

Because of the violent character of pumice formation, pumice deposits may consist of large masses but more commonly consist of pumice fragments ranging from gravel to sand size. If pumice vesicle walls are thin, the density of pumice may be lower than that of water. The pumice vesicles are not normally connected, so water will not flow through the pumice, and pumice will commonly float on water indefinitely. Very fine-grained granular pumice is essentially volcanic ash and is called pumicite.

Pumice occurs in geologically young volcanic regions. In the United States, geologically young volcanic rocks occur only in and west of the Rocky Mountains, although volcanic ash deposits, blown eastward across the Rocky Mountains in the late Genozoic era, occur in the Great Plains. Some important pumice-producing areas are Mono Craters and Siskiyou, California; Flagstaff, Arizona; and Bend, Oregon. In Europe, important deposits occur in Italy, Germany, and Greece. Pumice is a relatively low-priced and low-density product. As a raw material, therefore, it cannot be economically shipped to markets distant from the source, so international trade is limited. However, because of increased demand for 2- to 8-centimeter, low-density pumice for fabric treatment, sources of pumice have been developed in Turkey, Latin America, and Indonesia.

History

The term “pumice” comes from pumex, Latin for foam. The resource has figured prominently in world history, specifically in the Mediterranean region. In fact, pumice has recently been studied as a historical place marker. Chemical tests allow scientists to trace pumice samples to their place of origin and therefore deduce whether or not different cultures of the Mediterranean were in contact with each other. For example, pumice used in Egypt can be traced to volcanoes of Greek island origin, linking the two civilizations as trading partners.

In 1883, the eruption of Krakatau, one of the strongest and most devastating volcanic eruptions on record, occurred in modern-day Indonesia. The sea was blanketed in pumice and rose approximately 1.5 meters. Other prominent historical events involving pumice include the eruption of Mount Vesuvius, in which pumice buried the city of Pompeii, and the eruption of a volcano on the island of Thíra, which occurred in 1500 or 1600 b.c.e.

Pumice was also used in champlevé, a finishing process for art pieces that involves enameling, first popularized by the Celts in Europe during the Roman period. Ferdinand von Hochstetter, a German geologist commissioned by the government of New Zealand to complete a geological survey of the country, first deduced that pumice was the result of volcanic expulsion of lava, based on his observations at Lake Taupo.

Obtaining Pumice

Pumice is essentially frozen or solidified volcanic lava bubbles and can therefore be extracted with relative ease. open-pit mining is the primary method of extraction. Pumice can then be crushed into a powder or shaped into blocks, depending on its intended use.

Uses of Pumice

By far, the largest volume use of pumice is for lightweight concrete aggregate. The use of cast concrete or concrete block incorporating significant quantities of pumice reduces structure weight and bearing-strength requirements for load-bearing elements of a structure. As compared with normal concrete blocks, lightweight blocks are easier to handle and are superior in terms of heat and sound insulation. Loose granular pumice, in fact, is used as insulation.

Small blocks of pumice, normally sawed to shape, are used as abrasives, and pumicite is a common ingredient of polishes, scouring powders, and other fine abrasives. Pumice fragments 2 to 8 centimeters in diameter and lighter than water are used to “stonewash” finished garments of stiff fabric such as heavyweight denim. The process softens the fabric. Other uses of pumice and pumicite relate to their high surface area per unit of volume. These uses include absorbents, fillers, and filter media.