It's extremely porous, with a foamy appearance. Crushing pumice rock into a powder produces a substance called pumicite or simply volcanic ash. Pumice forms when super-heated, pressurized molten rock violently erupts from a volcano. Gases dissolved in magma mainly water and carbon dioxide form bubbles when pressure suddenly decreases, in much the same way carbon dioxide bubbles form upon opening a carbonated drink.
The magma swiftly cools, producing a solid foam. While pumicite may be produced by crushing pumice, it also occurs naturally. Fine-grained pumicite forms when magma containing a high concentration of dissolved gases suddenly depressurizes and cools. Pumice forms so quickly that its atoms often don't have time to organize into crystals. Sometimes there are crystals present in pumice, but most of the structure is amorphous, producing a volcanic glass called a mineraloid.
Pumice consists of silicates and aluminates. The silicic and felsic matter may include rhyolite, dactite, andesite, phonolite, pantellerite, trachyte, and less commonly basalt.
While pumice occurs in a variety of colors, it is almost always pale. Colors include white, gray, blue, cream, green, and brown. The pores or vesicles in the rock take two forms.
Some vesicles are roughly spherical, while others are tubular. Probably the most significant property of pumice is its low density. Pumice tends to be so light that it floats on water until its vesicles fill and it eventually sinks. Before it sinks, pumice can float for years, potentially forming huge floating islands.
Pumice rafts from the eruption of Krakatoa drifted around 20 years. Pumice rafting disrupts shipping and is important in the dispersal of marine organisms to new locations. Pumice occurs in everyday products and has many commercial uses. Stone-washed jeans are made by washing the denim with pumice rocks. The Greeks and Romans rubbed the rocks on their skin to remove unwanted hair.
Because the rocks retain water, they are valued in horticulture to grow cacti and succulents. Pumice is a lighter color, has a porosity near 90 percent and is less dense; scoria is denser with larger bubbles and thicker bubble walls and sinks rapidly unlike pumice, which initially floats.
If there are large amounts of gas present, pumice is created; when there is less gas, associated with less viscous magma, scoria is formed. Pumice can be formed rapidly and, in the past, large pumice rafts have been created from underwater volcanic eruptions like during the volcanic activity near Tonga in Collin Fitzsimmons has been writing professionally since , specializing in finance and the stock market.
Fitzsimmons earned a bachelor's degree in economics from the University of Virginia. Pumice, Volcanoes and Density Pumice is a unique rock, noted for its light weight and low density dry pumice can float in water. A List of Extrusive Igneous Rocks. How to Identify Obsidian. What is Pumice Used For? Pumice products: A variety of health and beauty products that contain pumice. They include the famous "Lava Soap" that cleans dirty hands with tiny pieces of pumice abrasive, a foot scrub cream that works as an exfoliant to smooth "sandal feet," two pumice stones, and a sponge with embedded pumice abrasive.
The largest use of pumice in the United States is the production of lightweight concrete blocks and other lightweight concrete products. When this concrete is mixed, the vesicles remain partially filled with air.
That reduces the weight of the block. Lighter blocks can reduce the structural steel requirements of a building or reduce the foundation requirements.
The trapped air also gives the blocks a greater insulating value. The second most common use of pumice is in landscaping and horticulture. The pumice is used as a decorative ground cover in landscaping and planters. It is used as drainage rock and soil conditioner in plantings.
Pumice and scoria are also popular rocks for use as substrates in hydroponic gardening. Pumice has many other uses. Together these account for less than a few percent of consumption in the United States, but these are the products that most people think of when they hear the word "pumice. Lots of people have found small pumice pebbles in the pockets of brand new "stone washed jeans," and almost everyone has seen the famous "Lava Soap" that uses pumice as an abrasive.
Below we list these and some of the other minor uses of pumice in no particular order. Pumice Reticulite: Reticulite is a basaltic pumice in which all of the bubbles have burst, leaving a honeycomb structure. Photograph by J. Griggs, USGS image. The best way to learn about rocks is to have specimens available for testing and examination.
Pumice is produced in two forms: rock pumice and pumicite. The word can be used synonymously with "volcanic ash. About , metric tons of pumice and pumicite were mined in the United States in The producing states were, in order of decreasing production:. All of the pumice production in the United States occurs west of the Mississippi River.
In , most of the pumice for consumption in the eastern United States was imported from Greece. In the eastern United States, expanded aggregate, produced by heating specific types of shale under controlled conditions, is used as a substitute for pumice in lightweight aggregate, horticultural, and landscaping applications.
Hendley II, and Peter H. Crangle, Jr. Find Other Topics on Geology. Maps Volcanoes World Maps. Rock, Mineral and Fossil Collections. Hardness Picks.
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