Pumice is a very low-density, form-like, mostly light-colored glassy volcanic rock with a vesicular texture, not a mineral. This rough, porous rock forms from mostly silicic, high-viscosity, volatile-rich lava during an explosive volcanic eruption. The explosive eruption ejects blobs of lava, which rapidly froth up, cools, and solidifies, entrapping the numerous gas bubbles in a glassy matrix.
The origin of the name is pūmex, which means pumice and is related to spūma, meaning foam, denoting the foam-like or vesicular appearance of these volcanic glass rocks and not their mineralogy. On the other hand, pumiceous means resembling pumice.
Some of the uses of pumice are in making lightweight aggregate, horticulture (gardening and landscaping), personal care (pumice stones and soaps), polishing and abrasive products, dental care, and water filtration, among others.
Learn about pumice, including what it is, how it forms, its mineralogy, and its chemical composition. We will also discuss occurrence, mining, its uses, and prices.
Quick facts and properties
- Name: Pumice
- Pronunciation: /ˈpʌmɪs/
- Rock type: Extrusive igneous or volcanic rock
- Colors: White, creamy white, grayish, green-brown, blue, or black
- Texture: Vesicular
- Porosity: 70 > 90%
- Hardness: 5-5.5 on the Mohs hardness scale but has a lower compressive strength than the obsidian or non-vesiculated counterpart.
- Specific gravity: <1 and depends on vascularity and amount of glass between voids.
- Chemical composition: Mostly silicic but can be intermediate or rarely basaltic.
- Mineral composition: Glassy, but it may be aphanitic.
- Cooling rate: Rapid, which prevents mineral formation/crystallization
- Optical properties: it has translucent glass that forms the walls of the cavities.
- Tectonic environment: Subduction zones (such as the Pacific Ring of Fire) on the convergent boundary and intracontinental rifts and hotspots.
Related terms
Some of the related terms and their meanings include the following:
1. Pumicite
Pumicite is fine-grained pumice, usually with small particles – grains, flakes, shards, or threads – less than 4mm in size. Sometimes, those not more than 0.25mm that often remain suspended in the air for a long time and can travel far are pumice dust. Pumicite commonly forms when eruptions have lots of gases.
2. Pumice rafts
Floating pumice that can be lapilli-size to kilometers in diameter floating in water resulting from an eruption in islands or submarine environments. Examples include 1979, 1944, and 2006 from the Home Reef volcano eruptions in Tonga created rafts that went hundreds of kilometers to Fiji and Krakatau in 1883, whose rafts reached the coast of Africa after floating for 20 years. Such rafts do support and disperse marine life.
Perhaps some of the largest pumice rafts are 300 miles (482 km) long and over 30 miles (48km) wide, spotted in August 2012 near Raoul Island by a Royal New Zealand Navy member, possibly from the 2012 Havre seamount eruption, as reported by NBC News.
3. Pumice cones
These are cinder cones (steep-sloped hills) formed predominantly by pumice falling near the vent.
What is pumice?
Pumice is felsic to intermediate low-density, highly vesiculated glassy volcanic rock with a frothy appearance, rarely mafic with or without crystals. Acidic or silica-rich pumice are light-colored, i.e., whitish, creamy to light gray; intermediate will be a bit dark, i.e., dark gray, blue, greenish-brown, while those poor in silica (mafic) will be blackish.
These volcanic rocks measure from minute particles to lapilli to larger blocks or stones and have macroscopic to microscopic vesicles, with most having microvesicles that give a fibrous or silky fiber.
Large cavities are most likely nucleated first and grow by diffusion of supersaturated gases through the vesicle wall while still in molten form. In contrast, coexisting smaller nucleated later and didn’t get time to grow before they solidified.
Pumice vesicles may be spherical, spheroidal, flattened, subparallel, tubular, or rodlike, surrounded by thin, filament, fiber, or threadlike glass, with the smallest often round/subspherical. The shape of the voids depends on flow and vapor pressure inside the bubbles. For instance, spherical indicates high vapor pressure during formation, while fibrous lower pressure.
On the other hand, distortion, such as elongation and flattening of vesicles, indicates flow or movement (stretch and shear) before solidification/vitrification.
Most natural pumice has 70-80% bubbles and a surface area of 0.5 m2/g. If uniform-sized spheres, the maximum packing will be 74% of the total rock volume, and for nonuniform, 85%. Rod-shaped vehicles can pack up to 93%. Vesicle packing above 93% occurs in fibrous pumice only.
Factors like rate of ascent, volatile concentration, solubility, and magma viscosity will impact vesicular density. Their size depends on the volumetric expansion of fluid present, viscosity (resists bubble wall stretching), and coalescence (merging of bubbles). Also, diffusion of volatiles into the bubble to add more mass, rate of magma ascent/decompression, Ostwald ripening, and concentration and solubility of volatiles play roles.
Pumice composition
Here is the chemical and mineral composition of pumice:
1. Chemical composition
Pumice is a frothy volcanic glass that forms mainly from highly viscous felsic or silicic to intermediate magmas like rhyolitic, andesitic, dacitic, trachytic, pantellerite, or phonolitic magmas and rarely from mafic magmas like basalt. For instance, Tenerife in the Canary Islands, Germany, has phonolitic pumices.
Thus, pumice composition will mainly be silica (SiO2) 55-77 wt.% and considerable amounts of aluminum oxide (Al2O3). Also, they have potassium oxide (K2O), sodium oxide (Na2O), calcium oxide CaO, magnesium oxide MgO, sulfur trioxide (SO3), and iron oxide (Fe2O3) in varying amounts depending on which magma it is.
However, if of mafic origin, then it will have a lower amount of silica, 45–52 wt.%, with more of the mafic elements like calcium, iron, and magnesium and less of the felsic constituents like sodium and potassium.
On the other hand, rhyolitic magmas, which are felsic, will tend to have more silica, sodium, and potassium and less of the mafic elements like calcium, iron, and magnesium. Intermediate magmas will have a composition in between mafic and felsic.
Going to specific examples, the chemical composition of pumice from El-Arish, North Sinai, Egypt, according to Ismail et al. (2014), is silicon dioxide 70.97%, titanium oxide 0.14%, aluminum 14.24%, iron oxide 1.88%, calcium oxide 1.37%, magnesium oxide 0.35%, potassium 4.46%, sodium 4.02% and others 2.41%. This example was probably rhyolitic, going by the higher amount of silica.
2. Pumice mineral composition
Pumice is predominantly frothy volcanic glass (amorphous solid). The rapid cooling and solidification prevented the crystallization of minerals. However, it may have small crystals of feldspar, hornblende, zircon, augite, plagioclase, orthopyroxene, etc. These minerals formed at greater depths at a slower cooling rate as the magma rose.
However, some pumice specimens may have some minute microlite or quartz if it comes to near a solid form as they ascent due to loss of volatiles.
For instance, during the 1980 Mt. St. Hellen eruption, samples had 30% hornblende, plagioclase, and orthopyroxene phenocrysts formed at depth, while in-between vesicle groundmass is a volcanic glass without microlite or quartz. This shows cooling was so rapid that crystals didn’t develop.
Lastly, cavities may fill with minerals such as zeolites, opal, calcite, and prehnite, deposited by seepage of hydrothermal solutions forming amygdules.
How does pumice form?
Pumice forms when frothy (vapor or gas-filled bubbles) molten (liquid) lava rapidly cools and freezes, entrapping numerous escaping gas bubbles and forming a vesiculated or form-like glassy rock.
Usually, a rise in magma or lava results in depressurization. As pressure drops, dissolved volatiles (water and gases) in magma or lava will exsolve due to reduced solubility, forming bubbles that will expand as they rise and escape.
These rapidly exsolving and expanding gases produce the force that blasts lava into the air. An analogy is opening a Coke or beer. It will froth because pressure drops and gas exsolves, forming bubbles trying to escape.
This rock forms during an explosive eruption of high-viscosity lava via rapid cooling and solidification of frothy 1) molten fragments spewed into the air and 2) magma flows.
1. From frothy airborne lava
It is the most common way that pumice forms and the most hazardous since they, together with other pyroclasts, will eventually fall like rainfall.
How exactly does it happen? It is simple. As superheated, highly pressurized, and highly viscous magma supercharged with volatiles rises and pressure drops.
Depressurization causes volatiles will exsolve and expand, causing an explosive eruption that will eject pyroclasts, including frothy lava, ash, blocks, and bombs or accidental clasts, into the air. While airborne, rapid cooling and solidification of frothy molten lava fragments will entrap or freeze the gas bubbles surrounded by thin glassy walls or matrix.
2. Lava flows
Towards the end of a volcanic eruption or involves highly viscous or pasty lava, some lava may flow down the volcano. Depressurization will make volatiles present exsolve, expand, and move towards the surface, making the near-surface layer frothy. Rapid cooling and quenching of the foamy surface will form pumice.
Pumice from lava flows will be interlayered by obsidian if of rhyolitic composition. Similarly, you will find obsidian with pale pumiceous layers.
3. Where does it form?
Pumice can form in continental, submarine, or marine environments during explosive volcanic eruptions on high-viscosity magmas, mostly felsic to andesitic. These eruptions will eject pyroclasts with pumice, ash, and other fragments and may also form pyroclastic flows of these ejecta.
Pumice often occurs in Plinian, the most powerful or explosive eruption with columns above 15km, common in evolved magmas like dacite, phonolitic, or rhyolitic, high in volatile content. Notable examples that are mostly Plinian include Mount St. Helens (May 18, 1980) and c (1912) in the US, Mount Vesuvius (79AD) in Italy, Krakatoa (1883), Mount Tambora (1815) in Indonesia and Mount Pinatubo (1991) in the Philippines.
However, pumice ejection also occurs in Surtsey explosive eruption in shallow lakes and seas. Examples include Examples of Surtsey (Iceland) and Taal Volcano (Philippines), Fukutoku-Okanoba (Japan) Hunga Tonga–Hunga Haʻapai (Tongan archipelago) (also Plinian). Some of these eruptions are phreatomagmatic, such as Surtsey.
Lastly, Vulcanian eruptions are known to eject pumice, but their eruption column doesn’t go beyond 15 km, while Peléan eruptions often create pumice cones, as seen at Mount Lamington (2021). However, Strombolian and Hawaiian eruptions will typically produce scoria.
4. Associated formation hazards
Plinian explosions, common in evolved magma like dacite, rhyolite, and phonolitic with the highest volatile content, produce various pyroclasts with copious pumice volume, eruption column goes above 15km, unlike Vulcanian. The falling pumice, volcanic ash, and other pyroclasts can be hazardous, like how the Mt. Vesuvius eruption in 79AD buried residents of Pompeii.
Even ash and pumice flows are deadly, but not as falls. They form as the eruption column is nearing collapse, including Nuées ardentes, form part of a ground-hugging potion. A pyroclastic flow with more than 50% pumice lapilli and blocks is a pumice flow.
Lastly, floating rafts and marine or submarine eruptions may be shipping hazards and affect marine life.
Where does pumice occur?
In 2019, USGS.GOV reported that the United States produced about 510,000 tons of pumice, whose estimated processed value was USD 17 million, mainly from Nevada, Oregon, New Mexico, Arizona, Kansas, Idaho, and California and imported about 110,000 tons from Greece.
Italy, Greece, Iran, Syria, and Chile are leading producers of pumice. In other European countries, it occurs in Spain, France, Germany, Turkey, Iceland, Hungry, and Russia (Kamchatka Peninsula), while in Asia, it is in Indonesia, Joran, Saudi Arabia, Japan, and Afghanistan.
Also, it occurs in New Zealand, the Caribbean Islands, Uganda, Algeria, Ethiopia, Chile, Cameroon, Ecuador, Guadeloupe, Guatemala, etc.
Open pit mining
Pumice exists as loose aggregate on the Earth’s surface, and mining is done by open pit without blasting, making it simple and environment friendly. The process requires removing overlaying dirt with machinery and scalping screens to rid any impure surficial materials. Then, you will use power shovels and boulders in its mining.
Once mined, it runs through crushes, shakers, and screens for crushing, separation, and grading, depending on the use before packaging. Some may require cutting into blocks without crushing.
What is pumice used for?
The neutral pH, inert, porosity, abrasive, and lightweight non-compact properties of pumice make it usable in many areas. Also, you can crush it without losing its properties.
Some of the uses of pumice include:
- Low-weight aggregate: One of the leading uses of pumice is making aggregate for low-weight concrete, masonry units/blocks, and other assorted lightweight building materials. It insulates against heat, reducing power requirements and reducing the need for structural steel to be lighter,
- Horticulture and landscaping: It’s porous, neutral, improves drainage, and holds nutrients, making it good for adding into soils, including succulents, cacti, aroids, etc., or as a hydroponic gardening medium. It will make tillage easier, reduce erosion, doesn’t compact, and naturally improves soil fertility. Also, you can use it for landscaping, as a decorative mulch, or making ground stable in high-traffic areas.
- Beauty and personal care: Pumice stone is a popular gentle skin exfoliator to remove dead skin (including on the bottom of the feet, knees, elbows, and calluses), thin warts, prevent ingrown hair, stretchmarks, and other pedicure uses. Also, they help de-pill or remove lint from sweaters or furniture and remove pet hair, while pulverized or crushed forms make body and hand scrubs like Aesop and GoJo.
- Cleaning products: You can use a pumice stone, powder, or pastes to scrub porcelain and tile to remove rust, limescale, rings from hard water, etc., instead of vinegar, baking soda, borax, or other cleaning agents. Some mild scrubbing products also have it.
More uses are in paint, rubber, and cement (pozzolan) additive, an industrial abrasive and polishing material, in pencil erasers, treating stonewashed jeans, and rubbing compounds. Also, it uses water and wastewater filtration, blast mitigation, abrasive polishes, agrochemical carriers, dentistry, chinchilla dust baths, etc.
What is the cost of pumice?
According to USGS.GOV, the price of pumice per ton averaged USD 33 per ton if locally sourced and about USD 44 if imported. However, pumice stones for horticulture (gardening or planting) will start from about US$1.5 per quartz, depending on size, brand, and where you buy it.
If you need a pumice stone for feet, scrubbing toilet, etc., prices range from USD 2-10 per piece. Those with handles may cost a little more. You should find them in CVS, Walmart, Target, Amazon, Etsy.com, Lowe’s, Walgreens, and other big box stores or pharmacies. However, Electric ones will cost USD10-50.
Frequently asked questions
Rhyolitic pumice is light gray even though its vesicle-free counterpart is black because bubble formation expands glass film between these bubbles, which will refract and diffuse light to make it appear gray. An analogy would be the breaking waves that form a whitecap in dark seawater.
Unlike scoria, this rock floats in water, its mafic counterpart, because of the very high vesiculation with isolated gas-trapped voids. It is the high viscosity that makes these gas bubbles isolated. However, once waterlogged after many years, it will sink. Once at the bottom of the ocean, they will slowly weather and get incorporated into ocean oozes and mud.
The term refers to reticulite, thread-laced scoria with 95-98% voids, not spheres but three-dimensional polyhedral-shaped with very thin intervening glass filaments that sink in water.
References
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Okrusch, M., & Frimmel, H. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits (1st ed.). Springer.
- E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Brown, R. J., & Calder, S. E. (2004). Pyroclastics. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5). Elsevier Academic
- Fisher, R. V., & Schmincke, H.U. (1984). Pyroclastic rocks (1st ed.). Springer-Vlg.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology (2nd ed.). Pearson Education.
- Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub.
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology (15th ed.). McGraw-Hill/Education, Inc.
- Ismail, A. I., El-Shafey, O. I., Amr, M. H., & El-Maghraby, M. S. (2014). Pumice characteristics and their utilization on the synthesis of mesoporous minerals and on the removal of heavy metals. International Scholarly Research Notices, 2014, 1–9. https://doi.org/10.1155/2014/259379
- Ries, N. F. G., & Buettner. (2020). The world of mineral deposits: A beginner’s Guide to Economic Geology (1st ed.) Springer International Publishing.