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Specimen of highly porous pumice from Teide volcano on Tenerife, Canary Islands. Density of specimen approx 0.25 g/cm³. Scale is in centimeters.
As commonly found on the Eastern coast of Australia
Illustrates the porous nature in detail
A 10 centimetre (6 inch) piece of pumice supported by a rolled-up U.S. 20-dollar bill demonstrates its very low density.
Pumice from Kos, Greece

Pumice (pronounced /ˈpʌmɨs/) is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases (including water and CO2) dissolved in the lava, so that they rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink is opened). The simultaneous cooling then freezes the bubbles in the matrix.



Pumice is composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. It is commonly, but not exclusively of silicic or felsic to intermediate in composition (e.g., rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but basaltic and other compositions are known. Pumice is commonly pale in color, ranging from white, cream, blue or grey, to green-brown or black. It forms when volcanic gases exsolving from viscous magma nucleate bubbles which cannot readily decouple from the viscous magma prior to chilling to glass. Pumice is a common product of explosive eruptions (plinian and ignimbrite-forming) and commonly forms zones in upper parts of silicic lavas. Pumice has an average porosity of 90%, and initially floats on water.

Scoria differs from pumice in being denser, with larger vesicles and thicker vesicle walls; it sinks rapidly. The difference is the result of the lower viscosity of the magma that forms scoria. When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite. Pumice is considered a glass because it has no crystal structure. Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water. After the explosion of Krakatoa, rafts of pumice drifted through the Pacific Ocean for up to 20 years, with tree trunks floating among them.[1] In fact, pumice rafts disperse and support several marine species.[2] In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts, some as large as 30 kilometres that floated hundreds of kilometres to Fiji. [3]

There are two main forms of vesicles. Most pumice contains tubular microvesicles that can impart a silky or fibrous fabric. The elongation of the microvesicles occurs due to ductile elongation in the volcanic conduit or, in the case of pumiceous lavas, during flow. The other form of vesicles are subspherical to spherical and result from high vapor pressure during eruption.


Pumice is widely used to make lightweight concrete or insulative low-density breeze blocks. When used as an additive for cement, a fine-grained version of pumice called pozzolan is mixed with lime to form a light-weight, smooth, plaster-like concrete. This form of concrete was used as far back as Roman times. Roman engineers used it to build the huge dome of the Pantheon and as construction material for many aqueducts.

It is also used as an abrasive, especially in polishes, pencil erasers, cosmetic exfoliants, and to produce stone-washed jeans. "Pumice stones" are often used in beauty salons during the pedicure process to remove dry and excess skin from the bottom of the foot as well as calluses. It was also used in ancient Greek and Roman times to remove excess hair.[4] Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners (such as Lava soap) as a mild abrasive.

External links


  1. ^ DeVantier, L.M. (September 1992). "Rafting of tropical marine organisms on buoyant coralla". Marine Ecology Progress Series 86: 301–302. doi:10.3354/meps086301. Retrieved 2007-07-14. ""trunks and pumice that washed ashore at (Keeling) Atoll in the early 1900's had been drifting for some 20 years, since the eruption of Krakatoa 1000 km to the northeast in 1883 (Wood-Jones 1912)."". 
  2. ^ Bryan, S.E.; A. Cook, J.P. Evans, P.W. Colls, M.G. Wells, M.G. Lawrence, J.S. Jell, A. Greig, R. Leslie (2004). "Pumice rafting and faunal dispersion during 2001–2002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga". Earth and Planetary Science Letters 227: 135–154. doi:10.1016/j.epsl.2004.08.009. Retrieved 2007-07-14. ""The abundance and variety of fouling taxa, coupled with the long dispersal trajectory (>3500 km) and period of pumice floatation (≥1 year), confirm the importance of sea-rafted pumice as a long-distance dispersal mechanism for marine organisms"". 
  3. ^ "New Island and Pumice Raft in the Tongas". NASA Earth Observatory. National Aeronautics and Space Administration. 16 November 2006. 
  4. ^ Sherrow, V. 2001. For appearance' sake: the historical encyclopedia of good looks. Greenwood, 312pp., ISBN 1573562041

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

PUMICE (Lat. pumex, spumex, spuma, froth), a very porous, froth-like, volcanic glass. It is an igneous rock which was almost completely liquid at the moment of effusion and was so rapidly cooled that there was no time for it to crystallize. When it solidified the vapours dissolved in it were suddenly released and the whole mass swelled up into a froth which immediately consolidated. Had it cooled under more pressure it would have formed a solid glass or obsidian; in fact if we take fragments of obsidian and heat them in a crucible till they fuse they will suddenly change to pumice when their dissolved gases are set free. Hence it can be understood that pumice is found only in recent volcanic countries. Artificial substances resembling pumice can be produced by blowing steam through molten glass or slag, and when a mass of slag is suddenly cooled by being tipped into the sea (as is the case at the blast furnaces of Whitehaven in Cumberland) it swells up into a pumiceous form so light and full of vesicles that it will float on water. Any type of lava, if the conditions are favourable, may assume the pumiceous state; but basalts and andesites do not so often occur in this form as do trachytes and rhyolites. Pumices are most abundant and most typically developed from acid rocks; for which reason they usually accompany obsidians, in fact in Lipari and elsewhere the base of a lava flow may be black obsidian while the upper portion is a snow white pumice.

Small crystals of various minerals occur in many pumices; the commonest are felspar, augite, hornblende and zircon. If they are abundant they greatly diminish the economic value of the rock, as they are hard and wear down more slowly than the glassy material; consequently they produce scratches. The cavities of pumice are sometimes rounded, but may also be elongated or tubular owing to the flowing movement of the solidifying lava. The glass itself forms threads, fibres and thin partitions between the vesicles. Rhyolite and trachyte pumices are white, contain 60 to 75% of silica and the specific gravity of the glass is 2.3 to 2.4; andesite pumices are often yellow or brown; while pumiceous basalts, such as occur in the Sandwich Islands, are pitch black when perfectly fresh.

Good pumice is found in Iceland, Hungary, Nevada, Teneriffe, New Zealand, Pantellaria and the Lipari Islands. The last-named are the chief sources of pumice for the arts and manufactures. At Campo Bianco in Lipari there is an extinct volcanic cone with a breached crater from which a dark stream of obsidian has flowed. For industrial purposes the best varieties are obtained from Monte Pelato and Monte Chirica. The pumice is extracted by means of shafts and tunnels driven through the soft incoherent stone. It is brought out in blocks of irregular shape and size and is trimmed into slabs and graded into several qualities before it is exported to Canneto, which is the centre of the pumice trade. The workmen say that the good pumice occurs in beds or veins, which are probably lava flows and are separated by valueless rock or by obsidian. The value depends entirely on the regularity, size and shape of the steam cavities and on the absence of minute crystals. From time immemorial the extraction and sale of pumice have been one of the principal sources of wealth to the inhabitants of this island. An inferior pumice, known in Lipari as Alessandrina, is used for smoothing oilcloth. Though all the Aeolian Isles are volcanic no pumice is exported from any of the others. In Iceland, Teneriffe and Hungary pumice also occurs, but not in sufficient quantity or of such quality as to render it worth working on a large scale. It is estimated that in Lipari there are 170 pumice quarries (or mines) giving employment to 1200 persons and producing 6000 tons of pumice per annum. The price varies with the quality: from 3 lire per Too kilogrammes for the commonest sorts to 200 or 300 lire for the best pieces, the average being about 15 lire. Much pumice is also used nowadays in the form of a fine powder, produced by crushing the rock, and forms an ingredient of metal polishes and some kinds of soap. It is often confounded with diatom earth or tripoli powder, but can easily be recognized by the aid of the microscope or by simple chemical tests.

Among the older volcanic rocks pumice occurs, but usually has its cavities filled up by deposits of secondary minerals introduced by percolating water; hence it is of no value for industrial purposes. Pumice, in minute fragments, has been shown to have an exceedingly wide distribution over the earth's surface at the present day. It occurs in all the deposits which cover the floor of the deepest portion of the oceans, and is especially abundant in the abysmal red clay. In some measure this pumice has been derived from submarine volcanic eruptions, but its presence is also accounted for by the fact that pumice will float on water for months, and is thus distributed over the sea by winds and currents. After a long time it becomes waterlogged and sinks to the bottom, where it gradually disintegrates and is incorporated in the muds and oozes which are gathering there. After the great eruption of Krakatoa in 1883 banks of pumice covered the surface of the sea for many miles and rose in some cases for four or five ft. above the water level. In addition to this much finely broken pumice was thrown into the air to a great height and was borne away by the winds, ultimately settling down in the most distant parts of the continents and oceans. (J. S. F.)

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Simple English

volcano on Tenerife, Canary Islands. Density of specimen approx 0.25 g/cm³. Scale is in centimeters.]]
File:Pumice stone
Illustrates the porous nature in detail

Pumice is a term for a lightweight volcanic rock. It is a solidified frothy lava, created when highly-heated, pressured molten rock is shot out from a volcano. Its light weight is due to air spaces, which are caused by fast cooling and loss of pressure. The loss of pressure makes bubbles by lowering the boiling point of the lava (alike to the bubble-creation when a carbonated drink is opened). The cooling then freezes the bubbles in the matrix.


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