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Pumice



 

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Pumice is a textural term for a volcanic rock that is a solidified foam 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 occurrences of basaltic and other compositions are known. Pumice is commonly pale in color, ranging from white, cream or grey, but can be green brown or black. It forms when 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 formed 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 km that floated hundreds of miles 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 vapour pressure during eruption.

Uses

Pumice is widely used to make lightweight concrete or insulative low-density 'breeze-block' type bricks. 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.

It is also used as an abrasive, especially in polishes, cosmetics exfoliants, and for stone-washed jeans. "Pumice stones" are often used in salons during the pedicure process to remove dry and excess skin from the bottom of the foot as well as calluses. Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners as a mild abrasive. Perhaps the most famous product advertised to contain pumice is Lava soap. It is a heavy-duty hand soap, sold in both bar and liquid form, for mechanics and others who get very dirty hands.

See also

References

  1. ^ DeVantier, L.M. (September 1992). "Rafting of tropical marine organisms on buoyant coralla". Mar. Ecol. Prog. Ser 86: 301-302. Retrieved on 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. Retrieved on 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, 2006. 
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pumice". A list of authors is available in Wikipedia.
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