Pumice

Pumice is a textural term for a volcanic rock that is a solidified frothy lava 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, 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. [cite journal |last=DeVantier |first=L.M. |authorlink= |coauthors= |year=1992 |month=September |title=Rafting of tropical marine organisms on buoyant coralla |journal= Mar. Ecol. Prog. Ser |volume=86 |issue= |pages=301–302 |id= |url=http://www.int-res.com/articles/meps/86/m086p301.pdf |accessdate=2007-07-14 |quote="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)." |doi=10.3354/meps086301] In fact, pumice rafts disperse and support several marine species. [cite journal |last=Bryan |first=S.E. |authorlink= |coauthors=A. Cook, J.P. Evans, P.W. Colls, M.G. Wells, M.G. Lawrence, J.S. Jell, A. Greig, R. Leslie |year=2004 |month= |title=Pumice rafting and faunal dispersion during 2001–2002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga |journal=Earth and Planetary Science Letters |volume=227 |issue= |pages=135–154 |id= |url=http://earth.geology.yale.edu/~je84/mystuff/research/bryanetal2004.pdf |accessdate=2007-07-14 |quote="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"|doi=10.1016/j.epsl.2004.08.009] 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.cite news | title=New Island and Pumice Raft in the Tongas | url=http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17461| date=2006 | publisher=NASA Earth Observatory]

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, 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. Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners (such as Lava soap) as a mild abrasive.

ee also

* Volcanic glass

References

External links

* [http://blogs.smh.com.au/science/archives/2007/01/nsw_to_become_the_pumiced_land.html NSW to become the pumiced land] Retrieved 2007-01-28.