Pumice , called pumicite in its powdered or dust form, is a volcanic rock that consists
of highly vesicular rough textured volcanic glass, which may or may not contain crystals.
It is typically light colored. Scoria is another vesicular volcanic rock that differs from
pumice in having larger vesicles and thicker vesicle walls and being dark colored and denser.
Pumice is created when super-heated, highly pressurized rock is violently ejected from
a volcano. The unusual foamy configuration of pumice happens because of simultaneous
rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering
the solubility of gases that are dissolved in the lava, causing the gases to rapidly
exsolve. The simultaneous cooling and depressurization freezes the bubbles in the matrix. Eruptions
under water are rapidly cooled and the large volume of pumice created can be a shipping
hazard.
Properties 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,
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 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. In fact, pumice rafts disperse and support
several marine species. In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga
created large pumice rafts, some as large as 30 km that floated hundreds of kilometres
to Fiji. 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.
Uses Pumice is widely used to make lightweight
concrete or insulative low-density cinder 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 the production of 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. Finely ground pumice is added to some toothpastes
and heavy-duty hand cleaners as a mild abrasive. Pumice is also used as a growing substrate
for growing horticultural crops. Some brands of chinchilla dust bath are made of powdered
pumice. Owing to its high demand particularly for
water filtration, chemical spill containment, cement manufacturing, horticulture and increasingly
for the pet industry, the mining of pumice in environmentally sensitive areas has been
under more scrutiny after such an operation was stopped in the U.S. state of Oregon, at
Rock Mesa in the southern part of the Three Sisters Wilderness.
References
External links University of Oxford image of pumice. Retrieved
2010-09-27. Analytical identification of a single source
pumice from Greek shores and ancient sites in the Levant.
On the occurrence of a pumice-rich layer in Holocene deposits of western Peloponnesus,
Ionian Sea, Greece. A geomorphological and geochemical approach.