Pumice ( ), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely 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, thicker vesicle walls, and being dark-colored and denser. Pumice is created when super-heated, highly pressurized rock is rapidly 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 (including water and CO2) that are dissolved in the lava, causing the gases to rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink is opened). The simultaneous cooling and depressurization freeze the bubbles in a matrix. Pumice clasts are fragments of tephra that cooled in air or water. If pumice from an underwater volcanic eruption reaches the water surface, it can form pumice rafts on the water surface that can be a hazard for ships.
Properties
Pumice is composed of highly microvesicular volcanic glass with very thin, translucent bubble walls of pyroclastic 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 never mafic as it will be a rock called scoria or reticulite. Pumice is commonly pale in color, ranging from white, cream, blue or gray, to green-brown or black; for example, the 2021 Fukutoku-Okanoba eruption produced mostly gray pumice with small amounts (enclaves or individual clasts) of amber, brown and black pumice; the black color is caused by the presence of tiny crystals of magnetite (a black iron oxide mineral) scattered throughout the pumice. Pumice of a black color can be confused with black scoria. Pumice forms when volcanic gases exsolving from viscous magma form bubbles that remain within the viscous magma as it cools to glass. Pumice is a common product of explosive eruptions (plinian and ignimbrite-forming) and as a byproduct of submarine (surtseyan) eruptions of intermediate to felsic composition. Pumice has a porosity of 64–85% by volume and it floats on water, possibly for years, until it eventually becomes waterlogged and sinks. Trachytic pumice from the 2021 eruption of Fukutoku-Okanoba (a submarine volcano) has a porosity ranging from 73% to 91%, with an average of 78% for floating samples and 75% for submerged samples. There are two main forms of vesicle. 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 an eruption. If the vesicles run perpendicular to the length of the clast, it is typically referred to as tube pumice. It can have a close resemblance to wood. Tube pumice is also known by the terms fibrous pumice, woody pumice, and pipe pumice. Highly vitreous pumice is considered a volcanic glass because it typically lacks a proper crystal structure. It can have a large amount of phenocrysts and/or microlites. The white pumices erupted by Pinatubo in 1991 are examples of pumice with an unusually high phenocryst content. 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, pumice rafts drifted through the Indian Ocean for up to 20 years, with tree trunks floating among them. Pumice rafts disperse and support several marine species. In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts that floated hundreds of kilometres to Fiji. When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite. Pumicite consists of particles less than 4 mm (0.16 in) in size. Pumicite is ash-sized particles of pumice and usually forms in Plinian eruptions or other eruptions with unusually high magma fragmentation.
Comparison with scoria Scoria differs from pumice in being typically denser. With larger vesicles and thicker vesicle walls, scoria often sinks but not always. Some scoria especially if it is unusually vesicular or has thinner walls can float for brief periods. The difference is the result of the lower viscosity of the magma that forms scoria.
Reticulite is a type of highly vesicular scoria that originates from unusually tall lava fountains (at least 300 m (980 ft) high). Reticulite contains a fragmented network of glass fibers which interlock and forming its characteristic hexagonal structure. It is formed by the depletion of the vesicle walls and the retraction of magma into threads that create the previous polygons' outlines. The glass fibers are usually of a triangular or hexagonal shape which is a sign of chilling before the vesicles could even be rounded like in other types of vesicular rocks. A synonym of it is thread-lace scoria.
Gallery
Etymology Pumice is an igneous rock with a foamy appearance. The name is derived from the Latin word pumex (meaning "pumice"), which is related to the Latin word spuma meaning "foam". In former times, pumice was called spuma maris, meaning "froth of the sea" in Latin because the frothy material was thought to be hardened sea foam. Around 80 B.C., it was called lapis spongiae in Latin for its vesicular properties. Many Greek scholars decided there were different sources of pumice, one of which was in the sea coral category.
Distribution Pumice can be found all around the globe deriving from continental volcanic occurrences and submarine volcanic occurrences. Floating stones can also be distributed by ocean currents. As described earlier, pumice is produced by the eruption of explosive volcanoes under certain conditions; therefore, natural sources occur in volcanically active regions. Pumice is mined and transported from these regions. In 2011, Italy and Turkey led pumice-mining production at 4 and 3 million tons respectively; other large producers at or exceeding a million tonnes were Greece, Iran, Chile, and Syria. Total world pumice production in 2011 was estimated at 17 million tonnes.
… excerpt ends here. Continue reading the full article.
