Pumice
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 that are dissolved in the lava, causing the gases to rapidly exsolve. 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 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 of amber, brown and black pumice; the black color is caused by the presence of tiny crystals of magnetite 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 and commonly forms zones in upper parts of silicic lavas. 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 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.
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 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
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 mafic vesicular rock that originates from unusually tall lava fountains. It is a highly vesicular type of scoria that 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.
Etymology
Pumice is an igneous rock with a foamy appearance. The name is derived from the Latin word pumex 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.Asia
There are large reserves of pumice in Asian countries, for example Afghanistan, Indonesia, Japan, Syria, Iran, the Philippines and eastern Russia.Considerable amounts of pumice can be found at the Kamchatka Peninsula on the eastern flank of Russia. This area contains 19 active volcanoes and it lies in close proximity with the Pacific volcanic belt.
Asia is also the site of the second-largest volcanic eruption in the 20th century, the 1991 eruption of Mount Pinatubo, in the Philippines. Ash and pumice lapilli were distributed over a mile around the volcano. The June 15 eruption deposited two main types of anhydrite-bearing pumice: a dominant porphyritic, phenocryst-rich white pumice and a subordinate phenocryst-poor gray/tan pumice. These ejections filled trenches that once reached deep. So much magma was displaced from the vent that the volcano became a depression on the surface of the Earth.
A basaltic trachyandesitic to trachydacitic pumice and scoria deposit underlies Manila and is called the Diliman Tuff. It consists of an ignimbrite along with reworked and airfall pumiceous tuff units. It is part of the larger Guadalupe Tuff Formation and is the youngest layer of it. Magma mixing and mingling is shown by banded textures in some of the pumice fragments, considerable range in groundmass composition in a single pumice fragment. The source volcano of this deposit has not yet been discovered. It is chemically distinct from adjacent Taal Caldera and Laguna Caldera with respect to both major- and trace-element concentrations.
The Taal Volcano south of Manila has also produced several extensive ignimbrite forming eruptions. The earliest of this unit is dated to 670ka and is called the Sampaga Formation.
Another well-known volcano that has produced pumice is Krakatoa. An eruption in 1883 ejected so much pumice that kilometers of sea were covered in floating pumice and in some areas the pumice raft thickness above sea level was 1.5 meters.
Europe
Europe is the largest producer of pumice with deposits in Italy, Turkey, Greece, Hungary, Iceland, and Germany. Italy is the largest producer of pumice because of its numerous eruptive volcanoes. On the Aeolian Islands of Italy, the island of Lipari is entirely made up of volcanic rock, including pumice. Large amounts of igneous rock on Lipari are due to the numerous extended periods of volcanic activity from the Late Pleistocene to the Holocene.North America
Pumice can be found all across North America including on the Caribbean Islands. In the United States, pumice is mined in Nevada, Oregon, Idaho, Arizona, California, New Mexico and Kansas. U.S. production of pumice and pumicite in 2011 was estimated at 380,000 tonnes, valued at $7.7 million with approximately 46% coming from Nevada and Oregon. Idaho is also known as a large producer of pumice because of the quality and brightness of the rock found in local reserves.Mount Mazama in Oregon erupted 7,700 years ago and deposited of pumice and ash around the vent. The large amount of magma that was erupted caused the structure to collapse, forming a caldera now known as Crater Lake.
Mount St. Helens produced a large amount of dacitic pumice in 1980. Two main types were observed which are the white/tan and gray types. The white type has phenocrysts while being little or absent in microlite content unlike the gray type which has the presence of microlites. Vesicularity for the white pumice is higher with around 85.7% vesicularity and thus is less dense while the gray type is lower with around 72.2% vesicularity and has a higher density. The vesicles of the white pumice are larger and more interconnected while the vesicles of the gray pumice are smaller, with hindered expansion and coalescence.