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Pumice

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Pumice is a volcanic rock that consists of highly vesicular rough textural rock glass. It generally light colored. It is created when gas-saturated liquid magma erupts like a carbonated drink and cools so rapidly that the resulting foam solidifies into a glass full of gas bubbles. Pumices from silica-rich lavas are white, those from lavas with intermediate silica content are often yellow or brown, and rarer silica-poor that are black. The hollows in the froth can be rounded, elongated, or tubular, depending on the flow of the solidifying lava. The glassy material that forms it can be in threads, fibers, or thin partitions between the hollows. Although pumice is mainly composed of glass, small crystals of various minerals occur. Pumice has a low density due to its numerous air-filled pores. For this reason, it can easily float in water

Name origin: The names derived from the Latin word “pumex” which means foam and through history has been given many names because its formation was unclear.

Texture: Aphanitic and vesicular (contains abundant small gas cavities)

Composition: felsic (rhyolitic)

Color: white to light-gray or light-tan

Cooling Rate: rapid, extrusive

Intrusive Equivalent: granite

Other Characteristics: very light and will float on water

Minerals: Feldspar, augite, hornblende, zircon

Pumice Composition

Pumice is primarily Silicon Dioxide, some Aluminum Oxide and trace amounts pf other oxide. Mall crystals of various minerals occur in many pumices; the most common are feldspar, augite, hornblende, and zircon. The cavities (vesicles) of pumice are sometimes rounded and may also be elongated or tubular, depending on the flow of the solidifying lava. The occurring among old volcanic rocks, the cavities are usually filled with deposits of secondary minerals introduced by percolating water. The glass itself forms threads, fibres, and thin partitions between the vesicles. Rhyolite and trachyte pumices are white, andesite pumices often yellow or brown, and pumiceous basalts (such as occur in the Hawaiian Islands) pitch black.

It forms so quickly that its atoms often don’t have time to organize into crystals. Sometimes there are crystals present in pumice, but most of the structure is amorphous, producing a volcanic glass called a mineraloid.

Pumice Formation

The pumice is formed by contact with the lava water. This occurs most commonly near water or underwater volcanoes. When the hot magma comes into contact with water, rapid cooling and rapid pressure loss reduce bubble by forming lava. The cooling of the rock below the melting point of the rock means that the bubbles are trapped inside when the rock changes into a solid immediately after contact with water. Since the pumice is irreversible, it is sometimes like glass and the bubbles are held between the thin translucent bubble walls of the rock.

Volcanic Gases and Density

If volcanic gases coming from the lava before it cools rapidly, that process can be created to scoria and pumice  It is light colored, has a porosity of about 90 percent and is less dense; scoria is more dense with larger bubbles and thicker bubble walls and is quickly dipped in contrast to the floating boom. If there is too much gas, pumice is formed; When less gas is associated with less viscous magma, the scoria is formed. During the volcanic activity near Tonga, pumice swings from underwater volcanic eruptions were produced, and the pumice can be created quickly and in the past.

Shipping around the World

The actual shipping method depends on the end user. For example, a dentist buys his pumice in a small 2-ounce jar. A manufacturer of hand soap or a computer circuit board manufacturer will receive pumice shipments as shrink-wrapped pallets of 44 lb. bags (20 KG) that arrive via truck or ocean-going container. A public works department using pumice for water filtration may take delivery of their pumice in bulk rail cars. A concrete batch plant will accept a load of pumice (used as a natural pozzolan) in a pneumatic truck. The bottom line is that a company with a pumice deposit in demand world-wide, such as Hess Pumice, necessarily develops the expertise and ability to package pumice however the customer needs it packaged and to ship that pumice efficiently where it’s needed, anywhere in the world.

Where is it located

They are most abundant and most typically developed from felsic (silica-rich) igneous rocks; accordingly, they commonly accompany obsidian. It can be found all around the globe deriving from continental volcanic occurrence and submarine volcanic occurrence. Floating stones can also be distributed by ocean currents.As described earlier it is produced by the eruption of explosive volcanoes under certain conditions, therefore, natural sources occur in volcanically active regions. It is mined and transported from these regions. In 2011, Italy and Turkey led pumice mining production at 4 and 3 million tonnes respectively; other large producers at or exceeding a million tonnes were Greece, Iran, Chile and Syria.

Asia

There are large reserves of pumice in Asian countries including Afghanistan, Indonesia, Japan, Syria, Iran 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.

Europe

Europe is the largest producer of pumice with deposits in Italy, Turkey, Greece, Hungary and Iceland. 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.

North America

It can be found all across North America including on the Caribbean Islands. In the United States, This rock 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.

South America

Chile is one of the leading producers of Pumice in the world.[The Puyehue-Cordón Caulle are two coalesced volcanoes in the Andes mountains that ejected ash and pumice across Chile and Argentina. A recent eruption in 2011 wreaked havoc on the region by covering all surfaces and lakes in ash and pumice.

Africa

Kenya, Ethiopia and Tanzania have some deposits of pumice.

Australia

The Havre Seamount volcano produced the largest-known deep ocean volcanic eruption on Earth. The volcano erupted in July 2012 but remained unnoticed until enormous pieces of pumice were seen to be floating on the Pacific Ocean. Blankets of rock reached a thickness of 5 meters. Most of this floating rock is deposited on the North-West coast of New Zealand and the Polynesia islands action in 2011 was estimated at 17 million tones.

Characteristics and 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 (e.g., rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but basaltic and other compositions are known. It 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 form bubbles that remain within the viscous magma as it cools to glass. It is a common product of explosive eruptions (plinian and ignimbrite-forming) and commonly forms zones in upper parts of silicic lavas. It has a porosity of 64–85% by volume and it floats on water, possibly for years, until it is eventually waterlogged and sink.

There are two main forms of vesicles. Most of this rock 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.

Physical Properties

Chemical Name: Amorphous Aluminum Silicate
Hardness (MOHS): 6
pH: 7.2
Radioactivity: none
Loss on Ignition (LOI): 5%
Softening Point: 900 degrees C
Water Soluble Substances: 0.15%
Acid Soluble Substances: 2.9%
Reactivity: Inert
Appearance: white powder (GE brightness of 84)

Chemical Analysis (typical averages)

Silicon Dioxide: 76.2%
Aluminum Oxide: 13.5%
Ferric Oxide: 1.1%
Ferrous Oxide: 0.1%
Sodium Oxide: 1.6%
Potassium Oxide: 1.8%
Calcium Oxide: 0.8%
Titanium Oxide: 0.2%
Magnesium Oxide: 0.05%
Water: <1.0%

Pumice Uses

It is a unique rock, noted for its light weight and low density (dry pumice can float in water). It is commonly used in cement, concrete and breeze blocks and as an abrasive in polishes, pencil erasers, exfoliates and to produce stone-washed jeans. It is also used to remove dry skin from the bottom of the foot during the pedicure process at some beauty salons.

It is a very light weight, porous and abrasive material and it has been used for centuries in the construction and beauty industry as well as in early medicine.

It is also used as an abrasive, especially in polishes, pencil erasers, and the production of stone-washed jeans.

It was also used in the early book making industry to prepare parchment paper and leather bindings. There is high demand for pumice, particularly for water filtration, chemical spill containment, cement manufacturing, horticulture and increasingly for the pet industry.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Atlas-hornin.sk. (2019). Atlas of magmatic rocks. [online] Available at: http://www.atlas-hornin.sk/en/home [Accessed 13 Mar. 2019].
  • Helmenstine, Anne Marie, Ph.D. (2019, March 10). What Is Pumice Rock? Geology and Uses. Retrieved from https://www.thoughtco.com/pumice-rock-4588534
Cite this article as: Geology Science. (2019). Pumice. [online] Available at: http://geologyscience.com/rocks/igneous-rocks/extrusive-igneous-rocks/pumice/ [15th September 2019 ]
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