Specimen of highly porous pumice from Teide volcano on Tenerife, Canary Islands. Density of specimen approximately 0.25 g/cm³; scale in centimeters.

Pumice  /ˈpʌmɨs/ is a textural term for a volcanic rock which is a solidified frothy lava. Pumice is typically created when super-heated, highly pressurized rock is violently ejected from a volcano. Alternatively it can be formed when lava and water are mixed. Most pumice is light enough that it floats on water.

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 freezes the bubbles in the matrix.

Properties[link]

Illustrates the porous nature in detail.

Rocks from the Bishop tuff, uncompressed with pumice on left; compressed with fiamme on right.

A 15 centimeter (6 inch) piece of pumice supported by a rolled-up U.S. 20-dollar bill demonstrates its very low density.

A piece of processed pumice resting on a plastic bag.

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. 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 (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 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.^[1] In fact, pumice rafts disperse and support several marine species.^[2] 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.^[3]

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[link]

Pumice is widely used to make lightweight concrete or insulative low-density breeze 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.^[4] Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners (such as Lava soap) as a mild abrasive. Pumice is also used as a growing substrate for growing horticultural crops.

References[link]

External links[link]

• 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.

Landsat image of Lake Toba, Indonesia, the largest volcanic crater lake in the world.

Lake Pinatubo, Philippines, formed after the 1991 eruption of Mount Pinatubo.

Space shuttle imagery of Manicouagan Reservoir / Manicouagan impact crater, Canada, the largest impact crater lake in the world.

Heaven Lake, North Korea / China.

Mount Aso crater lake, Japan.

Taal volcano in Lake Taal, Philippines.

Irazú crater lake, Costa Rica.

Maderas crater lake (Ometepe Island), Nicaragua.

Crater Lake in Oregon, USA.

Cuicocha, Ecuador.

Niuafo'ou crater lake, Tonga.

Katmai crater lake, Alaska, USA.

Kelimutu crater lake, Indonesia.

Kerið crater lake, Iceland.

A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar; less commonly and with lower association to the term a lake may form in an impact crater caused by a meteorite, or in an artificial explosion caused by humans. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not made. Crater lakes covering active (fumarolic) volcanic vents are sometimes known as volcanic lakes, and the water within them is often acidic, saturated with volcanic gases, and cloudy with a strong greenish color. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment.

Lava lakes, a rare phenomenon where a volcanic crater can sustainably contain a significant volume of molten lava, are discussed in a separate article.

Formation[link]

Volcanic crater lakes[link]

Crater lakes form as precipitation within the rim fills the created depression. The water level rises until an equilibrium is reached between the rate of incoming and outgoing water. Sources of water loss singly or together, may include evaporation, subsurface seepage, and in places, surface leakage or overflow when the lake level reaches the lowest point on its rim. At such a saddle location, the upper portion of the lake is contained only by its adjacent natural volcanic dam; continued leakage through or surface outflow across the dam can erode its included material, thus lowering lake level until a new equilibrium of water flow, erosion and rock resistance is established. If the volcanic dam portion erodes rapidly or fails catastrophically, the occurrence produces a breakout or outburst flood. With changes in environmental conditions over time, the occurrence of such floods is common to all natural dam types.

A well-known crater lake, which bears the same name as the geological feature, is Crater Lake in Oregon, USA. It is located in the caldera of Mount Mazama. It is the deepest lake in the United States with a depth of 594 m (1,949 ft). Crater Lake is fed solely by falling rain and snow, with no inflow or outflow at the surface, and hence is one of the clearest lakes in the world.^[1]

The highest volcano in the world, 6,893 metres (22,615 ft) Ojos del Salado in Chile, has a permanent crater lake about 100 metres (300 ft) in diameter at an elevation of 6,390 m (20,960 ft) on its eastern side.^[2] This is most likely the highest lake of any kind in the world.

Due to their unstable environment, some crater lakes exist only intermittently. Caldera lakes in contrast can be quite large and long-lasting; for instance, Lake Toba (Indonesia) formed after its eruption around 70,000 years ago and has an area of over 1,000 square kilometres.

While many crater lakes are picturesque, they can also be deadly. Gas discharges from Lake Nyos (Cameroon) suffocated 1,800 people in 1986, and crater lakes such as Mount Ruapehu's (New Zealand) often contribute to destructive lahars.

Certain bodies of water, although their formation is directly related to volcanic activity, are not usually referred to as crater lakes, including:

• Lakes created by volcanic dams due to lava flowing outside of the volcanic edifice/caldera (such as Garibaldi Lake in Canada, Fuji Five Lakes in Japan),
• Closed atoll lagoons (such as Clipperton lagoon), whose formation process also implies subsequent biogeomorphologic processes,
• Ponds encountered at the bottom of waterfalls occuring in volcanic canyons in a volcanic context but not within a volcanic edifice/caldera (such as Trou de Fer on Réunion Island).

Other kinds of crater lakes[link]

Lakes can also fill impact craters, but these are not usually referred to as crater lakes except in a few isolated cases. Example of such impact crater lakes include Manicouagan in Canada, Lake Bosumtwi in Ghana and Siljan in Sweden.

More rarely they can also fill craters caused by artificial explosions, such as the radioactive Lake Chagan in Kazakhstan.

Other geological features that can be mistaken for a crater lake[link]

Some geomorphological features, when filled with water, can sometimes be confused with crater lakes:

• Pingos (soil-covered ice mounts) whose summital part has collapsed,^[3]
• Sinkholes (karstic holes, also called cenotes), such as Otjikoto Lake in Namibia.

Some circular open-pit mines can also present a similar appearance, such as Big Hole in Kimberley, South Africa, a diamond mine (in a kimberlite pipe) where water has accumulated in the artificially created depression.

List of volcanic crater lakes[link]

This list is incomplete; you can help by expanding it.

List of meteor crater lakes[link]

List of artificial crater lakes[link]

List of crater lakes of unclear origin[link]

See also[link]

• Lava lake
• Volcanic crater
• Caldera
• Types of volcanic eruptions
• Maar
• Atoll
• Impact crater

References[link]

Wikimedia Commons has media related to: Crater lakes

Further reading[link]

• Delmelle, Pierre; Bernard, Alain (2000). "Volcanic Lakes". In Sigurdsson, Haraldur. Encyclopedia of Volcanoes. San Diego: Academic Press. 1999. pp. 877–895. ISBN 0-12-643140-X 
• Varekamp, Johan C.; Rowe, Gary L. , Jr. (eds.) (2000). "Crater Lakes". Journal of Volcanology and Geothermal Research 97 (1–4): pp. 1–508. http://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%235962%232000%23999029998%23197405%23FLA%23&_cdi=5962&_pubType=J&view=c&_auth=y&_acct=C000029718&_version=1&_urlVersion=0&_userid=582538&md5=1911beba4cc7a30b37f932dac1925ca9.  (entire volume about crater lakes)
• Pasternack, G. B.; Varekamp, J. C. (1997). "Volcanic lake systematics I. Physical constraints". Bulletin of Volcanology 58 (7): pp. 526–538. http://www.springerlink.com/content/fe99wvduu106pwpu/?p=bd7fb6cabac34210ace7b7edcfd9f13a&pi=2. 
• Kusakabe, Minoru (ed.) (1994). "Geochemistry of Crater Lakes". Geochemical Journal 28 (3): pp. 137–306. http://www.terrapub.co.jp/journals/GJ/frame/28.html.  (entire issue about chemistry of crater lakes)

External links[link]

• IAVCEI Commission of Volcanic Lakes
• IAVCEI Commission of Volcanic Lakes: Some fundamentals about Crater Lakes
• The Science of Volcanic Lakes
• Volcanic Lakes of the World
• USGS Hawaiian Volcano Observatory: Water on volcanoes: heavy rain and crater lakes
• USGS Cascades Volcano Observatory: Volcanic Lakes
• The Science of Volcanic Lakes, Greg Pasternack, U. California Davis
• Crater Lake National Park documentation in Building Oregon: Architecture of Oregon & the Pacific Northwest archive