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volcano
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{{about|the geological feature||Volcano (disambiguation)|and|Volcanic (disambiguation)}}{{broader|Volcanism}}{{short description|A rupture in the crust of a planetary-mass object that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface}}{{pp|small=yes}}{{cleanup HTML|date=February 2019}}{{Use mdy dates|date=May 2012}}A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle.WEB,weblink Earthquakes, Volcanoes, and Tsunamis, Resources for Environmental Literacy, NSTA Press / Archive.Org, 2007, April 22, 2014, yes,weblink" title="web.archive.org/web/20120913041924weblink">weblink September 13, 2012, Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging, and most are found underwater. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can also form where there is stretching and thinning of the crust's plates, e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism.BOOK, Plates vs. Plumes: A Geological Controversy, Foulger, Gillian R., 2010, 978-1-4051-6148-0, Wiley-Blackwell, Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth. Volcanoes are usually not created where two tectonic plates slide past one another.File:Erupción en el volcán Sabancaya, Perú.jpg|thumb|SabancayaSabancayaFile:Apaneca, El Salvador - panoramio (47).jpg|thumb|Cordillera de Apaneca volcanic range in El Salvador. The country is home to 170 volcanoes, 23 which are active, including two calderas, one being a supervolcanosupervolcanoFile:MtCleveland ISS013-E-24184.jpg|thumb|Cleveland Volcano in the Aleutian Islands of Alaska photographed from the International Space StationInternational Space StationErupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere (or troposphere); however, they also absorb heat radiated from the Earth, thereby warming the upper atmosphere (or stratosphere). Historically, volcanic winters have caused catastrophic famines.File:Pinatubo ash plume 910612.jpg|thumb|An eruption of Mount PinatuboMount PinatuboFile:Lava fountain USGS page 30424305-068 large.JPG|thumb|right|Fountain of lava erupting from a volcanic cone in Hawaii, 1983]]File:Ile Barren, 1995.jpg|thumb|Aerial view of the Barren Island, Andaman Islands, IndiaIndiaFile:Mount Shasta satellite view Jan 2014 - Zoomed.jpg|thumb|Satellite image of Mount ShastaMount Shasta

Etymology

The word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology.WEB,weblink Volcano, Mind over Magma: The Story of Igneous Petrology, Davis A., Young, January 2016, January 11, 2016, yes,weblink" title="web.archive.org/web/20151112071530weblink">weblink November 12, 2015, mdy-all, The study of volcanoes is called volcanology, sometimes spelled vulcanology.

Plate tectonics

(File:Spreading ridges volcanoes map-en.svg|thumb|upright=1.4|Map showing the divergent plate boundaries (oceanic spreading ridges) and recent sub-aerial volcanoes)

Divergent plate boundaries

At the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Because the crust is very thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion (without transfer of heat or matter) and the partial melting of the mantle, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans; therefore, most volcanic activity on the Earth is submarine, forming new seafloor. Black smokers (also known as deep sea vents) are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed; for example, Iceland.

Convergent plate boundaries

Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges, under the continental plate, forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, thus creating magma. This magma tends to be extremely viscous because of its high silica content, so it often does not attain the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Typical examples are Mount Etna and the volcanoes in the Pacific Ring of Fire.

Hotspots

Hotspots are volcanic areas believed to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting. Because tectonic plates move across them, each volcano becomes dormant and is eventually re-formed as the plate advances over the postulated plume. The Hawaiian Islands are said to have been formed in such a manner; so has the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate above the hot spot. This theory, however, has been doubted.

Volcanic features

{{See|Types of volcanoes}}File:Lakagigar Iceland 2004-07-01.jpg|right|thumbnail|Lakagigar fissure vent in Iceland, the source of the major world climate alteration of 1783–84, has a chain of volcanic cones along its length.]]File:Skjaldbreidur Herbst 2004.jpg|right|thumbnail|SkjaldbreiðurSkjaldbreiðurThe most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano. The features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material (including lava and ash) and gases (mainly steam and magmatic gases) can develop anywhere on the landform and may give rise to smaller cones such as Pu{{okina}}u {{okina}}Ō{{okina}}ō on a flank of Hawaii's Kīlauea.Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn, and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is actually a vent of an igneous volcano.

Fissure vents

Volcanic fissure vents are flat, linear fractures through which lava emerges.

Shield volcanoes

Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.

Lava domes

Lava domes are built by slow eruptions of highly viscous lava. They are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount Saint Helens, but can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lava generally does not flow far from the originating vent.

Cryptodomes

Cryptodomes are formed when viscous lava is forced upward causing the surface to bulge. The 1980 eruption of Mount St. Helens was an example; lava beneath the surface of the mountain created an upward bulge which slid down the north side of the mountain.

Volcanic cones (cinder cones)

File:Green Izalco Volcano.JPG|thumb|Izalco volcano, the youngest volcano in El Salvador. Izalco erupted almost continuously from 1770 (when it formed) to 1958, earning it the nickname of "Lighthouse of the Pacific".]]Volcanic cones or cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico, Caja del Rio is a volcanic field of over 60 cinder cones.Based on satellite images it was suggested that cinder cones might occur on other terrestrial bodies in the Solar system too; on the surface of Mars and the Moon.JOURNAL, Wood, C.A., Cindercones on Earth, Moon and Mars, Lunar and Planetary Science, X, 1979, 1370–1372,weblink 1979LPI....10.1370W, JOURNAL, Meresse, S., Costard, F.O., Mangold, N., Masson, P., Neukum, G., Formation and evolution of the chaotic terrains by subsidence and magmatism: Hydraotes Chaos, Mars, 10.1016/j.icarus.2007.10.023, Icarus, 194, 2, 487, 2008, 2008Icar..194..487M, JOURNAL, 10.1016/j.icarus.2011.11.030, A unique volcanic field in Tharsis, Mars: Pyroclastic cones as evidence for explosive eruptions, 2012, Brož, P., Hauber, E., Icarus, 218, 1, 88, 2012Icar..218...88B, JOURNAL, Lawrence, S.J., Stopar, J.D., Hawke, B.R., Greenhagen, B.T., Cahill, J.T.S., Bandfield, J.L., Jolliff, B.L., Denevi, B.W., Robinson, M.S., Glotch, T.D., Bussey, D.B.J., Spudis, P.D., Giguere, T.A., Garry, W.B., LRO observations of morphology and surface roughness of volcanic cones and lobate lava flows in the Marius Hills, 10.1002/jgre.20060, Journal of Geophysical Research: Planets, 118, 4, 615, 2013, 2013JGRE..118..615L,

Stratovolcanoes (composite volcanoes)

File:Volcano scheme.svg|thumb|upright=1.4|Cross-section through a stratovolcano (vertical scale is exaggerated): {{Div col|colwidth=30em}}{hide}ordered list
|1=Large magma chamber
|2=Bedrock
|3=Conduit (pipe)
|4=Base
|5=Sill
|6=Dike
|7=Layers of ash emitted by the volcano
|8=Flank
|9=Layers of lava emitted by the volcano
|10=Throat
|11=Parasitic cone
|12=Lava flow
|13=Vent
|14=Crater
|15=Ash cloud
stratovolcanoStratovolcanoes or composite volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that gives rise to the name. Stratovolcanoes are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash, and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process repeats. Classic examples include Mount Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.Throughout recorded history, ash produced by the explosive eruption of stratovolcanoes has posed the greatest volcanic hazard to civilizations. Not only do stratovolcanoes have greater pressure buildup from the underlying lava flow than shield volcanoes, but their fissure vents and monogenetic volcanic fields (volcanic cones) also have more powerful eruptions because they are often under extension. They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars.BOOK,weblink Volcanoes: Global Perspectives, John P., Lockwood, Richard W., Hazlett, 2010, 978-1-4051-6250-0, 552, Large pieces of tephra are called volcanic bombs. Big bombs can measure more than 4 feet(1.2 meters) across and weigh several tons.Berger, Melvin, Gilda Berger, and Higgins Bond. "Volcanoes-why and how ." Why do volcanoes blow their tops?: Questions and answers about volcanoes and earthquakes. New York: Scholastic, 1999. 7. Print.

Supervolcanoes

{{see also|List of largest volcanic eruptions}}A supervolcano usually has a large caldera and can produce devastation on an enormous, sometimes continental, scale. Such volcanoes are able to severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. They are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States); Lake Taupo in New Zealand; Lake Toba in Sumatra, Indonesia; and Ngorongoro Crater in Tanzania. Because of the enormous area they may cover, supervolcanoes are hard to identify centuries after an eruption. Similarly, large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted (even though the lava flow is non-explosive).

Underwater volcanoes

{{See also|Subaqueous volcano}}Submarine volcanoes are common features of the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above the ocean's surface. In the ocean's deep, the tremendous weight of the water above prevents the explosive release of steam and gases; however, they can be detected by hydrophones and discoloration of water because of volcanic gases. Pillow lava is a common eruptive product of submarine volcanoes and is characterized by thick sequences of discontinuous pillow-shaped masses which form under water. Even large submarine eruptions may not disturb the ocean surface due to the rapid cooling effect and increased buoyancy of water (as compared to air) which often causes volcanic vents to form steep pillars on the ocean floor. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break the ocean surface as new islands or floating pumice rafts.

Subglacial volcanoes

Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lava on top collapses, leaving a flat-topped mountain. These volcanoes are also called table mountains, tuyas, or (uncommonly) mobergs. Very good examples of this type of volcano can be seen in Iceland, however, there are also tuyas in British Columbia. The origin of the term comes from Tuya Butte, which is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near the boundary with the Yukon Territory.

Mud volcanoes

Mud volcanoes or mud domes are formations created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high.

Erupted material

File:Lava channel overflow.JPG|thumb|Pāhoehoe lava flow on Hawaii. The picture shows overflows of a main lava channellava channelFile:DenglerSW-Stromboli-20040928-1230x800.jpg|thumb|The Stromboli stratovolcano off the coast of SicilySicilyFile:Vulkan Chaparrastique, El Salvador 2013 01.JPG|thumb|(San Miguel (volcano)]], El Salvador. On December 29, 2013, San Miguel volcano, also known as "Chaparrastique", erupted at 10:30 local time, spewing a large column of ash and smoke into the sky; the eruption, the first in 11 years, was seen from space and prompted the evacuation of thousands of people living in a 3 km radius around the volcano.)File:Sanmiguel amo 2013363 lrg.jpg|thumb|Ash plume from San Miguel (volcano) "Chaparrastique", seen from a satellite, as it heads towards the Pacific Ocean from the El Salvador Central AmericaCentral America

Lava composition

Another way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into four different compositions:BOOK, Casq, R.A.F., Wright, J.V., Volcanic Successions, 1987, Unwin Hyman Inc, 978-0-04-552022-0, 528,
  • If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic.
    • Felsic lavas (dacites or rhyolites) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome.
    • Because siliceous magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 Â°C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaska's Valley of Ten Thousand Smokes, formed by the eruption of Novarupta near Katmai in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit. Volcanic ash that is light enough to be erupted high into the Earth's atmosphere may travel many kilometres before it falls back to ground as a tuff.
  • If the erupted magma contains 52–63% silica, the lava is of intermediate composition.
    • These "andesitic" volcanoes generally only occur above subduction zones (e.g. Mount Merapi in Indonesia).
    • Andesitic lava is typically formed at convergent boundary margins of tectonic plates, by several processes:
      • Hydration melting of peridotite and fractional crystallizationFile:Sarychev Peak eruption on 12 June 2009, oblique satellite view.ogv|thumb|right|Sarychev Peak eruption, Matua IslandMatua Island
      • Melting of subducted slab containing sediments{{citation needed|reason=I believe that full-slab melting has been rejected, and it is more mineral destabiliztion; tag here to incite me or someone else to find a ref. Same comment left at Andesite. |date=April 2011}}
      • Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or lava flow.
  • If the erupted magma contains 45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic lavas occur in a wide range of settings:
  • Some erupted magmas contain WEB,weblink Enceladus, Saturn's Moon, Smith, Yvette, March 15, 2012, Image of the Day Gallery, NASA, 4 July 2014, The ejecta may be composed of water, liquid nitrogen, ammonia, dust, or methane compounds. Cassini–Huygens also found evidence of a methane-spewing cryovolcano on the Saturnian moon Titan, which is believed to be a significant source of the methane found in its atmosphere.WEB,weblink Hydrocarbon volcano discovered on Titan, June 8, 2005, Newscientist.com, October 24, 2010, It is theorized that cryovolcanism may also be present on the Kuiper Belt Object Quaoar.
A 2010 study of the exoplanet COROT-7b, which was detected by transit in 2009, suggested that tidal heating from the host star very close to the planet and neighboring planets could generate intense volcanic activity similar to that found on Io.WEB, Jaggard, Victoria, "Super Earth" May Really Be New Planet Type: Super-Io, National Geographic web site daily news, National Geographic Society, February 5, 2010,weblink March 11, 2010,

Traditional beliefs about volcanoes

{{Expand section|date=October 2016}}{{See also|Popocatepetl and Iztaccihuatl}}Many ancient accounts ascribe volcanic eruptions to supernatural causes, such as the actions of gods or demigods. To the ancient Greeks, volcanoes' capricious power could only be explained as acts of the gods, while 16th/17th-century German astronomer Johannes Kepler believed they were ducts for the Earth's tears.JOURNAL
, Micheal
, Williams
, November 2007
, Hearts of fire
, Morning Calm
, 11–2007
, 6
,
One early idea counter to this was proposed by Jesuit Athanasius Kircher (1602–1680), who witnessed eruptions of Mount Etna and Stromboli, then visited the crater of Vesuvius and published his view of an Earth with a central fire connected to numerous others caused by the burning of sulfur, bitumen and coal.
Various explanations were proposed for volcano behavior before the modern understanding of the Earth's mantle structure as a semisolid material was developed. For decades after awareness that compression and radioactive materials may be heat sources, their contributions were specifically discounted. Volcanic action was often attributed to chemical reactions and a thin layer of molten rock near the surface.

Gallery

File:In Deep (16153202370).jpg |In the aftermath of the May 1980 eruption of Mount St. Helens in Washington State, a geologist is standing in a trench of ash on Coldwater Ridge about 18 inches deep.File:1984 Eruption (23125250919).jpg |An aerial view of the Pu‘u ‘O‘o fountain during episode 23 of the Pu‘u ‘O‘o–Kupaianaha Eruption File:An Eruptive Situation (22195124630).jpg |A geologist collecting spatter samples while fountains erupt southwest of the base of Pu‘u ‘O‘o.

See also

{{div col|colwidth=30em}}
  • {{annotated link|Global Volcanism Program}}
  • {{annotated link|List of extraterrestrial volcanoes}}
  • {{annotated link|Maritime impacts of volcanic eruptions}}
  • {{annotated link|Prediction of volcanic activity}}
  • {{annotated link|Timeline of volcanism on Earth}}
  • {{annotated link|Volcanic Explosivity Index}}
  • {{annotated link|Volcano Number}}
  • {{annotated link|Volcano observatory}}
{{div col end}}

References

{{Reflist}}

Further reading

  • Macdonald, Gordon and Agatin T. Abbott. (1970). Volcanoes in the Sea. University of Hawaii Press, Honolulu. 441 p.
  • BOOK, Marti, Joan, Ernst, Gerald., yes, Volcanoes and the Environment, Cambridge University Press, 2005, 978-0-521-59254-3,
  • Ollier, Cliff. (1988). Volcanoes. Basil Blackwell, Oxford, {{ISBN|0-631-15664-X}} (hardback), {{ISBN|0-631-15977-0}} (paperback).
  • Sigurðsson, Haraldur, ed. (1999). Encyclopedia of Volcanoes. Academic Press. {{ISBN|0-12-643140-X}}. This is a reference aimed at geologists, but many articles are accessible to non-professionals.

External links

{{Commons category|Volcanoes}}{{Wikivoyage|Volcanoes}}{{Library resources box}} {{Volcanoes}}{{List of volcanoes}}{{Authority control}}

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