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Shield volcano
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=== Structure === {{Shield volcano diagram|right}} Shield volcanoes are distinguished from the three other major volcanic types—[[stratovolcano]]es, [[lava dome]]s, and [[cinder cone]]s—by their structural form, a consequence of their particular [[magmatic]] composition. Of these four forms, shield volcanoes erupt the least [[viscosity|viscous]] lavas. Whereas stratovolcanoes and lava domes are the product of highly viscous flows, and cinder cones are constructed of [[explosive eruption|explosively eruptive]] [[tephra]], shield volcanoes are the product of gentle [[effusive eruption]]s of highly fluid lavas that produce, over time, a broad, gently sloped eponymous "shield".<ref name=usgs-types>{{cite web|title=Principal Types of Volcanoes|url=http://pubs.usgs.gov/gip/volc/types.html|publisher=United States Geological Survey|access-date=30 December 2013|author=John Watson|date=1 March 2011}}</ref><ref name=hvw-shield /> Although the term is generally applied to [[basaltic]] shields, it has also at times been applied to rarer [[wikt:scutiform|scutiform]] volcanoes of differing magmatic composition—principally [[pyroclastic shield]]s, formed by the accumulation of fragmentary material from particularly powerful explosive eruptions, and rarer [[felsic]] lava shields formed by unusually fluid felsic magmas. Examples of pyroclastic shields include [[Billy Mitchell (volcano)|Billy Mitchell]] volcano in [[Papua New Guinea]] and the [[Purico complex]] in [[Chile]];<ref>{{cite gvp|vnum=355094|name=Purico Complex|access-date=30 December 2013}}</ref><ref>{{cite gvp|vnum=255011|name=Billy Mitchell|access-date=30 December 2013}}</ref> an example of a felsic shield is the [[Ilgachuz Range]] in [[British Columbia]], Canada.<ref>{{cite book|last1=Wood|first1=Charles A.|last2=Kienle|first2=Jürgen|page=133|title=Volcanoes of North America: United States and Canada|year=1990|publisher=[[Cambridge University Press]]|location=[[Cambridge]], England|isbn=0-521-43811-X}}</ref> Shield volcanoes are similar in origin to vast [[Volcanic plateau#Lava plateau|lava plateau]]s and [[flood basalt]]s present in various parts of the world. These are eruptive features which occur along linear [[fissure vent]]s and are distinguished from shield volcanoes by the lack of an identifiable primary eruptive center.<ref name=usgs-types/> [[Volcano#Active|Active]] shield volcanoes experience near-continuous eruptive activity over extremely long periods of time, resulting in the gradual build-up of edifices that can reach extremely large dimensions.<ref name="hvw-shield"/> With the exclusion of flood basalts, mature shields are the largest volcanic features on Earth.<ref name="volcworld">{{cite web|title=Shield Volcanoes|url=http://volcano.und.edu/vwdocs/vwlessons/volcano_types/shield.htm |publisher=University of North Dakota|access-date=22 August 2010 |archive-url = https://web.archive.org/web/20070808133055/http://volcano.und.edu/vwdocs/vwlessons/volcano_types/shield.htm |archive-date = 8 August 2007}}</ref> The summit of the largest [[subaerial]] volcano in the world, [[Mauna Loa]], lies {{convert|4169|m|ft|0|abbr=on}} above [[sea level]], and the volcano, over {{convert|60|mi|km|-1|abbr=on}} wide at its base, is estimated to contain about {{convert|80000|km3|cumi|-3|abbr=on}} of basalt.<ref name="usgs-shield"/><ref name="hvw-shield"/> The mass of the volcano is so great that it has slumped the [[lithosphere|crust]] beneath it a further {{convert|8|km|mi|0|abbr=on}}.<ref name="gspp">{{cite book|chapter-url=https://books.google.com/books?id=wRosAQAAIAAJ&pg=PA95|chapter=Subsidence of the Hawaiian Ridge|author=J.G. Moore|title=Volcanism in Hawaii |series=Geological Survey Professional Paper |volume=1350 |year=1987}}</ref> Accounting for this subsidence and for the height of the volcano above the [[sea floor]], the "true" height of Mauna Loa from the start of its eruptive history is about {{convert|17170|m|ft|-3|abbr=on}}.<ref>{{cite web|title=How High is Mauna Loa?|url=http://hvo.wr.usgs.gov/volcanowatch/archive/1998/98_08_20.html|publisher=Hawaiian Volcano Observatory – United States Geological Survey|access-date=5 February 2013|date=20 August 1998}}</ref> [[Mount Everest]], by comparison, is {{convert|8848|m|ft|0|abbr=on}} in height.<ref>{{cite web|title=Nepal in new bid to finally settle Mount Everest height|url=https://www.bbc.co.uk/news/science-environment-17191400|work=BBC News|access-date=10 December 2012|author=Navin Singh Khadka|date=28 February 2012}}</ref> In 2013, a team led by the [[University of Houston]]'s [[William Sager]] announced the discovery of [[Tamu Massif]], an enormous extinct [[submarine volcano|submarine]] volcano, approximately {{convert|450|by|650|km|mi|abbr=on}} in area, which dwarfs all previously known volcanoes on Earth. However, the extents of the volcano have not been confirmed.<ref>{{cite magazine|title=New Giant Volcano Below Sea Is Largest in the World|url=http://news.nationalgeographic.com/news/2013/09/130905-tamu-massif-shatsky-rise-largest-volcano-oceanography-science/|archive-url=https://web.archive.org/web/20130906013208/http://news.nationalgeographic.com/news/2013/09/130905-tamu-massif-shatsky-rise-largest-volcano-oceanography-science/|url-status=dead|archive-date=September 6, 2013|magazine=National Geographic|access-date=31 December 2013|author=Brian Clark Howard|date=5 September 2013}}</ref> Although Tamu Massif was initially believed to be a shield volcano, Sanger and his colleagues acknowledged in 2019 that Tamu Massif is not a shield volcano.<ref name="Sanger_et_al_2019">{{cite journal | title=Oceanic plateau formation by seafloor spreading implied by Tamu Massif magnetic anomalies | author=Sanger, W. | display-authors=et al. | journal=Nature Geoscience | year=2019 | volume=12 | issue=8 | pages=661–666 | doi=10.1038/s41561-019-0390-y| bibcode=2019NatGe..12..661S }}</ref> Shield volcanoes feature a gentle (usually 2° to 3°) slope that gradually steepens with elevation (reaching approximately 10°) before flattening near the summit, forming an overall upwardly convex shape. These slope characteristics have a [[correlation]] with age of the forming lava, with in the case of the Hawaiian chain, steepness increasing with age, as later lavas tend to be more alkali so are more viscous, with thicker flows, that travel less distance from the summit vents. <ref name=Moore1992>{{cite journal|last1 =Moore|first1 =J.G|last2 =Mark|first2 =R.K.|year =1992|title =Morphology of the Island of Hawaii|journal =GSA Today|volume =2|issue =12|pages =257–262|bibcode =1992GSAT....2..257M|url=https://pubs.usgs.gov/publication/70207943|access-date=1 May 2024}}</ref> In height they are typically about one twentieth their width.<ref name="hvw-shield">{{cite web|title=How Volcanoes Work: Shield Volcanoes|url=http://www.geology.sdsu.edu/how_volcanoes_work/shieldvolc_page.html|publisher=San Diego State University|access-date=30 December 2013|archive-date=2 January 2014|archive-url=https://web.archive.org/web/20140102135754/http://www.geology.sdsu.edu/how_volcanoes_work/shieldvolc_page.html|url-status=dead}}</ref> Although the general form of a "typical" shield volcano varies little worldwide, there are regional differences in their size and morphological characteristics. Typical shield volcanoes found in California and Oregon measure {{convert|3|to|4|mi|km|0|abbr=on}} in diameter and {{convert|1500|to|2000|ft|m|-2|abbr=on}} in height,<ref name=usgs-types/> while shield volcanoes in the central Mexican [[Michoacán–Guanajuato volcanic field]] average {{convert|340|m|ft|-2|abbr=on}} in height and {{convert|4100|m|ft|-2|abbr=on}} in width, with an average slope angle of 9.4° and an average volume of {{convert|1.7|km3|cumi|1|abbr=on}}.<ref name=jvgr-1994>{{cite journal|last=Hasenaka|first=T.|title=Size, distribution, and magma output rate for shield volcanoes of the Michoacán-Guanajuato volcanic field, Central Mexico|journal=[[Journal of Volcanology and Geothermal Research]]|date=October 1994|volume=63|issue=2|pages=13–31|doi=10.1016/0377-0273(94)90016-7|bibcode=1994JVGR...63...13H}}</ref> [[Rift zone]]s are a prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandic cousins<ref name=hvw-shield /> can be attributed to rift eruptions. Fissure venting is common in Hawai{{okina}}i; most Hawaiian eruptions begin with a so-called "wall of fire" along a major fissure line before centralizing to a small number of points. This accounts for their asymmetrical shape, whereas Icelandic volcanoes follow a pattern of central eruptions dominated by [[Caldera|summit calderas]], causing the lava to be more evenly distributed or symmetrical.<ref name="usgs-shield"/><ref name=hvw-shield /><ref name="hvw-Hawaiian"/><ref name="worldbook">{{cite book|title=World Book: U {{·}} V {{·}} 20|publisher=Scott Fetzer|year=2009|pages=438–443|url=http://worldbookonline.com|isbn=978-0-7166-0109-8|access-date=22 August 2010|location=Chicago}}</ref>
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