Editing Shield volcano (section)

== Geology ==
=== Structure ===
{{Shield volcano diagram|right}}

Shield volcanoes are distinguished from the three other major volcanic types&mdash;[[stratovolcano]]es, [[lava dome]]s, and [[cinder cone]]s&mdash;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&mdash;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>

=== Eruptive characteristics ===
<!-- todo: this image should be de-cluttered and adopted into a proper labelled image, like the structural opener -->
{{multiple image | direction = vertical | width = 300 |image2=Hawaiian Eruption-numbers.svg|caption2=Diagram of a [[Hawaiian eruption]]. (key: 1. [[Ash plume]] 2. [[Lava fountain]] 3. [[Volcanic crater|Crater]] 4. [[Lava lake]] 5. [[Fumarole]]s 6. [[Lava|Lava flow]] 7. Layers of [[lava]] and [[volcanic ash|ash]] 8. [[Stratum]] 9. [[Sill (geology)|Sill]] 10. [[Magma]] conduit 11. [[Magma chamber]] 12. [[Dike (geology)|Dike]]) [[:File:Hawaiian Eruption-numbers.svg|Click for larger version]].}}

Most of what is currently known about shield volcanic eruptive character has been gleaned from studies done on the volcanoes of [[Hawaii (island)|Hawai{{okina}}i Island]], by far the most intensively studied of all shields because of their scientific accessibility;<ref name=epsl-2013>{{cite journal|title=A new model for the growth of basaltic shields based on deformation of Fernandina volcano, Galápagos Islands|journal=[[Earth and Planetary Science Letters]]|date=September 2013|volume=377–378|pages=358–366|doi=10.1016/j.epsl.2013.07.016|author1=Marco Bagnardia |author2=Falk Amelunga |author3=Michael P. Poland |bibcode=2013E&PSL.377..358B}}</ref> the island lends its name to the slow-moving, effusive eruptions typical of shield volcanism, known as [[Hawaiian eruption]]s.<ref name="Oxford-2003">{{cite journal |last1=Regelous |first1=M. |last2=Hofmann |first2=A. W. |last3=Abouchami |first3=W. |last4=Galer |first4=S. J. G. |year=2003 |title=Geochemistry of Lavas from the Emperor Seamounts, and the Geochemical Evolution of Hawaiian Magmatism from 85 to 42 Ma |journal=Journal of Petrology |volume=44 |issue=1 |pages=113–140 |doi=10.1093/petrology/44.1.113|bibcode=2003JPet...44..113R |doi-access=free }}</ref> These eruptions, the least explosive of volcanic events, are characterized by the effusive emission of highly fluid basaltic lavas with low [[volcanic gas|gaseous content]]. These lavas travel a far greater distance than those of other eruptive types before solidifying, forming extremely wide but relatively thin magmatic sheets often less than {{convert|1|m|ft|0|abbr=on}} thick.<ref name="usgs-shield"/><ref name=hvw-shield /><ref name="hvw-Hawaiian">{{cite web|title=How Volcanoes Work: Hawaiian Eruptions|url=http://www.geology.sdsu.edu/how_volcanoes_work/Hawaiian.html|publisher=San Diego State University|access-date=27 July 2014|archive-date=3 March 2001|archive-url=https://web.archive.org/web/20010303161907/http://www.geology.sdsu.edu/how_volcanoes_work/Hawaiian.html|url-status=dead}}</ref> Low volumes of such lavas layered over long periods of time are what slowly constructs the characteristically low, broad profile of a mature shield volcano.<ref name="usgs-shield">{{cite web|last=Topinka|first=Lyn|title=Description: Shield Volcano|url=http://vulcan.wr.usgs.gov/Glossary/ShieldVolcano/description_shield_volcano.html|publisher=USGS|access-date=21 August 2010|date=28 December 2005}}</ref>

Also unlike other eruptive types, Hawaiian eruptions often occur at decentralized [[fissure vent]]s, beginning with large "curtains of fire" that quickly die down and concentrate at specific locations on the volcano's rift zones. Central-vent eruptions, meanwhile, often take the form of large lava fountains (both continuous and sporadic), which can reach heights of hundreds of meters or more. The particles from lava fountains usually cool in the air before hitting the ground, resulting in the accumulation of cindery [[scoria]] fragments; however, when the air is especially thick with [[pyroclast]]s, they cannot cool off fast enough because of the surrounding heat, and hit the ground still hot, accumulating into [[Volcanic cone#Spatter cone|spatter cone]]s. If eruptive rates are high enough, they may even form splatter-fed lava flows. Hawaiian eruptions are often extremely long-lived; [[Puʻu ʻŌʻō]], a cinder cone of [[Kīlauea]], erupted continuously from January 3, 1983, until April 2018.<ref name="hvw-Hawaiian"/>

Flows from Hawaiian eruptions can be divided into two types by their structural characteristics: [[pāhoehoe]] lava which is relatively smooth and flows with a ropey texture, and [[ʻaʻā]] flows which are denser, more viscous (and thus slower moving) and blockier. These lava flows can be anywhere between {{convert|2|and|20|m|ft|-1|abbr=on}} thick. {{okina}}A{{okina}}ā lava flows move through pressure&mdash; the partially solidified front of the flow steepens because of the mass of flowing lava behind it until it breaks off, after which the general mass behind it moves forward. Though the top of the flow quickly cools down, the molten underbelly of the flow is buffered by the solidifying rock above it, and by this mechanism, {{okina}}a{{okina}}ā flows can sustain movement for long periods of time. Pāhoehoe flows, in contrast, move in more conventional sheets, or by the advancement of lava "toes" in snaking lava columns. Increasing viscosity on the part of the lava or [[shear (geology)|shear stress]] on the part of local topography can morph a pāhoehoe flow into an ʻaʻā one, but the reverse never occurs.<ref name="hvw-basaltic">{{cite web|title=How Volcanoes Work: Basaltic Lava|url=http://www.geology.sdsu.edu/how_volcanoes_work/Basaltic_lava.html|publisher=San Diego State University|access-date=2 August 2010|archive-date=8 October 2018|archive-url=https://web.archive.org/web/20181008083242/http://www.geology.sdsu.edu/how_volcanoes_work/Basaltic_lava.html|url-status=dead}}</ref>

Although most shield volcanoes are by volume almost entirely Hawaiian and basaltic in origin, they are rarely exclusively so. Some volcanoes, such as [[Mount Wrangell]] in Alaska and [[Cofre de Perote]] in Mexico, exhibit large enough swings in their historical magmatic eruptive characteristics to cast strict categorical assignment in doubt; one geological study of de Perote went so far as to suggest the term "compound shield-like volcano" instead.<ref name=jvgr-2010>{{cite journal|title=Evolution and hazards of a long-quiescent compound shield-like volcano: Cofre de Perote, Eastern Trans-Mexican Volcanic Belt|journal=[[Journal of Volcanology and Geothermal Research]]|date=30 November 2010|volume=197|issue=4|pages=209–224|doi=10.1016/j.jvolgeores.2009.08.010|author=Gerardo Carrasco-Núñeza|display-authors=etal|bibcode=2010JVGR..197..209C}}</ref> Most mature shield volcanoes have multiple cinder cones on their flanks, the results of tephra ejections common during incessant activity and markers of currently and formerly active sites on the volcano.<ref name=volcworld /><ref name=hvw-Hawaiian /> An example of these parasitic cones is at Puʻu ʻŌʻō on Kīlauea<ref name="worldbook"/>&mdash;continuous activity ongoing since 1983 has built up a {{convert|2290|ft|m|0|abbr=on}} tall cone at the site of one of the longest-lasting rift eruptions in known history.<ref>{{cite web|title=Summary of the Pu'u 'Ō 'ō-Kupaianaha Eruption, 1983-present|url=http://hvo.wr.usgs.gov/kilauea/summary/|publisher=United States Geological Survey - Hawaii Volcano Observatory|access-date=5 February 2011|date=4 October 2008}}</ref>

The Hawaiian shield volcanoes are not located near any [[plate tectonics|plate boundaries]]; the volcanic activity of this island chain is distributed by the movement of the oceanic plate over an upwelling of magma known as a [[Hotspot (geology)|hotspot]]. Over millions of years, the tectonic movement that moves continents also creates long volcanic trails across the seafloor. The Hawaiian and Galápagos shields, and other hotspot shields like them, are constructed of oceanic island basalt. Their lavas are characterized by high levels of [[sodium]], [[potassium]], and [[aluminium]].<ref name=uoo-galapagos />

Features common in shield volcanism include [[lava tube]]s.<ref name="vhp-shield">{{cite web|title=VHP Photo Glossary: Shield volcano|url=http://volcanoes.usgs.gov/images/pglossary/ShieldVolcano.php|publisher=USGS|access-date=23 August 2010|date=17 July 2009}}</ref> Lava tubes are cave-like volcanic straights formed by the hardening of overlaying lava. These structures help further the propagation of lava, as the walls of the tube [[Thermal insulation|insulate]] the lava within.<ref name="usgs-lava tubes">{{cite web|last=Topinka|first=Lyn|title=Description: Lava Tubes and Lava Tube Caves|url=http://vulcan.wr.usgs.gov/Glossary/LavaTubes/description_lava_tubes.html|publisher=USGS|access-date=23 August 2010|date=18 April 2002}}</ref> Lava tubes can account for a large portion of shield volcano activity; for example, an estimated 58% of the lava forming Kīlauea comes from lava tubes.<ref name="vhp-shield"/>

In some shield volcano eruptions, basaltic lava pours out of a long fissure instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broad [[plateau]]. Plateaus of this type exist in Iceland, Washington, Oregon, and Idaho; the most prominent ones are situated along the [[Snake River]] in Idaho and the [[Columbia River]] in Washington and Oregon, where they have been measured to be over {{convert|1|mi|km|0|abbr=on}} in thickness.<ref name="usgs-shield"/>

Calderas are a common feature on shield volcanoes. They are formed and reformed over the volcano's lifespan. Long eruptive periods form cinder cones, which then collapse over time to form calderas. The calderas are often filled up by progressive eruptions, or formed elsewhere, and this cycle of collapse and regeneration takes place throughout the volcano's lifespan.<ref name=volcworld />

Interactions between water and lava at shield volcanoes can cause some eruptions to become [[Phreatic eruption|hydrovolcanic]]. These explosive eruptions are drastically different from the usual shield volcanic activity<ref name=volcworld /> and are especially prevalent at the waterbound volcanoes of the [[Hawaiian–Emperor seamount chain|Hawaiian Isle]]s.<ref name=hvw-Hawaiian />

<gallery heights="100" widths="150" class="center">
File:Aa large.jpg|[[A'a|{{okina}}A{{okina}}a]] advances over solidified pāhoehoe on [[Kīlauea]], [[Hawaii (island)|Hawai{{okina}}i]]
File:Pahoeoe fountain original.jpg|A pāhoehoe [[lava fountain]] on [[Kīlauea]] erupts
File:Erta-ale lac-de-lave 2001.jpg|A [[lava lake]] in the [[caldera]] of [[Erta Ale]], an active shield volcano in [[Ethiopia]]
File:Pāhoehoe lava meets Pacific.jpg|Pāhoehoe flows enter the Pacific Ocean on [[Hawaii (island)|Hawai{{okina}}i island]]
File:Puu Oo cropped.jpg|[[Puʻu ʻŌʻō]], a parasitic [[cinder cone]] on [[Kīlauea]], [[lava fountain]]ing at dusk in June 1983, near the start of its eruptive cycle
File:Thurston Lava Tube, Big Island.jpg| Nāhuku, a [[lava tube]] on [[Hawaii (island)|Hawai{{okina}}i island]], now a tourist attraction in the [[Hawaiʻi Volcanoes National Park]] 
</gallery>