Editing Volcano (section)

===Lava flows===
{{Main|Lava flow}}
[[File:Rinjani 1994 cropped.jpg|thumb|[[Mount Rinjani]] eruption in 1994, in [[Lombok]], [[Indonesia]]]]The form and style of an eruption of a volcano is largely determined by the composition of the lava it erupts. The viscosity (how fluid the lava is) and the amount of dissolved gas are the most important characteristics of magma, and both are largely determined by the amount of silica in the magma. Magma rich in silica is much more viscous than silica-poor magma, and silica-rich magma also tends to contain more dissolved gases.

Lava can be broadly classified into four different compositions:<ref>{{cite book|last1=Casq|first1=R.A.F.|url=https://archive.org/details/volcanicsuccessi0000casr/page/528/mode/2up|title=Volcanic Successions|last2=Wright|first2=J.V.|date=1987|publisher=Unwin Hyman Inc|isbn=978-0-04-552022-0|page=528|url-access=registration}}</ref>

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* If the erupted [[magma]] contains a high percentage (>63%) of [[silica]], the lava is described as ''[[felsic]]''. Felsic lavas ([[dacite]]s or [[rhyolite]]s) are highly [[viscous]] and are erupted as domes or short, stubby flows.{{sfn|Philpotts|Ague|2009|p=70–72}} [[Lassen Peak]] in California is an example of a volcano formed from felsic lava and is actually a large lava dome.<ref>{{cite web|title=Volcanoes|url=https://www.nps.gov/lavo/learn/nature/volcanoes.htm|website=Lassen Volcanic National Park California|publisher=National Park Service|access-date=November 27, 2020}}</ref>

:Because felsic magmas are so viscous, they tend to trap volatiles (gases) that are present, which leads to explosive volcanism. [[Pyroclastic flow]]s ([[ignimbrite]]s) are highly hazardous products of such volcanoes since they hug the volcano's slopes and travel far from their vents during large eruptions. Temperatures as high as {{convert|850|C|}}<ref>{{cite book|last1=Fisher|first1=Richard V.|last2=Schmincke|first2=H.-U.|title=Pyroclastic rocks|date=1984|publisher=Springer-Verlag|location=Berlin|isbn=3540127569|pages=210–211}}</ref> 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 many meters thick.{{sfn|Philpotts|Ague|2009|p=73–77}} [[Alaska]]'s [[Valley of Ten Thousand Smokes]], formed by the eruption of [[Novarupta]] near [[Mount Katmai|Katmai]] in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit.<ref>{{cite web|title=Exploring the Valley of Ten Thousand Smokes|url=https://www.nps.gov/katm/planyourvisit/exploring-the-valley-of-ten-thousand-smokes.htm|website=Katmai National Park and Preserve, Alaska|publisher=National Park Service|access-date=November 27, 2020}}</ref> Volcanic ash that is light enough to erupt high into the [[Earth's atmosphere]] as an [[eruption column]] may travel hundreds of kilometres before it falls back to ground as a fallout [[tuff]]. Volcanic gases may remain in the [[stratosphere]] for years.{{sfn|Schmincke|2003|p=229}}

:Felsic magmas are formed within the crust, usually through the melting of crust rock from the heat of underlying mafic magmas. The lighter felsic magma floats on the mafic magma without significant mixing.{{sfn|Philpotts|Ague|2009|pp=15-16}} Less commonly, felsic magmas are produced by extreme [[fractional crystallization (geology)|fractional crystallization]] of more mafic magmas.{{sfn|Philpotts|Ague|2009|p=378}} This is a process in which mafic minerals crystallize out of the slowly cooling magma, which enriches the remaining liquid in silica.

* If the erupted magma contains 52–63% silica, the lava is of ''[[intermediate composition]]'' or ''[[andesitic]]''. Intermediate magmas are characteristic of stratovolcanoes.{{sfn|Schmincke|2003|p=143}} They are most commonly formed at [[convergent boundaries]] between [[tectonic plate]]s, by several processes. One process is the hydration melting of mantle peridotite followed by fractional crystallization. Water from a subducting [[Slab (geology)|slab]] rises into the overlying mantle, lowering its melting point, particularly for the more silica-rich minerals. Fractional crystallization further enriches the magma in silica. It has also been suggested that intermediate magmas are produced by the melting of sediments carried downwards by the subducted slab.<ref>{{cite journal|last1=Castro|first1=Antonio|title=The off-crust origin of granite batholiths|journal=Geoscience Frontiers|date=January 2014|volume=5|issue=1|pages=63–75|doi=10.1016/j.gsf.2013.06.006|bibcode=2014GeoFr...5...63C|doi-access=free}}</ref> Another process is magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir before emplacement or lava flow.{{sfn|Philpotts|Ague|2009|p=377}}
* If the erupted magma contains <52% and >45% silica, the lava is called ''[[mafic]]'' (because it contains higher percentages of [[magnesium]] (Mg) and iron (Fe)) or [[basalt]]ic. These lavas are usually hotter and much less viscous than felsic lavas. Mafic magmas are formed by partial melting of the dry mantle, with limited fractional crystallization and assimilation of crustal material.{{sfn|Philpotts|Ague|2009|p=16}}

:Mafic lavas occur in a wide range of settings. These include [[mid-ocean ridge]]s; [[Shield volcanoes]] (such the [[Hawaiian Islands]], including [[Mauna Loa]] and [[Kilauea]]), on both [[oceanic crust|oceanic]] and [[continental crust]]; and as continental [[flood basalt]]s.

* Some erupted magmas contain ≤45% silica and produce ''[[ultramafic]]'' lava. Ultramafic flows, also known as [[komatiite]]s, are very rare; indeed, very few have been erupted at Earth's surface since the [[Proterozoic]], when the planet's heat flow was higher. They are (or were) the hottest lavas, and were probably more fluid than common mafic lavas, with a viscosity less than a tenth that of hot basalt magma.{{sfn|Philpotts|Ague|2009|p=24}}
Mafic lava flows show two varieties of surface texture: {{okina}}A{{okina}}a (pronounced {{IPA|haw|ˈʔaʔa|}}) and [[pāhoehoe]] ({{IPA|haw|paːˈho.eˈho.e|}}), both [[Hawaiian language|Hawaiian]] words. {{okina}}A{{okina}}a is characterized by a rough, clinkery surface and is the typical texture of cooler basalt lava flows. Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Pāhoehoe flows are sometimes observed to transition to {{okina}}a{{okina}}a flows as they move away from the vent, but never the reverse.{{sfn|Schmincke|2003|pp=131-132}}

More silicic lava flows take the form of block lava, where the flow is covered with angular, vesicle-poor blocks. [[Rhyolitic]] flows typically consist largely of [[obsidian]].{{sfn|Schmincke|2003|pp=132}}