Editing Volcano (section)

==Plate tectonics==
{{Main|Plate tectonics}}

According to the theory of plate tectonics, Earth's [[lithosphere]], its rigid outer shell, is broken into sixteen larger and several smaller plates. These move continuously at a slow pace, due to [[convection]] in the underlying ductile [[Earth's mantle|mantle]], and most volcanic activity on Earth takes place along plate boundaries, where plates are converging (and lithosphere is being destroyed) or are diverging (and new lithosphere is being created).<ref>{{cite book|last1=Schmincke|first1=Hans-Ulrich|title=Volcanism|date=2003|publisher=Springer|location=Berlin|isbn=9783540436508|pages=13–20|url=https://books.google.com/books?id=XHlJrFAhth4C&pg=PA13}}</ref>

During the development of geological theory, certain concepts that allowed the grouping of volcanoes in time, place, structure and composition have developed that ultimately have had to be explained in the theory of plate tectonics. For example, some volcanoes are ''[[Polygenetic volcanic field|polygenetic]]'' with more than one period of activity during their history; other volcanoes that become extinct after erupting exactly once are ''[[Monogenetic volcanic field|monogenetic]]'' (meaning "one life") and such volcanoes are often grouped together in a geographical region.<ref>{{cite web|last=Hsu-Buffalo|first=Charlotte|title=Do monogenetic volcanoes threaten the southwestern US?|date=November 4, 2021|work=Futurity|url=https://www.futurity.org/monogenetic-volcano-2652672-2/|access-date=July 21, 2023}}</ref>

===Divergent plate boundaries===
{{Main|Divergent boundary}}
[[File:Spreading ridges volcanoes map-en.svg|thumb|upright=1.8|Map showing the divergent plate boundaries (oceanic spreading ridges) and recent sub-aerial volcanoes (mostly at convergent boundaries)]]

At the [[mid-ocean ridge]]s, two [[list of tectonic plates|tectonic plates]] diverge from one another as hot mantle rock creeps upwards beneath the thinned [[oceanic crust]]. The decrease of pressure in the rising mantle rock leads to [[adiabatic]] expansion and the [[partial melting]] of the rock, causing volcanism and creating new oceanic crust. Most [[divergent plate boundaries]] are at the bottom of the oceans, and so most volcanic activity on Earth is submarine, forming new [[seafloor]]. [[Black smoker]]s (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, such as [[Iceland]].{{sfn|Schmincke|2003|pp=17-18, 276}}<ref name=mor/>

===Convergent plate boundaries===
{{Main|Convergent boundary}}

[[Subduction]] zones are places where two plates, usually an oceanic plate and a continental plate, collide. The oceanic plate subducts (dives beneath 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 reach the surface but [[Igneous intrusion|cools and solidifies at depth]]. When it does reach the surface, however, a volcano is formed. Thus subduction zones are bordered by chains of volcanoes called [[volcanic arc]]s. Typical examples are the volcanoes in the Pacific [[Ring of Fire]], such as the [[Cascade Volcanoes]] or the [[Japanese Archipelago]], or the eastern islands of [[Indonesia]].{{sfn|Schmincke|2003|pp=18,113-126}}<ref name=dkp/>

===Hotspots===
{{Main|Hotspot (geology)}}

[[Hotspot (geology)|Hotspots]] are volcanic areas thought to be formed by [[mantle plume]]s, which are hypothesized to be columns of hot material rising from the core-mantle boundary. As with mid-ocean ridges, the rising mantle rock experiences decompression melting which generates large volumes of magma. Because tectonic plates move across mantle plumes, each volcano becomes inactive as it drifts off the plume, and new volcanoes are created where the plate advances over the plume. The [[Hawaiian Islands]] are thought to have been formed in such a manner, as has the [[Snake River Plain]], with the [[Yellowstone Caldera]] being part of the North American plate currently above the [[Yellowstone hotspot]].{{sfn|Schmincke|2003|pp=18,106-107}}<ref name=bgs/> However, the mantle plume hypothesis has been questioned.<ref name=Foulger>{{cite book|title=Plates vs. Plumes: A Geological Controversy|last1=Foulger|first1=Gillian R.|year=2010|isbn=978-1-4051-6148-0|publisher=Wiley-Blackwell}}</ref>

===Continental rifting===
{{Main|Rift}}

Sustained upwelling of hot mantle rock can develop under the interior of a continent and lead to rifting. Early stages of rifting are characterized by [[flood basalt]]s and may progress to the point where a tectonic plate is completely split.<ref>{{cite book|last1=Philpotts|first1=Anthony R.|last2=Ague|first2=Jay J.|title=Principles of igneous and metamorphic petrology|date=2009|publisher=Cambridge University Press|location=Cambridge, UK|isbn=9780521880060|pages=380–384, 390|edition=2nd}}</ref>{{sfn|Schmincke|2003|pp=108-110}} A divergent plate boundary then develops between the two halves of the split plate. However, rifting often fails to completely split the continental lithosphere (such as in an [[aulacogen]]), and failed rifts are characterized by volcanoes that erupt unusual [[Alkaline magma series|alkali lava]] or [[carbonatite]]s. Examples include the volcanoes of the [[East African Rift]].{{sfn|Philpotts|Ague|2009|pp=390-394,396-397}}