Editing Magma (section)

=== Effects of water and carbon dioxide ===
The change of rock composition most responsible for the creation of magma is the addition of water. Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800&nbsp;°C in the presence of excess water, but near 1,500&nbsp;°C in the absence of water.<ref>{{cite journal | last1 = Grove | first1 = T. L. | last2 = Chatterjee | first2 = N. | last3 = Parman | first3 = S. W. | last4 = Medard | first4 = E. | year = 2006 | title = The influence of H<sub>2</sub>O on mantle wedge melting. | journal = Earth and Planetary Science Letters | volume = 249 | issue = 1–2| pages = 74–89 | doi = 10.1016/j.epsl.2006.06.043 | bibcode = 2006E&PSL.249...74G }}</ref> Water is driven out of the oceanic [[lithosphere]] in [[subduction zone]]s, and it causes melting in the overlying mantle. Hydrous magmas with the composition of basalt or andesite are produced directly and indirectly as results of dehydration during the subduction process. Such magmas, and those derived from them, build up [[island arc]]s such as those in the [[Pacific Ring of Fire]].<ref>{{citation|doi=10.1029/2001RG000108|title=Subduction zones|journal=Reviews of Geophysics|volume=40|issue=4|pages=24–31|year=2002|last1=Stern|first1=Robert J.|bibcode=2002RvGeo..40.1012S|s2cid=15347100 |doi-access=free}}</ref> These magmas form rocks of the [[calc-alkaline]] series, an important part of the [[continental crust]].{{sfn|Philpotts|Ague|2009|pp=374-380}} With low density and viscosity, hydrous magmas are highly buoyant and will move upwards in Earth's mantle.<ref name="Hydrous-magma">{{cite journal | last1 = Drewitt | first1 = J. W. E. | last2 = Walter | first2 = M. J. | last3 = Brodholt | first3 = J. P. | last4 = Muir | first4 = J. M. R. | last5 = Lord | first5 = O. T. | title= Hydrous silicate melts and the deep mantle H2O cycle| journal = Earth and Planetary Science Letters | volume = 581 | page=117408 | date=2022 |doi=10.1016/j.epsl.2022.117408| bibcode = 2022E&PSL.58117408D | s2cid = 246777976 | doi-access = free | hdl = 1983/5cc45839-38b0-45a2-ba4b-5ff2436ad9a1 | hdl-access = free }}</ref>

The addition of [[carbon dioxide]] is relatively a much less important cause of magma formation than the addition of water, but genesis of some [[normative mineralogy|silica-undersaturated]] magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experiments document that the peridotite solidus temperature decreases by about 200&nbsp;°C in a narrow pressure interval at pressures corresponding to a depth of about 70&nbsp;km. At greater depths, carbon dioxide can have more effect: at depths to about 200&nbsp;km, the temperatures of initial melting of a carbonated peridotite composition were determined to be 450&nbsp;°C to 600&nbsp;°C lower than for the same composition with no carbon dioxide.<ref>{{cite journal | last1 = Dasgupta | first1 = R. | last2 = Hirschmann | first2 = M. M. | year = 2007 | title = Effect of variable carbonate concentration on the solidus of mantle peridotite | journal = American Mineralogist | volume = 92 | issue = 2–3| pages = 370–379 | doi = 10.2138/am.2007.2201 | bibcode = 2007AmMin..92..370D | s2cid = 95932394 }}</ref> Magmas of rock types such as [[nephelinite]], [[carbonatite]], and [[kimberlite]] are among those that may be generated following an influx of carbon dioxide into mantle at depths greater than about 70&nbsp;km.<ref>{{cite journal |last1=Wyllie |first1=Peter J. |last2=Huang |first2=Wuu-Liang |title=Influence of mantle CO2 in the generation of carbonatites and kimberlites |journal=Nature |date=September 1975 |volume=257 |issue=5524 |pages=297–299 |doi=10.1038/257297a0|bibcode=1975Natur.257..297W |s2cid=4267906 }}</ref>{{sfn|Philpotts|Ague|2009|pp=259-261, 394-397}}