Editing Magma (section)

=== Decompression ===
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Decompression melting occurs because of a decrease in pressure.<ref name=Wilson>{{cite book |title=Understanding the Earth |author1=Geoff C. Brown |author2=C. J. Hawkesworth |author3=R. C. L. Wilson |page=93 |url=https://books.google.com/books?id=Kgk4AAAAIAAJ&pg=PA93 |isbn=0-521-42740-1 |edition=2nd |year=1992 |publisher=Cambridge University Press}}</ref> It is the most important mechanism for producing magma from the upper mantle.{{sfn|Philpotts|Ague|2009|p=593}}

The [[solidus (chemistry)|solidus]] temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. [[Peridotite]] at depth in the [[Earth's mantle]] may be hotter than its solidus temperature at some shallower level. If such rock rises during the [[mantle convection|convection]] of solid mantle, it will cool slightly as it expands in an [[adiabatic process]], but the cooling is only about 0.3&nbsp;°C per kilometer. Experimental studies of appropriate [[peridotite]] samples document that the solidus temperatures increase by 3&nbsp;°C to 4&nbsp;°C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from the upward movement of solid mantle is critical in the evolution of the Earth.{{sfn|Philpotts|Ague|2009|pp=591-599}}

Decompression melting creates the ocean crust at [[mid-ocean ridges]], making it by far the most important source of magma on Earth.{{sfn|Philpotts|Ague|2009|p=593}} It also causes [[volcanism]] in intraplate regions, such as Europe, Africa and the Pacific sea floor. Intraplate volcanism is attributed to the rise of [[mantle plume]]s or to intraplate extension, with the importance of each mechanism being a topic of continuing research.<ref>{{cite journal |last1=Homrighausen |first1=S. |last2=Geldmacher |first2=J. |last3=Hoernle |first3=K. |last4=Rooney |first4=T. |title=Intraplate Volcanism |journal=Encyclopedia of Geology |date=2021 |pages=52–59 |doi=10.1016/B978-0-12-409548-9.12498-4|isbn=9780081029091 |s2cid=240954389 }}</ref>