Editing Proterozoic (section)

==Subduction processes==
The Proterozoic Eon was a very tectonically active period in the Earth's history. Oxygen changed the chemistry allowing for extensive geological changes. Volcanism was also extensive resulting in more geologic changes.

The late Archean Eon to Early Proterozoic Eon corresponds to a period of increasing crustal recycling, suggesting [[subduction]]. Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6&nbsp;[[billion years ago|Ga]].<ref name=Kearey>{{cite book |last1=Kearey |first1=P. |last2=Klepeis |first2=K. |last3=Vine |first3=F. |year=2008 |title=Precambrian Tectonics and the Supercontinent Cycle |series=Global Tectonics |edition=Third |pages=361–377 }}</ref>

The occurrence of [[eclogite]] (a type of [[metamorphic rock]] created by high pressure, >&nbsp;1&nbsp;GPa), is explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor the formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon.<ref>{{cite journal |last=Bird |first=P. |year=2003 |title=An updated digital model of plate boundaries |journal=Geochemistry, Geophysics, Geosystems |volume=4 |issue=3 |page=1027 |doi=10.1029/2001GC000252 |doi-access=free |bibcode=2003GGG.....4.1027B }}</ref>

As a result of remelting of [[basalt]]ic [[oceanic crust]] due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes.

The long-term tectonic stability of those [[craton]]s is why we find [[continental crust]] ranging up to a few billion years in age.<ref>{{cite book |last=Mengel |first=F. |year=1998 |title=Proterozoic History |series=Earth System: History and variablility |volume=2 }}</ref> It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the [[Phanerozoic]].<ref name=Kearey/> Studies by Condie (2000)<ref>{{cite journal |last=Condie |first=K. |year=2000 |title=Episodic continental growth models: Afterthoughts and extensions |journal=Tectonophysics |volume=322 |issue=1 |pages=153–162 |doi=10.1016/S0040-1951(00)00061-5 |bibcode=2000Tectp.322..153C }}</ref> and  Rino et al. (2004)<ref>{{cite journal |last1=Rino |first1=Shuji |last2=Komiya |first2=Tsuyoshi |last3=Windley |first3=Brian F. |last4=Katayama |first4=Ikuo |last5=Motoki |first5=Akihisa |last6=Hirata |first6=Takafumi |date=August 2004 |title=Major episodic increases of continental crustal growth determined from zircon ages of river sands; implications for mantle overturns in the Early Precambrian |journal=Physics of the Earth and Planetary Interiors |volume=146 |issue=1–2 |pages=369–394 |doi=10.1016/j.pepi.2003.09.024|bibcode=2004PEPI..146..369R |s2cid=140166194 }}</ref> suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period.<ref name=Kearey/>

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