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[QUOTE]Originally posted by DD'eDeN: [QB] Beginnings of Plate Tectonics / Continental Drift 3ba [ Interesting early date, I hadn't known if it began much later due to cosmic collisions and change in sun temps/orbital speed ] Archean upper crust transition from mafic to felsic marks the onset of plate tectonics Ming Tang, Kang Chen & Roberta L Rudnick 2016: Science 351:372-5 doi 10.1126/science.aad5513 The Archean Eon witnessed - the production of early continental crust, - the emergence of life & - fundamental changes to the atmosphere. The nature of the first continental crust (the interface between the surface & deep Earth) has been obscured by the weathering, erosion & tectonism that followed its formation. We used Ni/Co & Cr/Zn ratios in Archean terrigenous sedimentary rocks & Archean igneous/meta-igneous rocks, to track the bulk MgO composition of the Archean upper continental crust. This crust evolved - from a highly mafic bulk composition before 3000 Ma - to a felsic bulk composition by 2500 Ma. This compositional change was attended by a 5-fold increase in the mass of the upper continental crust, due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3000 Ma. New crustal clues from old rocks Brent Grocholski 2016 Science 351:350-1 doi 10.1126/science.351.6271.350-f The ghost of continental crust long eroded away may exist in certain element ratios found in Archean rocks. Tang cs used Ni/Co & Cr/Zn ratios as a proxy for the MgO that long ago weathered away in Earth's oldest rocks. This allowed a reconstruction of rock composition, which appears to be very different from that of the crust today. The shift to contemporary crust composition occurred after the Archean, suggesting the onset of plate tectonics. [/QB][/QUOTE]
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