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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
One of the biggest unanswered questions in geology is how and when modern plate tectonics started. Plate tectonics is uncontroversial going back to about 700 million years ago, which is the age of the oldest blueschists. These are rocks that only form in cool steep subduction zone environments that are hallmarks of modern plate tectonics. But past that, there's mountains of different research arguing for dates between 700 million years ago and over 4 billion years ago, and everything in between. If rocks indicative of subduction and mordern-ish tectonics were forming back then, they must not have been preserved for us to see today. Lots of geologists nowadays seem to be settling on a date around 2.7-3 billion years ago for the initiation of modern-style plate tectonics, as there's lots of other evidence from ancient mountain-building events and the evolving chemistry of rocks over time.
Some reasons early Earth didn't have plate tectonics are a) it was still too damn hot for the density contrasts that allow for steep subduction, and b) the continental crust needs time to form - the Earth formed from a homogeneous ball of material and then differentiated into layers, and that takes time. Forming the first granitic continental crust from the original basaltic proto-crust also takes time, and the geology behind how this might have happened is still debated.
Source: I have a Masters in geology, tectonics specifically. I'm sure /u/CrustalTrudger can chime in if I've made any mistakes.
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