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Originally Posted by aurora
It may also be that the Earth's crust was the right composition for plate tectonics only after the Mar's size impactor formed our Moon. I have seen that proposed as well.
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There are rift valleys on other worlds (Valles Marineris on Mars for example) that could have been the start of plate tectonics if there had been enough heat to keep the process going. I don't think a giant impactor is needed.