Plate tectonics started at least 3.5 billion years ago

Yosemite National Park’s Half Dome

ROCK OF AGES  Plate tectonics may have started as early as 3.5 billion years ago on Earth. That recycling of Earth’s surface leads to the separation of light-colored continental crust (exposed here at Yosemite National Park’s Half Dome) from darker, denser oceanic crust.  

© Basil Greber

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