Earth's internal convection. Diagram of a sectioned Earth, showing the convection (yellow arrows) in the mantle that drives volcanic activity, crustal spreading, and the movement of the continents. The Earth's core is at lower left. The first continents arose where oceanic crust accumulated and thickened over areas of mantle down-welling (continental crust shown at centre right). Stable continents first formed around 2.5 billion years ago, 2 billion years after the Earth formed. It would be another 2 billion years before complex marine and land life would evolve, with plate tectonics continually reforming the shape of the continents.
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