The image on the left depicts what Earth might have looked like more than 3 billion years ago in the early Archean. The orange shapes represent the magnesium-rich proto-continents before plate tectonics started, although it is impossible to determine their precise shapes and locations. The ocean appears green due to a high amount of iron ions in the water at that time. The timeline traces the transition from a magnesium- rich upper continental crust to a magnesium-poor upper continental crust. Credit: Ming Tang/University of Maryland Image credit: Ming Tang/University of Maryland
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