Archean Vs. Modern Earth (IMAGE) University of Maryland Caption The image at 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 (Fe[II]) ions in the water at that time. The timeline traces the transition from a magnesium-rich (mafic) upper continental crust (UCC) to a magnesium-poor (felsic) UCC. Credit Ming Tang/UMD Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.