Large oceans of water and a rich atmosphere with just the right elements to support life set Earth apart from its close cousins in the solar system. Earth is also the only planet that has an active outer layer made of large tectonic plates that grind together and dip beneath each other, activity that gives rise to mountains, volcanoes, earthquakes and large continents of land.

Geologists have long debated when these processes, collectively known as plate tectonics,  first got underway. Some scientists propose that the process began as early as 4.5 billion years ago, shortly after Earth?s formation. Others suggest a much more recent start, within the past 800 million years.

 A new study from UC Santa Barbara and the University of Maryland (UMD) provides new geochemical evidence for a middle ground between these two extremes.