The era encompasses 90% of earth's history. some of the oldest rocks ever discovered on earth were found in western greenland and were dated at approximately billion years using isotopic dating methods. the break-up of the most recent supercontinent of is responsible for the current arrangement of continents. during the precambrian time, the oceans and atmosphere formed around earth, and the concentration of oxygen gas eventually reached the point where it could support aerobic as the earth cooled, layers formed as gravity pulled the dense material into the interior of the planet, creating a of dense nickel and iron.