Archaean Earth. Illustration of the Earth as it may have been in the Archaean Eon (3.8 to 2.5 billion years ago) in its early history. At this time there was considerably more volcanic activity on Earth, its crust was beginning to cool and its heat flow was far greater. The composition of both the atmosphere and oceans were very different. The columnar formations in the sea are stromatolites. These formed from the precipitation of calcium carbonate by cyanobacteria. The oldest fossil stromatolites date to the early Archaean Eon.