The oldest part of Earth history was spectacular, but is hard to characterize, because there is not much record in the rocks. The Hadean was the first half billion years, when the core separated, the moon formed and the earth was still being constantly battered by meteorite impacts. We can only find traces of the Hadean, some zircon minerals. These tell us that there was already water on earth then. When the crust began to form we get more complete rock records, like the Canadian Shield. This is Archean time.
This map shows the rocks of the Canadian shield in red and orange. These are mainly Archean in age and represent the first crustal rocks on Earth. The central Upper Peninsula is part of the Canadian Shield.
Archean rocks contain the first clear record of life on earth. We see cyanobacteria fossils in the form of stromatolites, very similar to stromatolites of today’s tropical oceans. Earth’s atmosphere contained little or no oxygen at that time, but the cyanobacteria produced oxygen which began to slowly increase the oxygen in the atmosphere. This buildup of oxygen continued throughout the Proterozoic, which lasted almost half of all earth history.
One important question: Are stromatolites biogenic?
