Hadean, Earth evolution

Since the terrestrial rock record has hitherto failed to furnish unequivocal evidence of prebiotic organic chemistry, early chemical evolution was almost certainly confined to the Hadean era bracketed by the time of Earth s formation ( 4.5 billion years ago) and the onset of the sedimentary record about 3.8 billion years ago (Figure 29). 

What is the most difficult research job in all of science It is the task of deducing the history of Earth s evolution in its first two billion years, during the Hadean eon from 4.5 to 4.0 Ga (IGa = 10 years before present) and the Archean eon from 4.0 to 2.5 Ga. Detectives of ancient rocks are faced with a lack of clues to investigate. Most of Earth s surface is covered by water. The cores of the continents are probably underlain by rocks older than 2.5 Ga but they are hidden by a veneer of younger sediments. Even in localities where Hadean-Archean rocks are readily available, alteration by subsequent geologic events has obscured the record of conditions under which they were formed. It is revealing to compare and contrast the detailed information available in a textbook of Earth s modem ecologies with the sparse collection of facts accepted by disciplinary consensus for the Hadean-Archean eras. 

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