Today, 70.8% of the Earth is covered in water (± a few tenths of a percent depending on how you account for lakes).1 How has this figure changed over the history of the Earth, and why?
Of course, if we go back to the early Hadean Eon, there were no oceans at all, so obviously there was an upward trend, at least initially - but what was the nature of that upward trend? And, once the oceans were more or less "done" forming, was the fraction of Earth covered by water reasonably constant, or not?
Can't say much about the later epochs, but the 'hellish Hadean' epoch without oceans is kind of an outdated idea.
We have zircon records indicating that crust and oceanic formation was already done to some unknown extent 4.4 Gyrs ago. The zircon analyzed in the nature paper given originates from interaction with sub-crustal material in a watery environment at this date.
Since this percentage is an area ratio, it fluctuates with sea level. Consequently it has fluctuated quite a few during the Phanerozoic (see Haq et al. 1988; Miller et al. 2005 for instance).
Now if we're talking in term of oceanic/continental crust ratio, the present day continental crust age repartition shows that, by 2 Ga, ca. 60% of the present day continental crust was already formed (see e. g. Hawkesworth & Kemp 2006) meaning that most of the variation in oceanic/continental crust occurred prior to the middle Paleoproterozoic.
Haq et al. 1988. Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level changes. SEPM Special Publication, 42: 71-108.
Hawkesworth & Kemp 2006. Evolution of the continental crust. Nature, 443: 811-817.
Miller et al. 2005. The Phanerozoic Record of Global Sea-Level Change. Science, 310: 1293-1298.
