Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 34

fundamental proportions of land area and ocean area—would have
been utterly different to today's familiar patterns. An alien planet on
our own Earth indeed.
But what do the rocks tell us about the nature of the sea on the
early Earth? From 3.8 billion years ago we find preserved rock strata,
from which one can say that there was, then, surface water. The earli-
est rocks of all, terribly mangled (but still partly decipherable) rocks
from Greenland, are made up of particles of former sediment—sand
and mud—that could only have been laid down in water, although
whether the water bodies were lakes or seas, shallow or deep, is
unclear.
A little later, there is some evidence that the early Earth may have
possessed deep water. In the 3.5 billion-year-old rocks of the Pilbara
area of Australia, there is a particular combination of strata, including
what were fine, silica-rich muds, pebble layers, and slumped, con-
torted deposits that have been interpreted as deep-sea floor
deposits—the kind of sediments that pile up on the edge of an ocean
floor today. 30
A true ocean? Perhaps. It is the best evidence yet found, but it is by
no means conclusive, for rock patterns of that kind simply indicate
that there has been transport of sediment from shallow to deeper
water, without specifying how shallow is shallow, and how deep is
deep. There is work to do still, to decipher the shape and depth of
those ancient seas.
When would the plate tectonics have started to usher in the kind of
planet, and the kind of ocean basins, that we know today? A number
of curious changes seem to have taken place around 3.2 billion years
ago, still in the depths of the Archaean Eon. Changes occur in zircon
crystals, and in the chemistry of the element hafnium 31 that suggest a
change in the rate at which the Earth's crust was being formed. The
first undoubted traces of modern-style mountain building likewise