Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 29

mantle. At some point, sufficient water had been added to allow the
Earth's plate tectonic motor to begin to run. Once triggered, this
motor has been running ever since, smoothing and modulating the
processes of volcanism and earthquakes, and creating the Earth's
mountain chains and ocean basins. It set the scene, quite certainly, for
the evolution and maintenance of life.
Plate tectonics, for all the periodic death and destructiveness caused
by the great volcanoes, earthquakes, and tsunamis associated with its
working, is still a remarkably gentle process compared with all the
alternatives. In essence, it allows the Earth's heat to be released
smoothly and evenly, and it recycles and renews much of the Earth's
surface, piece by piece.
The 'folding in', as Albarède put it, of the Earth's surface water deep
into the interior is something of a mystery process, for there is little
direct evidence of that 'start-up' phase of plate tectonics—although
that has not stopped people thinking about how this might have
happened (see the section 'The Last Great Readjustment'). Once
started though, plate tectonics would certainly have been helped by
the waters of the early oceans that literally acted as a lubricant (as they
still do) to help ease the path for descending crustal plates at subduction
zones.
As regards the oceans, however, the beginnings were likely any-
thing but smooth. For most of the next billion years, rocks were still
raining out of the sky.
Perilous Times for the Hadean Oceans
The 'late veneer' that arguably modified the Earth's crust and brought
in the world's water stores was not the end of the meteorite bombard-
ment. It takes quite a while to clear a young star system of its debris,
or to arrange the remains of that debris—as in the asteroid belt—in
more or less stable orbits. And so, over the millions of years that