Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 37

lead to further production of early crust. For 'mantle plumes' one
might, in Moore and Webb's parlance, now say 'heat pipes'. These are
also vertical, cylindrical conduits for magma ascent. By whichever
name, this kind of 'precursor tectonics' might have been more effec-
tive at producing continental crust than the plate tectonics that
seemingly succeeded it some 3 billion years ago. Well before the heat-
pipe model was dreamed up, one commonly cited pattern was that
continents grew rapidly until about 3 billion years ago, and then the
rate of their growth slowed. Perhaps that, given this new idea, is not
a coincidence.
What caused the change to a plate tectonic Earth? It is tempting to
link this with the arrival of water on Earth via comets, and its infold-
ing into the depths of the Earth. If such infolding of material (both
rock and water) into the mantle happened on a heat-pipe Earth, this
might have increased the water content of the mantle to a point at
which 'modern' plate tectonics started. Or perhaps the transition was
simply caused by the cooling of the Earth to a threshold where cold
ocean crustal slabs could sink. It is still very early days for the science
of deciphering the early Earth.
The hypothesized 34 switchover from heat-pipe Earth to plate tec-
tonic Earth has been envisaged by Moore and Webb as rapid—a flip
from one planetary state to another. With a new mechanism of heat
loss—pouring magma out on to the surface from ever-open cracks,
thousands of kilometres long, the heat pipes would have congealed
and died. Perhaps, though, mantle plumes might be regarded as the
remaining few. Or perhaps heat pipes will return as shadows of their
former grandeur towards the end of our Earth (see Chapter 8). For
now though, we can take up the extraordinary story of our familiar
Earth, and of how it creates ever-changing shapes in which oceans
can be held.