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the water that used to be continually stirred into them, will begin
to become more viscous and less able to flow. The extraordinary
mechanism of Earthly plate tectonics would, geologically soon, grind
to a halt.
It would be one of the most fascinating episodes of our extraordi-
nary planet. It would be a planet not quite in its death throes (although
precious little if anything would be left of the Earth's biology by then),
for it would still have a few billion years of existence ahead of it. Just
how would it work? The machinery of plate tectonics is unlikely to
simply just stop, smoothly and without fuss. Any time travellers
incautious enough to venture into the time of the great change would
likely see some extraordinary landscapes, as ocean plates were driven
towards an obstacle—the ocean trenches—that they could no longer
pass through, or some parts of ocean ridges stalled while others kept
splitting apart.
The painful transition into an Earth without plate tectonics would
probably be followed by an eerie calm for perhaps some tens of mil-
lions of years. The Earth would be adjusting to a new situation. For
even though its inner fires would be waning it would still have a heat
release problem, and with the halting of plate tectonics that would
need another solution.
One of the new features of the Earth without oceans and with grid-
locked plate tectonics will be a change in the pattern of volcanism. As
we have seen, volcanism is not just an accidental by-product of plate
tectonics. It is a planetary heat control mechanism; a means of releas-
ing the large amounts of heat being produced within the Earth, as
radioactive elements continue to decay within the Earth's interior.
There will still be too much heat being produced, even late in the
Earth's existence as the amounts of radioactivity diminish, to be sim-
ply lost by conduction to the surface. The standard means of plane-
tary heat loss is to melt rock in the interior and allow that to escape to
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