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was the last great clearing of the solar system, and, once it passed, the
oceans of the Earth were no longer under the threat of sudden vapour-
ization. Other threats were to emerge in the succeeding eons, but the
oceans could now adjust themselves to the contours of an evolving
Earth; an Earth that might have been preparing to evolve
radically
.
The Last Great Readjustment
How did plate tectonics start—and from what? With little scope for
an answer on Earth, one has to look farther afield for clues. Funda-
mentally, plate tectonics is a means of releasing planetary heat. If a
planet is small and cold—like the Moon today—simple conduction of
heat through the rock mass will suffice. But with a hotter body that
won't do, because rock is a poor conductor of heat. So the heat builds
up until the rock melts, and the molten rock makes its way to the
surface where it can radiate its heat out to space. The question then
becomes
how
does the magma travel.
The Earth in its early days would have been much hotter than now,
with a lot of heat still retained from the energy of the impacts that
constructed it, and with more internal radioactivity. A comparison
has been drawn, remarkably, with one of Jupiter's moons, Io. This
might be far from the Sun but, stretched and squeezed by the immense
tidal forces exerted on it by its giant parent planet, its interior is heated
so much that the flux of heat from interior to surface is about 40 times
that of the modern Earth. This heat is released as magma outpourings
on to the surface, and Io is by some way our solar system's most active
planet. There is no evidence that its volcanism is linked with any form
of super-energetic plate tectonics. Rather, the magma must find its
way to the surface through simple vertical conduits—or, as they have
been called, heat pipes.
The hot early Earth, prior to plate tectonics starting up, was
essentially a heat-pipe planet too, according to the US geologists
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