Geoscience Reference
In-Depth Information
So, the question becomes, how has the convection rate of the mantle
changed through time? The convection rate is driven, to a first approxi-
mation, by the temperature gradient from the interior to the surface of
Earth; the higher the gradient, the faster the convection. (You can see
how this works with a simple home experiment if you follow this end-
note.
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) The gradient in temperature, in turn, depends on the rate at
which heat is generated in Earth. The middle of Earth is estimated to be
a toasty 5500°C (although this is not known precisely). Some of the
high temperature is related to the tremendous heat generated as Earth
was first accreted from smaller “planetesimals” way back in time.
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In-
deed, in the later stages of this process, a massive object, something like
the size of Mars, is believed to have struck Earth, spitting of the Moon
as a result. This collision would have reduced Earth to a molten mass.
Heat is also generated through the radioactive decay of elements
within the mantle and core. Because radioactive isotopes decay into
nonradioactive chemical forms, there has been a steady decrease in the
radioactivity of Earth through time, with a concomitant decrease in
heat production.
All in all, the loss of heat from the early formation of Earth, as well as
a reduction in the heat produced by radioactive decay, should have re-
sulted in a cooling of the planet's interior through time. This, in turn,
should have led to slower rates of mantle convection. Therefore, one can
make the case that the rate of H
2
release has decreased through time in
the face of slower convection. The rate of decrease in the H
2
flux through
time is of extreme importance here, but unfortunately, this rate is still
pretty much guesswork. If we assume, for example, that the H
2
release
rate has decreased linearly with the flow of heat from the mantle, we
make one set of predictions, and poor ones at that, because the history
of heat flow through time is not that well known. If the H
2
release rate
decreased in a different proportion to heat flow, we make another set of
predictions. Quite frankly, we lack the insights to judge which rate of
decrease is most appropriate, but some reduction in the H
2
flux with
time to the surface environment is reasonably certain.
Despite these difficulties, many researchers, including myself, have
tried to play this game. Indeed, we must if the goal is to understand the
evolution of Earth surface chemistry through time, and the connection
