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that of plate tectonics, implying wide convective
cells -- large aspect ratio. A large horizontal wave-
length of convection can significantly reduce the
efficiency of heat transfer. The likely variations
of continental insulation and wavelength with
time imply important variations of the heat flow
on timescale of 100 Ma.
The proposition that the pattern of convec-
tion is preferentially the one that maximizes the
heat transfer efficiency is not supported by exper-
iments or calculations. Large plates and large
convective cells do not favor maximal heat trans-
fer efficiency. The present situation -- smallish
continents and midocean ridges well distributed
over the surface of the Earth -- probably leads to
an abnormally present high heat loss. Something
is probably minimized or maximized in the oper-
ation of plate tectonics but efficiency of heat loss
is not the control parameter. If the present day
heat flow through the surface is higher than aver-
age, there may be no heatflow paradox.
stumbling over the difficulty imposed
by convection models
and their interpreta-
tion of mantle tomography. This is similar to
the dispute between Lord Kelvin and the geol-
ogists regarding the age of the Earth. The geo-
physical approach uses simplified
parameter-
ized models of mantle cooling
or approx-
imations of the depth dependence of thermal
properties. The calculated rate of cooling of the
Earth at the beginning of its history is too rapid
to allow a sufficient present-day secular cooling
rate to explain the mismatch. Geochemical esti-
matesofU,ThandKinthemantlearetoolow
to explain the observed present mantle heat loss.
Cooling rates can be lowered by continental insu-
lation, by chemically layered mantle structures
and by having large aspect ratio convection cells,
dictated by surface plates rather than the depth
of the mantle. The heat transfer rate strongly
depends on the wavelength of convection. The
length scale of convection in Earth's mantle is
