Heat - The Solid Earth - page 287

This temperature is clearly too high for the real Earth because the temperature at
the visible surface of the Sun is only about 5700 ◦ C-the Earth is not a star. The
model is unrealistic since, in fact, heat is not conducted but convected through
the mantle, and the heat-generating elements are concentrated in the upper crust
rather than being uniformly distributed throughout the Earth. These facts mean
that the actual temperature at the centre of the Earth is much lower than this
estimate. Convection is important because it allows the surface heat flow to
exploit the entire internal heat of the Earth, instead of just the surface portions
of a conductive Earth (see Section 6.1 for Kelvin's conduction calculation of the
age of the Earth).
Another fact that we have neglected to consider is the decrease of the radioac-
tive heat generation with time. Equation (7.48) can be solved for an exponential
time decay and a non-uniform distribution of the internal heat generation; the
temperature solutions are rather complicated (see Carslaw and Jaeger (1959),
Section 9.8, for some examples) and still are not applicable to the Earth because
heat is convected through mantle and outer core rather than conducted.
It is thought that the actual temperature at the centre of the Earth is about
7000 ◦ C, on the basis of available evidence: thermal and seismic data, laboratory
behaviour of solids at high temperatures and pressures and laboratory melting of
iron-rich systems at high pressures.
7.5 Oceanic heat flow
7.5.1 Heat flow and the depth of the oceans
Figure 7.6 shows the mean heat flow as a function of age for the three major
oceans. The average heat flow is higher over young oceanic crust but exhibits a
much greater standard deviation than does that over the older ocean basins. This
decrease of heat flow with increasing age is to be expected if we consider hot
volcanic material rising along the axes of the mid-ocean ridges and plates cooling
as they move away from the spreading centres. The very scattered heat-flow
values measured over young oceanic crust are a consequence of the hydrothermal
circulation of sea water through the crust (which is also discussed in Section
9.4.4). Heat flow is locally high in the vicinity of hot-water vents and low where
cold sea water enters the crust. Water temperatures approaching 400 ◦ Chave been
measured at the axes of spreading centres by submersibles, and the presence of hot
springs on Iceland (which is located on the Reykjanes Ridge) and other islands
or regions proximal to spreading centres is well known. As will be discussed in
Chapter 9, the oceanic crust is formed by the intrusion of basaltic magma from
below. Contact with sea water causes rapid cooling of the new crust, and many
cracks form in the lava flows and dykes. Convection of sea water through the
cracked crust occurs, and it is probable that this circulation penetrates through
most of the crust, providing an efficient cooling mechanism (unless you drive a
Volkswagen, your car's engine is cooled in the same manner). As the newly formed