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Figure 7.14. (a) Extreme
thermal models used to
calculate equilibrium
geotherms beneath an
ocean, O 1 and O 2 , and
beneath an old stable
continent, C 1 and C 2 . Heat
flows Q 0 and Q d are in
mW m −2 ; heat generation
A 0 is in Wm −3 . (b)
Predicted geotherms for
these models. Thin
dashed lines, oceanic
geotherms; thin solid
lines, continental
geotherms; heavy solid
line, equilibrium
geotherm for the PSM
plate model, taking into
account the small-scale
convection occurring in
the thermal boundary
layer (see Fig. 7.10). Grey
shading, region of
overlap. The heavy
dashed line is an error
function for the geotherm
of age 70 Ma (see Section
7.5.2). The mantle
temperature T a is taken as
1300 C. (After Parsons
and McKenzie (1978) and
Sclater et al . (1981b).)
lithosphere, C 1 and C 2 . These have been calculated by using the one-dimensional
heat-conduction equation. The extensive region of overlap of these four geotherms
indicates that, on the basis of surface measurements, for depths greater than about
80 km there need be little difference in equilibrium temperature structure beneath
oceans and continents. All the proposed oceanic thermal models (Section 7.5)
fall within the shaded region of overlap. The solid line is the geotherm for the
oceanic-plate model in Fig. 7.10. The heavy dashed line is the geotherm for the
simple error-function model of Section 7.5.2.Figure 7.15 shows thermal models
of oceanic and old continental lithospheres.
7.7 The adiabat and melting in the mantle
The previous sections have dealt in some detail with the temperatures in the con-
tinental and oceanic lithosphere and with attempts to estimate the temperatures
in the mantle and core, assuming that heat is transferred by conduction. For the
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