Geology Reference
In-Depth Information
The heat flow from the core is not only hard to estimate, it is entangled with
questions about the thermal evolution of the core and mantle, the rate of growth of
the inner core, the generation of the Earth's magnetic field, and the controversial
possibility of some radioactivity in the core. These topics are well beyond the scope
of this topic, so only the main points will be noted here.
Constraints on the heat flow from the core have been deduced from the energy
requirements of the geodynamo. There are considerable uncertainties, particularly
in the thermal conductivity in the core [65] and in the energy dissipation associated
with the dynamo. Lay
et al.
[51] quote a range of 3-8 TW for the heat flow out
of the core, but some studies deduce values up to 13 TW [64, 66]. Higher heat
flows would imply a relatively young inner core. This in turn would require even
higher heat flows in the past, because the dynamo is less efficient without inner
core crystallisation, and implausibly high core temperatures are then implied to
have existed early in Earth's history. To avoid this difficulty, it has been proposed
that the core contains radioactive
40
K, which would generate extra heat and prevent
the core from cooling so rapidly. However, the geochemical conditions required
to sequester potassium into the core ought to have left other clear geochemical
signatures that are not observed, so this possibility is doubtful. Details of these
arguments, with references, can be found in Davies [65] and Lay
et al.
[51]. Davies
[65] presented thermal evolution models of the core in which all constraints were
plausibly met and the present heat loss from the core is 5-7 TW. We will encounter
a similar evolution model in Chapter 9 and Appendix B.
Thus the estimates of core heat loss and of plume heat flow near the bottom
of the mantle are comparable. This supports Morgan's proposal that plumes come
from a thermal boundary layer at the base of the mantle.
7.3 The dynamics and form of mantle plumes
Although the preceding discussion is based on observations and fairly direct infer-
ences from the observations, without appeal to theory, in fact there is a well-
developed physical theory of thermal plumes. It is based on experiments, numer-
ical models and fairly simple theoretical ideas calibrated by the experiments and
models. The result is a good understanding of how thermally buoyant upwellings in
the mantle ought to behave. This understanding is good enough to be quantitatively
predictive and therefore to be tested. Assertions by some plume sceptics that the
plume hypothesis is infinitely adaptable and so not testable and not scientific are
therefore not true.
On the other hand, there are observations of non-plate volcanic activity that
rather obviously do not fit the predictions of the thermal plume model. Geochemical
interpretations of hotspots indicate that plumes do not have the same composition