Page 402 - Global Tectonics

384 CHAPTER 12

1600
1400

Number of observations 1000 Global
1200

800
600
400

200
0
0
Continents
0
Oceans
0 50 100 150 200 250
–2
Heat flow (mW m )

Figure 12.4 Comparison of the heat flow from continents and oceans (redrawn from Pollack et al., 1993, by permis-
sion of the American Geophysical Union. Copyright © 1993 American Geophysical Union).

enriched in the upper continental crust, and it has been mantle. Indeed, conduction cannot occur to any great
−2
estimated that their decay contributes 18–38 mW m to depth as the rate of heat transfer by this mechanism
the observed heat flow (Pollack & Chapman, 1977). is much slower than required. The feasibility and form

Consequently up to about 60% of the heat fl ow in of such convection is discussed in the following
continental regions may be generated within the upper sections.
10–20 km of the crust. The oceanic crust, however, is
virtually barren of radioactive isotopes, and only about
−2
4 mW m can be attributed to this source. Over 96% of
the oceanic heat fl ow must originate from beneath the 12.5 CONVECTION
crust, and so different processes of heat supply must act
beneath continents and oceans (Sclater & Francheteau, IN THE MANTLE
1970).
Thus, a large proportion of the continental heat
flow is from sources concentrated at a shallow depth,

and only a small sub-crustal component is required. 12.5.1 The convection process
Conversely, the majority of oceanic heat fl ow must

originate at sub-crustal levels. Because of the melting The nature of convective flow in the mantle is prob-
problems discussed above, this heat must be trans- lematic. Analytical solution is difficult because of the

ported under the influence of a low thermal gradient. complex rheological structure, including the presence

The mechanism of heat transfer by convection is the of a transition zone (Section 2.8.5), the presence of
only feasible process conforming to these constraints. heat sources within the convecting layer as well as

Therefore, although heat transfer by conduction takes beneath it, the influence of an overlying rigid litho-
place within the rigid lithosphere, heat transfer by sphere on the pattern of convection, and the fact that
convection must predominate in the sublithospheric the convecting layer has the form of a spherical shell.