Page 403 - Global Tectonics
P. 403

THE MECHANISM OF PLATE TECTONICS  385



            However, as a result of advances in numerical simula-  cooled from above, in which case there is a hot thermal
            tions and analogue modeling, and constraints on the   boundary layer at its base and a cold thermal bound-
            pattern of convection supplied by seismic tomography   ary layer at the top (Fig. 12.5a). However, it is possible
            and past and present plate motions, it is now possible   that one of these boundary layers may be weak or
            to derive considerable information on the convective   absent. In addition, the fl uid layer may be heated from
            process.                                     within (Fig. 12.5b,c). In Fig. 12.5b the lower boundary


               Convection in a fluid involves heat transport by   layer is missing and the fluid is heated internally. The

            motion of the fluid caused by positive or negative   cold dense fluid sinking from the top boundary layer


            buoyancy of some of the fluid, that is, horizontal   drives convection and the upwelling is passive rather
            density contrasts or gradients within it. The latter are   than buoyant; fluid has to move upwards to create

            typically produced by more dense downwellings from   space for the sinking cold fluid. The mantle is prob-

            a cold boundary layer or less dense upwellings from   ably more like Fig. 12.5c in that it is heated from below,
            a hot boundary layer, but they may also be of com-  by heat flowing from the core, and from within

            positional origin. Indeed one tends to think of a con-  by radioactivity. In Fig. 12.5 if the temperature of
            vecting fluid layer as being heated from below and   the lower boundary is fixed in each case then the


                     (a)                        COLD                           Temperature








                                                 HOT
                     (b)                        COLD                           Temperature



                                             Internal heating




                                              INSULATING

                     (c)                         COLD                          Temperature


                                             Internal heating





                                                 HOT

            Figure 12.5  Sketches of convecting fluid layers, and their associated temperature–depth profiles, illustrating the


            varying nature of the lower thermal boundary layer depending on the way in which the fluid layer is heated (from

            Davies, 1999. Copyright © Cambridge University Press, reproduced with permission).
   398   399   400   401   402   403   404   405   406   407   408