The convection process

The nature of convective flow in the mantle is problematic. Analytical solution is difficult because of the complex rheological structure, including the presence of a transition zone (Section 2.8.5), the presence of heat sources within the convecting layer as well as beneath it, the influence of an overlying rigid lithosphere on the pattern of convection, and the fact that the convecting layer has the form of a spherical shell.

However, as a result of advances in numerical simulations and analogue modeling, and constraints on the pattern of convection supplied by seismic tomography and past and present plate motions, it is now possible to derive considerable information on the convective process.

Convection in a fluid involves heat transport by motion of the fluid caused by positive or negative buoyancy of some of the fluid, that is, horizontal density contrasts or gradients within it. The latter are typically produced by more dense downwellings from a cold boundary layer or less dense upwellings from a hot boundary layer, but they may also be of compositional origin. Indeed one tends to think of a con-vecting fluid layer as being heated from below and cooled from above, in which case there is a hot thermal boundary layer at its base and a cold thermal boundary layer at the top (Fig. 12.5a). However, it is possible that one of these boundary layers may be weak or absent. In addition, the fluid layer may be heated from within (Fig. 12.5b,c). In Fig. 12.5b the lower boundary layer is missing and the fluid is heated internally. The cold dense fluid sinking from the top boundary layer drives convection and the upwelling is passive rather than buoyant; fluid has to move upwards to create space for the sinking cold fluid. The mantle is probably more like Fig. 12.5 c in that it is heated from below, by heat flowing from the core, and from within by radioactivity. In Fig. 12.5 if the temperature of the lower boundary is fixed in each case then the

COLD

HOT COLD

INSULATING COLD

Temperature

Temperature

Temperature

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).

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).

temperature profiles will be as shown to the right of the figure. If there is no heating from below the temperature in the interior of the fluid will be the same as that at the base of the fluid layer (Fig. 12.5b). If there is some heating from below, in addition to internal heating (Fig. 12.5c), then the interior of the converting fluid will have an intermediate temperature between cases (a) and (b). This results in a greater drop in temperature across the upper boundary layer, and a lower drop in temperature across the lower boundary layer, compared to case (a). This effect of internal heating, whereby the top boundary layer is strengthened and the bottom boundary layer weakened, may therefore be applicable to the mantle.

The effect of internal heating and the lack of a lower thermal boundary layer is illustrated in Fig. 12.6. This shows the results of two numerical models with parameters appropriate to the mantle. The three frames on the left relate to a model with heating from below and no internal heating and those on the right to a model with internal heating and no lower boundary layer. In the first case one can clearly see cold sinking columns and hot rising columns analogous to Fig. 12.5a. In the right hand case only downwellings are apparent and the upwellings are passive and widely distributed (cf. Fig. 12.5b).

Although instructive, these models probably do not accurately simulate convection in the mantle as they assume uniform viscosity throughout the convecting layer, a parallel-sided rather than a spherical layer, thermal convection alone and no phase changes within the fluid. In the Earth's mantle it is now thought that the viscosity increases with depth and that buoyancy is in part created by compositional variations. As discussed in Section 12.9 these two factors appear to stabilize the convective pattern for hundreds of millions of years, whereas the convective patterns developed in the models of Fig. 12.6 are clearly unstable over this period of time.

How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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