Geoscience Reference
In-Depth Information
separate lower mantle would be a possible repository. Likewise a lower-mantle
repository could account for the observation that there is much less
40
Ar in the
atmosphere and continental crust than should have been produced by the decay
of primordial
40
K. However, the tomographic images reveal that, although the
descent of some subducting plates is impeded at 670 km, in general the mantle
seems to be one system. The location and extent of any geochemical mantle
reservoir has not been established - a self-consistent model for the Earth that
reconciles geochemistry, plate tectonics and mantle convection remains a goal
and a subject for much research interest.
Advances in computer technology have benefited those making numerical
models of mantle convection. Much more realistic models than the simple rect-
angular two-dimensional models of Figs. 8.14 and 8.16 are now achievable.
Figure 8.18 and Plate 14 show a series of three-dimensional spherical convec-
tion models of the whole mantle. Figure 8.18(a) is the simplest model, with a
constant-viscosity incompressible mantle with all heating being internal. The
wavelength of the convection cells is short compared with the size of the Earth
and there are numerous downwellings. Figures 8.18(b)-(d) show the change in
convection pattern that takes place when the viscosity of the lower mantle is
increased to a more realistic value - thirty times that of the upper mantle. With
this change the wavelength of the convection cells increases and the flow itself
is dominated by sheets that extend right through the mantle. Figure 8.18(e) is
another constant-viscosity model, but for a compressible mantle, rather than an
incompressible mantle (Fig. 8.18(a)). Again this has numerous downwellings and
the flow has a short wavelength. If heating from the core is included in the model
(Fig. 8.18(f)), there is a thermal boundary layer at the base of the mantle and
the convection pattern is dominated by hot upwellings. Figure 8.18(g) shows the
major effects caused by inclusion of an endothermic phase change at 670 km.
Both downgoing and upwelling material is inhibited by the phase change, but
the wavelength of convection cells is not substantially affected and the overall
timescale of the flow is not significantly affected. Figure 8.18(h) shows that, just
as for an incompressible mantle (Fig. 8.18(b)), the wavelength increases when
the viscosity of the lower mantle is increased to thirty times that of the upper
mantle. Figure 8.18(i) shows the effect of including heating from the core on a
model with a compressible layered mantle. Now that three-dimensional models
such as those shown here can be made, it is possible to investigate separately and
together many of the physical parameters which may contribute to the way the
Earth's mantle convects. Important factors are the viscosity of the mantle, phase
changes in the mantle, sources of heat and the inclusion of the plates as the outer
boundary layer.
A three-dimensional numerical model of convection with an exothermic phase
change at 400 km and an endothermic phase change at 670 km resulted in a
layered convection pattern. The upper and lower shells are effectively separate:
downwelling cold sheets in the upper mantle do not penetrate the 670-km horizon
