Geoscience Reference
In-Depth Information
but collect above it. These sheets are typically several thousand kilometres apart,
a scale similar to, but less than, the spacing of subduction zones in the Earth.
This cold material is gravitationally unstable; when enough has collected at the
base of the upper mantle a catastrophic avalanche into the lower mantle ensues.
Sudden avalanches of cold material into the lower mantle may take place at several
locations at one time and descend as cylinders directly to the CMB. This pattern
of downwelling could explain the images of the mantle determined from seismic
tomography, which have extensive high-velocity regions at the CMB and a lower
mantle that is characterized by long-wavelength anomalies (Figs. 8.6 and 8.8). The
hot wide upwelling regions that developed in the upper mantle are not associated
with features in the lower mantle. Occasionally, however, narrow hot plumes of
material rise from the CMB and can penetrate the 670-km discontinuity to pass
into the upper mantle. These are, though, neither stable nor weak enough to be
analogous to plume hotspots in the real mantle. With ever-improved computer
modelling using realistic Rayleigh numbers (achieved by using lower and better
estimates of viscosity) and inclusion of the plates, it is expected that the phase
change may prove to be a major factor in the partial separation of the flow regimes
in the upper and lower mantle. Better numerical parameterization of the phase
changes should also enhance their effects; layering is enhanced by narrower
phase transitions and is sensitive to the magnitude and sign of the Clapeyron
slope - the exothermic change at depth 400 km acts against stratification while
the endothermic change at 670 km causes stratification. The metastability of
olivine in the subducting slab (Section 9.6.3) could have a considerable impact,
reducing the negative buoyancy of the slab as well as decreasing the heat released
at 400 km.
Figure 8.19 illustrates the dramatic effect that the Rayleigh number has on the
stratification of convection in a three-dimensional rectangular box. That layering
tends to develop at high Rayleigh numbers can be viewed in a simple manner as
being due to the thinning of the boundary layer and hence its decreasing ability
to penetrate the upper-lower-mantle phase boundary. It is presumed that, during
the Archaean, mantle temperatures were higher, the mantle viscosity was lower
and plate velocities were high: together these imply that the Rayleigh number
was higher. This could have had a major influence on the style of Archaean
mantle convection, with a totally stratified system operating until such time that
sufficient cooling had occurred for penetration of upper-mantle material into the
lower mantle to take place. It is important that factors such as depth-dependent
physical properties, temperature-dependent viscosity and plates on the surface
be included in realistic spherical-shell convection models in order to determine
whether these patterns of convection are similar to what actually occurs in the
mantle.
Thus it seems that the geochemists and the geophysicists may perhaps
both
be correct - the mantle
is
stratified, but descending cold lithosphere
can
peri-
odically can descend to the CMB, subducting plates
are
impeded at 670 km
