Geology Reference
In-Depth Information
13.7 Ma
305.6 Ma
966.1 Ma
0
Temperature (
°
C)
1500
0
log Viscosity
2.0
Figure 6.3. Convection beginning in a fluid with a stiff upper thermal boundary
layer. The left panels show temperature, in grey scale, and streamlines. The right
panels show the viscosity, relative to the least viscous fluid, in a logarithmic grey
scale. The viscosity depends on temperature, though it is capped at a maximum
of 100. It is also reduced, artificially, at either end of the left-hand plate.
spreading, but while the plate is intact there is no upwelling or downwelling that
involves the surface.
This inhibition of deformation within the lithosphere has a major effect on the
spatial
pattern
of convection in the mantle. It means that the plates determine where
the upwellings and downwellings of convection will be. This effect is illustrated in
Figure 6.3, which shows convection beginning in a fluid whose viscosity depends
on temperature, so that the viscosity in the thermal boundary layer is higher than
in the interior of the fluid. However, the viscosity is prescribed to be lower at two
places along the top boundary. This has the effect of separating the stiff thermal
boundary layer into two pieces, simulating plates. The left-hand plate is started with
a cold segment hanging down, so as to initiate flow. Subsequently the left-hand
plate continues to subduct at this location, and new fluid wells up at the left side
of the box and cools as it moves to the right, thus adding to and maintaining the
surface plate. The subducted portion of the plate sinks through the fluid and folds
onto the bottom of the box. Its viscosity stays relatively high for quite a long time
in this numerical model. (The convection in this model is less vigorous than actual
mantle convection, which is why the times shown on the boxes are quite long.)
The important point illustrated by Figure 6.3 is that the locations of the upwelling
and downwelling remain confined to the edges of the plate. No new drips form in
the middle of the left-hand plate, because the fluid within the plate is too stiff. This
contrasts with the behaviour shown in Figure 6.2, in which the distance between
upwellings and downwellings is similar to the depth of the box. If the viscosity
of the thermal boundary layer was the same as that in the interior in Figure 6.3,
