Geology Reference
In-Depth Information
218.3 Ma
349.6 Ma
441.9 Ma
587.0 Ma
536.7 Ma
738.7 Ma
0.0
Temperature
2840.
0.0
Temperature
2840.
Figure 6.4. Convection in a constant-viscosity fluid. In the left-hand panels the
fluid is heated through the base and cooled through the top, so there is a hot
thermal boundary layer at the bottom and a cool thermal boundary layer at the
top. In the right-hand panels the fluid is heated internally and cooled through the
top, so there is only a cool thermal boundary layer at the top, from which drips
fall erratically. (Internal heating simulates the effect of radioactive heating of the
mantle.)
drips would form well before the plate material reached the subduction zone. In
Figure 6.3, the convection 'cell' under the left-hand plate is about three times
wider than the depth of the box. This corresponds roughly with the situation in
the mantle. The mantle is about 3000 km deep, yet the larger plates are 7000 to
14 000 km across, two to four times the depth of the mantle. The ratio of cell width to
depth is often called the
aspect ratio
, and for the larger plates the aspect ratio is 2-4.
Actually the examples in Figures 6.2 and 6.3 are not strictly comparable because
in Figure 6.2 the vigour of convection is lower and it has a hot thermal boundary
layer at the bottom. However, the distinctly different behaviour in Figure 6.3 is
not due to either of these factors. This can be seen from Figure 6.4, which shows
convection in a fluid of constant viscosity at a vigour similar to that of Figure 6.3.
The left-hand panels have both thermal boundary layers (in other words, the fluid is
heated through the base as well as cooled through the top), whereas the right-hand
panels have only the top thermal boundary layer, as in Figure 6.3. As you can see,
the pattern of convection in Figure 6.4, right-hand panels, is quite different from
that in Figure 6.3. In Figure 6.4, drips form erratically at the top surface, and they
actually migrate laterally with time, which is why they are not vertical columns.
It is worth mentioning that two other factors may contribute to the large aspect
ratio of mantle convection [1]. One is that the lower mantle is inferred to have
