Geology Reference
In-Depth Information
1700.
Basalt barrier
No basalt
No barrier
1200.
0
4500
Time (Myr)
Figure 9.13. Comparison of the mean temperatures of the numerical models of
this chapter.
The overturns cease after about 1.8 Gyr in this model (about 2.7 Gyr ago). In other
models they ceased after 1.0 and 1.6 Gyr [156]. The reason they cease is that even-
tually the decline in radioactive heating allows plates to be slow enough and thick
enough to penetrate the basalt barrier regularly, thus preventing it from becom-
ing strong enough to block flow. The transition is quite marked (Figure 9.11(a)
and (b)), and once it happens the evolution soon converges on a whole-mantle
trajectory similar to previous models.
While layering persists, the overall heat loss is inhibited, as just noted - between
overturns the upper mantle cools rapidly and its plates slow, so that heat loss is
relatively low (Figure 9.11(b)). As a result, the mean temperature of the system stays
quite high. This is clearly evident in Figure 9.13, which compares the evolution
of the mean temperature in the numerical models of this chapter. Even after the
layering has ceased, the model of Figure 9.10 still has a higher mean temperature
than the others. This is because there is some inhibition in vertical flow through the
transition zone, due both to the vestiges of the basalt barrier (Figure 9.10, lower
right panel) and to the assumption in this model that the thermal displacement of
phase transformations also inhibits flow through the transition zone, though not so
much as to cause layering. (This mechanism has not been dealt with in this topic.
It is covered in
Dynamic Earth
[1].)
9.4.4 Continental collision
Continental collision is another source of behaviour more complicated than the
monotonic declines of Sections 9.1 and 9.2. It is a likely agent of episodicity [157],
though the episodicity is more likely to be regional than global. It is included
under this heading because it is due ultimately to the compositional buoyancy of
continental crust.
