Geology Reference
In-Depth Information
600 Myr
1500 Myr
2000 Myr
4500 Myr
0.0
1800.
0.0
10.0
Temperature
tracer concentration
Figure 9.10. Evolution of a model in which subducted oceanic crust is buoyant in
the depth range 660-750 km. The subducted crustal material accumulates around
660 km to form a 'basalt barrier' that separates mantle convection into two layers
in the first two panels. After Davies, Case 2 [156]. Copyright Elsevier Science.
Reprinted with permission.
The swings occur because the layering breaks down episodically, allowing hot
fluid from the lower mantle to flood into the upper mantle, suddenly raising its tem-
perature. This is confirmed in Figure 9.12, which shows a breakthrough in progress.
Layering is soon re-established, after which the upper mantle cools smoothly but
