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Figure 8.19. Convection
in a compressible three-
dimensional rectangular
model mantle in which
viscosity increases with
depth and includes phase
changes at 400 and 660
km. Rayleigh number: (a)
2
(a)
(b)
10 6 , (b) 1
10 7 , (c) 4
×
×
×
10 7 , (d) 6
10 7 , (e) 1
10 8
×
×
(c)
10 8 Note the
change in flow pattern
that takes place with
increasing Rayleigh
number: upper and lower
mantle become stratified,
with episodic avalanches
ceasing at higher Rayleigh
numbers. Colour scheme:
red, hotter; blue, colder.
Colour version Plate 15.
(Reprinted from Phys.
Earth Planet. Interiors , 86 ,
Yuen, D. A. et al .Various
influences on three-
dimensional mantle
convection with phase
transitions, 185-203,
Copyright (1994), with
permission from Elsevier.)
and (f) 4
×
(d)
(e)
(f)
and hotspots may originate at the CMB. There is clearly much to be learned
from further study of convection processes in increasingly realistic mantle
models: the dynamics of mantle convection remains the subject of much research
activity.
8.2.4 Forces acting on the plates
The cold upper thermal boundary layer which forms in models of thermal con-
vection of the mantle is assumed to represent the lithosphere. The motion of
these lithospheric plates relative to each other and the mantle is associated with
a number of forces, some of which drive the motion and some of which resist
the motion. Figure 8.20 shows the main driving and resistive forces. If the plates
are moving at a constant velocity, then there must be a force balance: driving
forces
=
resistive forces.
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