Geoscience Reference
In-Depth Information
cooling is not uniform, the boundary conditions
control the planform of convection. The fluid is
no longer free to self-organize. In most mantle
convection simulations the boundary conditions
are uniform and various patterns and styles
evolve that do not replicate conditions inside the
Earth or at the surface. In many published sim-
ulations the effect of pressure on material prop-
erties is ignored. The few simulations that have
been done with more realistic surface bound-
ary conditions and pressure-dependent proper-
ties are more Earth-like, but plate-like behavior
and realistic plate tectonics do not evolve nat-
urally from the equations of fluid mechanics.
Two-dimensional simulations, in planar or cylin-
drical configurations, cannot hope to capture the
complexity of 3D mantle convection.
Far-from-equilibrium systems
do strange things.
They become inordinately sensitive to exter-
nal or internal influences. Small changes can
yield huge, startling effects up to and including
reorganization of the entire system. We expect
self-organization in slowly driven interaction-
dominated systems. The resulting patterns do
not involve templates or tuning. The dynam-
ics in complex systems is dominated by mutual
interactions, not by individual degrees of free-
dom. Periods of gradual change or calm qui-
escence are interrupted by periods of hectic
activity. Such changes in the geologic record may
be due to plate interactions and plate reorga-
nization, rather than events triggered by man-
tle
An important attribute of plate tectonics is
the large amount of energy associated with
toroidal (strike-slip and transform fault) motion.
This does not arise directly from buoyancy forces
involved in normal convection. In a convecting
system the buoyancy potential energy is balanced
by viscous dissipation in the fluid. In plate tec-
tonics both the buoyancy and the dissipation
are generated by the plates. The slab provides
most of the driving buoyancy and this is bal-
anced almost entirely by slab bending and by
transform fault resistance as characterized by
the toroidal/poloidal energy partitioning of plate
motions. This further confirms the passive role
of the mantle.
In SOFFE systems, such as thermal convection,
it is not always clear what it is, if anything, that
is being minimized or maximized. At one time
it was thought that the pattern of convection in
a heated fluid was self-selected to maximize heat
flow but this did not turn out to be the case. The
plate system may reorganize itself to minimize
dissipation, but this is just one conjecture out
of many possibilities. It can do this most effec-
tively by changing the lengths, directions and
normal stresses across transform faults, reducing
the toroidal component of plate motions, localiz-
ing deformational heating, increasing the sizes of
plates, changing trench-rollback and ridge migra-
tion rates and so on. If plate dynamics is a
far-from-equilibrium system, sensitive to initial
conditions or fluctuations, then we expect each
planet to have its own style of behavior, and the
Earth to have behaved differently in the past and,
perhaps, during different supercontinent cycles.
One does not necessarily expect a simple thermal
history or a balance between heat production and
heat loss.
Dissipation is also involved in the style
of small-scale convection. Tabular or linear
upwellingevolveastheyriseto3Dplume-like
upwellings. Longitudinal and transverse rolls
develop under moving plates, possibly to min-
imize dissipation. Two-dimensional convection
simulations do not capture these 3D effects.
These are some of the reasons why an indepen-
dent style of convection, and source of heat,
is often invoked to explain 'hotspots.' Plate tec-
tonics on a sphere has additional out-of-plane
convection
(
mantle overturns
,
mantle
avalanches
).
Dissipation
Although plate-driving forces are now well under-
stood, the resisting or dissipative forces are the
source of self-organization. The usual thought is
that mantle viscosity is what resists plate and
convective motions. But
bending forces in
plates, for example, may control the
cooling rate of the mantle
. Sources of
dissipation include friction along faults, inter-
nal plate deformation, continent--continent col-
lision, and so on. These forces also generate local
sources of heating. Dissipative forces and local
stress conditions can change rapidly.
