Geoscience Reference
In-Depth Information
are needed to understand this fluid. The effect
of pressure suppresses the role of the lower ther-
mal boundary layer at the core--mantle--boundary
(CMB) interface.
The slow uplift of the surface of the Earth
in response to the removal of an ice cap or
drainage of a large lake changes the shape of
the Earth and the geoid; this is not only proof of
the fluid-like behavior of the mantle but also pro-
vides the data for estimating its viscosity. In con-
trast to everyday experience mantle convection
has some unusual characteristics. The container
has spherical geometry. The 'fluid' has stress-,
pressure- and temperature-dependent properties.
It is cooled from above and from within (slabs)
and heated from within (radioactivity) and
from below (cooling of the core and crystalliza-
tion of the inner core). The boundary conditions
and heat sources change with time. Melting and
phase changes contribute to the buoyancy and
provide additional heat sources and sinks. Mantle
convection is driven partly by plate motions and
partly by chemical buoyancy. The boundaries are
deformable rather than rigid. None of these char-
acteristics are fully treated in numerical calcu-
lations, and we are therefore woefully ignorant
of the style of convection to be expected in the
mantle. The cooling plates may well organize and
drive mantle convection, as well as themselves. A
mantle with continents on top will convect dif-
ferently from one with no continents.
The theory of convection in the mantle can-
not be decoupled from the theories of solids
and petrology. The non-Newtonian rheology, the
pressure and temperature sensitivity of viscosity,
thermal expansion, and thermal conductivity,
and the effects of phase changes and compress-
ibility make it dangerous to rely too much on
the intuition provided by oversimplified fluid-
dynamic calculations or laboratory experiments.
There are, however, some general characteris-
tics of convection that transcend these details.
Technical details of normal or classical thermal
convection can be found in
textbooks on man-
tle convection
. Plate tectonics and mantle
motions, however, are far from normal thermal
convection.
Geochemists consider convection and stir-
ring to be equivalent. They use
convecting
mantle
as convenient shorthand for what they
consider to be the
homogenous upper mantle
.
The underlying assumption is that midocean
ridge basalts, known for their chemical homo-
geneity, must come from a well-stirred mantle
reservoir.
Generalities
SOFFE systems are extraordinarily sen-
sitive to boundary
and initial conditions.
The corollary is that small differences between
computer or laboratory simulations, or between
them and the mantle, can completely change
the outcome. The effect of pressure suppresses
the role of the lower thermal boundary layer
(TBL) at the core--mantle--boundary (CMB) inter-
face. The state of stress in the lithosphere defines
the plates, plate boundaries and locations of mid-
plate volcanism. Fluctuations in stress, due to
changing boundary conditions, are responsible
for global plate reorganizations and evolution
of volcanic chains. In
Rayleigh-Bénard con-
vection
, by contrast, temperature fluctuations
are the important parameters. In plume theory,
plates break where heated or uplifted by hot
buoyant upwellings. Ironically, the fluid flows in
the
experiments by Bénard
, which motivated
the
Rayleigh theory of thermal convec-
tion
, were driven by surface tension, i.e. stresses
at the surface.
Computer simulations of mantle convection
have not yet included a self-consistent thermo-
dynamic treatment of the effect of temperature,
pressure, melting and volume on the physical
and thermal properties; understanding of the
'exterior' problem (the surface boundary con-
dition) is in its infancy. Plate tectonics itself
is implicated in the surface boundary condi-
tion. Sphericity, pressure and the distribution of
radioactivity break the symmetry of the prob-
lem and the top and bottom boundary condi-
tions play quite different roles than in the simple
calculations and cartoons of mantle dynam-
ics and geochemical reservoirs. Conventional
(Rayleigh--Bénard) convection theory may have lit-
tle to do with plate tectonics. The research oppor-
tunities are enormous.
