Geoscience Reference
In-Depth Information
The history of ideas
Convection can be driven by bottom heating,
top or side cooling, and by motions of the
boundaries. Although the role of the surface
boundary layer and
slab-pull
are now well under-
stood and the latter is generally accepted as
the prime mover in plate tectonics, there is a
widespread perception that active hot upwellings
from deep in the interior of the planet, inde-
pendent of plate tectonics, are responsible for
'extraordinary' events such as plate reorganiza-
tion, continental break-up, extensive magmatism
and events far away from current plate bound-
aries. Active upwellings from deep in the mantle
are viewed as controlling some aspects of surface
tectonics and volcanism, including reorganiza-
tion, implying that the mantle is not passive. This
is called the
plume mode of mantle convec-
tion
. This has been modeled by the injection of
hot fluids into the base of a tank of motionless
fluid.
Numerical experiments show that mantle
convection is controlled from the top by con-
tinents, cooling lithosphere, slabs and plate
motions and that plates not only drive and
break themselves but can control and reverse
convection in the mantle. Studies of the
time dependence in 3D spherical mantle
convection with continental drift
show
the extreme sensitivity to changes of conditions
and give results quite different from simpler sim-
ulations. Supercontinents and other large plates
generate spatial and temporal temperature vari-
ations. The migration of continents, ridges and
trenches cause a constantly changing surface
boundary condition, and the underlying man-
tle responds passively. Plates break up and move,
and trenches roll back because of forces on the
plates and interactions of the lithosphere with
the mantle. Density variations in the mantle
are, by and large, generated by plate tectonics
itself by slab cooling, refertilization of the man-
tle, continental insulation; these also affect the
forces on the plates. Surface plates are constantly
evolving and reorganizing although major global
reorganizations are infrequent. Plates are mainly
under lateral compression although local regions
having horizontal least-compressive axes may
be the locus of dikes and volcanic chains. The
Aegean plate is an example of a 'rigid' plate col-
lapsing, or falling apart, because of changes in
stress conditions.
The mantle is generally considered to convect
as a single layer (whole mantle convection), or at
most two. However, the mantle is more likely to
convect in multiple layers as a result of gravita-
tional sorting during accretion, and the density
difference between the mantle products of differ-
entiation.
Instabilities
Rayleigh--Taylor (RT) instabilities form when a
dense, heavy fluid occurs above a low-density
fluid, such as a layer of dense oil placed, care-
fully, on top of a layer of water. Two plane-
parallel layers of immiscible fluid are stable,
but the slightest perturbation leads to release of
potential energy, as the heavier material moves
down under the (effective) gravitational field, and
the lighter material is displaced upwards. As
the instability develops, downward-moving dim-
ples are quickly magnified into sets of inter-
penetrating RT fingers or plumes. This process
is evident not only in many examples, from boil-
ing water to weather inversions. In mantle geo-
physics, plumes are often modeled by inserting a
light fluid into a tank of a static higher density
fluid. This is meant to mimic the instability of a
hot basal layer. In the later situation, the insta-
bility develops naturally and the density con-
trast is limited. In the injection experiment, the
density contrast is imposed by the experimenter,
as is the scale of the upwelling. There is a dif-
ference between upwellings of intrinsically hot
basal layers and intrinsically light chemical lay-
ers. The former case sets up the lateral temper-
ature gradients that are the essence of thermal
convection.
Rise of deep diapirs
Delamination and sinking of garnet pyroxenite
cumulates, sinking of slabs, and upwelling of
mantle at ridges are important geodynamic pro-
cesses. Diapiric ascent, melt extraction and crys-
tal settling are important processes in igneous
petrology. Basic melts apparently separate from
magma chambers, or rising diapirs, at depths
as great as 90 km and possibly greater. Eclogite
