Geoscience Reference
In-Depth Information
occurs at greater depths for higher mantle tem-
peratures or lower melting temperatures, allow-
ing higher degrees of partial melting. Any form
of adiabatic decompression can give rise to par-
tial melting if the solidus is crossed. Partial melt-
ing can be initiated when the plate is thinned
by extension or by delamination of the lower
crust or removal of the lower part of the TBL.
Partial melting can thus occur by an increase in
temperature, by adiabatic decompression, or by
the depression of the solidus by the presence of
eclogite, CO
2
or water. These processes can occur
in a variety of tectonic environments, depend-
ing upon the background thermal state. Regions
of high heat flow, such as midocean ridges and
highly extended continental-crustal regions, are
characterized by geotherms with steep tempera-
ture gradients such that the base of the thermal
boundary layer lies well within the partial melt-
ing P-T field. In this case, partial melting occurs
where the mantle adiabat intersects the solidus.
Regions characterized by low heat flow, such as
the stable interiors of continents, are not prone
to melting as cold geotherms never intersect the
solidus of dry peridotite. Partial melting of the
mantle wedge overlying subducting slabs occurs
because the peridotite solidus has been depressed
by the addition of volatiles. Delaminated lower
continental crust also lowers the melting point
of the mantle that it sinks into.
and tearing resistance, mantle viscosity and
bot-
tom drag
. If convection currents dragged plates
around, the bottom drag force would be the
most important. However, there is no evidence
that this is a strong force, and even its sign is
unknown (driving or resisting drag). The ther-
mal and density variations introduced into the
mantle by subduction also generate forces on the
plates.
The pull of subducted slabs -- the slab-pull
force -- is thought to be the main driver of
the motions of Earth's tectonic plates and the
motions beneath the plates. A slab mechanically
attached to a subducting plate can exert a direct
pull on the plate; a detached slab may drive a
plate by causing a flow in the mantle that exerts
a shear traction on the base of the plate. A cold
slab can also set up thermal gradients that exert
forces on the plates. Slab pull forces may account
for about half of the total driving force on plates.
Slabs in the deeper mantle are supported by vis-
cous mantle forces and they may reach density
equilibration.
Mantle convection may also be driven primar-
ily by descent of dense slabs of subducted oceanic
lithosphere. Slab forces cause both subducting
and overriding plates to move toward subduction
zones and they are also responsible for the migra-
tion of trenches and ridges. Ridges and trenches
are stationary in the mantle reference frame only
in very idealized symmetric cases. Cooling plates
also exert forces that cause the plates to move
away from ridges and toward subduction zones.
One can alternatively think of mantle convection
as the passive response to plate-tectonic stresses
and thermal gradients created by plate tecton-
ics and lithospheric architecture. Although both
the plates and the underlying mantle are parts
of the same convecting system it is useful to
think of where most of the buoyancy and dissi-
pation in the system resides. In 'normal' convec-
tion most of the energy comes from outside the
system (bottom heating) and leaves at the top,
and the buoyancy (via thermal expansion) and
dissipation (viscosity) are distributed internally.
In the mantle, much of the energy is provided
from within (radioactivity) and much of the buoy-
ancy and dissipation occurs in the surface layer;
the upper and lower TBLs are not symmetric;
Forces
Plate tectonic and convective motions represent
a balance between driving forces and resisting
forces. Buoyancy is the main driving force but
there are a variety of resisting forces. Negative
buoyancy is primarily created by cooling at the
surface. Positive buoyancy is created by heating
and melting.
The creation of new plates at ridges, the subse-
quent cooling of these plates, and their ultimate
subduction at trenches introduce forces that
drive and break up the plates. They also intro-
duce chemical and thermal inhomogeneities into
the mantle. Plate forces such as
ridge push
-- a
misnomer for the pulling force created by a cool-
ing plate --
slab pull
and
trench suction
are basically
gravitational forces generated by cooling plates.
They are resisted by transform fault, bending
