Plate tectonics - Introducing Tectonics, Rock Structures and Mountain Belts - page 20

If, as seems probable, they represent the
sites of the upwelling hot currents of the
convection system, the lateral flow from
them would not always correspond to
the direction of motion of the plate, but
in some circumstances would oppose
it. Another problem is that the part of
the upper mantle system in which the
most vigorous flow would be expected
is the asthenosphere, which, by defini-
tion, is the weakest part of the mantle
and would seem to be incapable of
transmitting the force necessary to
move the much stronger lithosphere.
The origin of the forces that drive
plate motion, and which therefore
are ultimately responsible for crustal
structures, must therefore be sought
within the lithosphere itself. In the
Earth's large-scale convective system
that transfers heat, and thus energy,
to the surface, heat is lost through the
lithosphere mainly at the ocean ridges,
and it is at the subduction zones that
the cooled lithosphere is returned to
the mantle. Both structures are in a
state of gravitational imbalance, and
it is to this gravitational effect that
we must look to explain plate move-
ment. The forces thus generated are
called, respectively, the ridge-push
and slab-pull forces (Figure 3.14A).

The ridge-push force
An ocean ridge represents a large
volume of warmer, lower-density ma-
terial. The average ridge is around
500 km wide and up to 3 km high, and
so represents a considerable addi-
tional load on the ocean crust. This
extra vertical load is compensated by
the lower density of this whole section
of lithosphere, in the same way as the
mountain ranges are compensated,
as explained in Chapter 2 (see Figure
2.4) so that its total weight is the same
as that of the surrounding ocean floor.
Nevertheless, it is not in gravitational
equilibrium, since the gravitational
effect of this topographic high is to
attempt to restore the extra mass to
the general level of the ocean floor,
thus providing a lateral force tending
to make the extra mass flow sideways
(Figure 3.14B). Complete gravitational
equilibrium can only be achieved
if the various layers of the Earth are
of the same thickness throughout
and become less dense upwards.

by a (usually) smaller force towards
the trench exerted on the upper plate
of the subduction zone, due to its
attachment to the downgoing plate.
Rough calculations of the magnitude
of the forces created by these gravi-
tational effects seem to indicate that
they are the most likely source of the
driving force of the plates. The size of
the lateral force provided by material
flowing horizontally beneath the plates
( mantle drag - Figure 3.14A) is an order
of magnitude smaller and can probably
be discounted. Each of these forces is
subject to the effect of frictional forces
acting to oppose the plate motion, for
example, along transform faults, but
these are obviously not large enough
to materially affect the movement.

Plate interiors
The forces just described act through-
out each plate, as can be shown by the
fact that deformation occurs within
the plates as well as at their bounda-
ries, although to a much lesser extent.
Relatively low-magnitude intraplate
(i.e. within-plate) earthquakes are
widespread and usually caused by
movements along pre-existing weak-
nesses in the crust. The concept of the
'rigid' plate is therefore an approxi-
mation, since all plates, particularly
continental ones, suffer some internal
deformation. In the case of the central
Asian section of the Eurasian plate, this
internal deformation has been consid-
erable, as we shall see in Chapter 11.

The slab-pull force
The downward movement of a cooled
piece of oceanic lithosphere at a sub-
duction zone is due to the gravitational
effect of its greater density compared
to the adjoining mantle. Its effect is to
pull the rest of the oceanic lithosphere
to which it is attached along with it,
thus providing a sideways force towards
the ocean trench; this is known as the
slab-pull force. This force is opposed

warmer,
less dense
ridge push

colder,
denser

slab pull

A

mantle drag

Figure 3.14 A. Mechanisms for plate motion: sketch sections to illustrate: 1) ridge push: warmer, less
dense material of the ridge expands and exerts a sideways push on the adjacent plates; 2) slab pull:
cooler, denser material of the descending slab exerts a downward pull on the subducting plate; 3)
mantle drag: sideways drag of mantle convection current is likely to be much less effective than ridge
push or slab pull. B. The ridge push mechanism: the gravitational effect of the excess topographic
high of the ridge exerts a sideways force on the oceanic plates; the downward gravitational force of
this extra mass is balanced by the effect of the less dense mantle root.

B

excess volume