State of current knowledge
The importance of rheology for plates, plate boundaries,
and slabs in generating realistic plate tectonics is illustrated
by Stadler et al. (2010) and Alisic et al. (2012). These studies match plate motions globally and locally in the
no-net-rotation reference frame with very high-resolution
simulations that incorporate lithospheric thickening,
upper-mantle slab temperature profiles defined by seismicity, and lower-mantle buoyancy from seismic tomography
and complex nonlinear rheologies. The inclusion of
nonlinear rheology and variations in rheology at the plate
boundary are important for matching other observables,
such as the state of stress of the slab defined by deep earthquakes. The buoyancy of the slab remains the most important driving factor, but the details of the rheology of the
plate boundary, slab, and mantle are critical for matching
plate velocities and defining the slab-plate coupling.
At present, the slab pull force is understood as the weight
of the slab exerting a pulling stress on the plate it is attached
to. The buoyancy of the slab induces flow, but the slab is
strong enough to support its own weight and transmit all
the stresses to its attached plate, in other words act as a
stress guide (Elsasser, 1969). However, once the slab gets
into the lower mantle and warms up in a more viscous lower
mantle, it is no longer able to support its own weight and is
supported by viscous stresses in the mantle (Conrad and
Lithgow-Bertelloni, 2002; Stadler et al., 2010). This view
reconciles a mantle convection view of plate driving forces
with the more traditional approach of Forsyth and Uyeda
(1975), where the slab pull force pulls at the edge of the slab
and is resisted by viscous drag along the slab interface.
Ridge push is just another source of buoyancy acting
over the entire surface area of oceanic plates. It arises from
the cooling and thickening of the oceanic lithosphere as it
ages (Lister, 1975). At the ridge or for young oceanic lithosphere (<20–30 My), the density contrast with respect to
the mantle is negative and no flow is induced. This driving
force is again balanced by viscous stresses acting at the
bottom of the oceanic lithospheric plates.
The magnitude of individual forces depends on the tectonics of each plate and the local viscosity structure. Globally, the ratio of forces due to the excess buoyancy of slabs
including those in the lower mantle (slab pull) and those
due to lithospheric thickening (ridge push) is 70–80 %
to 30–20 % (Lithgow-Bertelloni and Richards, 1995;
Becker and O’Connell, 2001).
The effect of lateral variations in viscosity, deep continental roots, and parameterized slab strength (van
Summeren et al., 2012) shows that the effect of roots is
minimal and does not affect plate driving forces substantially, but the presence of a laterally varying asthenosphere
is critical and slab buoyancy and strength (slab pull)
remain the most important contribution to the force budget. Locally, however, the tractions exerted by large-scale
mantle upwellings can contribute significantly to the force
balance of individual plates (Lithgow-Bertelloni and
Silver, 1998).
Summary and conclusions
The plate-mantle system is one and indivisible. Plates are
part of mantle convection and interact with all parts of the
mantle through their buoyancy and rheology. The classic
view of plates decoupled from the mantle by a very
low-viscosity asthenospheric layer does not conform to
the present understanding. While the presence of the
asthenosphere is critical to making plates and matching
plate velocities (Tackley, 2000; Richards et al., 2001), it
does not preclude interaction with the deeper mantle as
far as the core-mantle boundary. The terms slab pull and
ridge push are parameterizations of the buoyancy and
strength of plates and slabs. Matching plate motions
and the state of stress of plates and slabs is critically
dependent on the rheology of the plates, slab, and mantle
as well as the plate boundary, which remains the most
uncertain physical property affecting the plate-mantle
system.
Bibliography
Alisic, L., Gurnis, M., Stadler, G., Burstedde, C., and Ghattas, O.,
2012. Multi-scale dynamics and rheology of mantle flow with
plates. Journal of Geophysical Research: Solid Earth
(1978–2012), 117, B10.
Anderson, D. L., 2007. The New Theory of the Earth. Cambridge:
Cambridge University Press.
Becker, T. W., 2006. On the effect of temperature and strain-rate
dependent viscosity on global mantle flow, net rotation, and driving forces. Geophysical Journal International, 167, 943–957.
Becker, T. W., and O’Connell, R. J., 2001. Predicting plate velocities with mantle circulation models. Geochemistry, Geophysics,
Geosystems, 2, 1060.
Bercovici, D., 2003. The generation of plate tectonics from mantle
convection. Earth and Planetary Science Letters, 205, 107–121.
Bird, P., 1998. Testing hypotheses on plate-driving mechanisms
with global lithosphere models including topography, thermal
structure, and faults. Journal of Geophysical Research: Solid
Earth (1978–2012), 103, 10115–10129.
Chapple, W. M., and Tullis, T. E., 1977. Evaluation of the forces that
drive the plates. Journal of Geophysical Research, 82,
1967–1984.
Chase, C. G., and Sprowl, D. R., 1983. The modern geoid and
ancient plate boundaries. Earth and Planetary Science Letters,
62, 314–320.
Coblentz, D. D., Zhou, S., Hillis, R. R., Richardson, R. M., and
Sandiford, M., 1998. Topography, boundary forces, and the
Indo-Australian intraplate stress field. Journal of Geophysical
Research: Solid Earth (1978–2012), 103, 919–931.
Conrad, C. P., and Lithgow-Bertelloni, C., 2002. How mantle slabs
drive plate tectonics. Science, 298, 207–209.
Dziewonski, A. M., Hager, B. H., and O’Connell, R. J., 1977.
Large-scale heterogeneities in the lower mantle. Journal of Geophysical Research, 82, 239–255.
Elsasser, W. M., 1969. Convection and stress propagation in the
upper mantle. In Runcorn, S. K. (ed.), The Application of Modern Physics to the Earth and Planetary Interiors. Hoboken:
Wiley-Interscience, pp. 1–41.
Elsasser, W. M., 1971. Sea-floor spreading as thermal convection.
Journal of Geophysical Research, 76, 1101–1112.
Forsyth, D. W., and Uyeda, S., 1975. On the relative importance of
driving forces of plate motions. Geophysical Journal of the
Royal Astronomical Society, 43, 163–200.
DRIVING FORCES: SLAB PULL, RIDGE PUSH
195
The importance of rheology for plates, plate boundaries,
and slabs in generating realistic plate tectonics is illustrated
by Stadler et al. (2010) and Alisic et al. (2012). These studies match plate motions globally and locally in the
no-net-rotation reference frame with very high-resolution
simulations that incorporate lithospheric thickening,
upper-mantle slab temperature profiles defined by seismicity, and lower-mantle buoyancy from seismic tomography
and complex nonlinear rheologies. The inclusion of
nonlinear rheology and variations in rheology at the plate
boundary are important for matching other observables,
such as the state of stress of the slab defined by deep earthquakes. The buoyancy of the slab remains the most important driving factor, but the details of the rheology of the
plate boundary, slab, and mantle are critical for matching
plate velocities and defining the slab-plate coupling.
At present, the slab pull force is understood as the weight
of the slab exerting a pulling stress on the plate it is attached
to. The buoyancy of the slab induces flow, but the slab is
strong enough to support its own weight and transmit all
the stresses to its attached plate, in other words act as a
stress guide (Elsasser, 1969). However, once the slab gets
into the lower mantle and warms up in a more viscous lower
mantle, it is no longer able to support its own weight and is
supported by viscous stresses in the mantle (Conrad and
Lithgow-Bertelloni, 2002; Stadler et al., 2010). This view
reconciles a mantle convection view of plate driving forces
with the more traditional approach of Forsyth and Uyeda
(1975), where the slab pull force pulls at the edge of the slab
and is resisted by viscous drag along the slab interface.
Ridge push is just another source of buoyancy acting
over the entire surface area of oceanic plates. It arises from
the cooling and thickening of the oceanic lithosphere as it
ages (Lister, 1975). At the ridge or for young oceanic lithosphere (<20–30 My), the density contrast with respect to
the mantle is negative and no flow is induced. This driving
force is again balanced by viscous stresses acting at the
bottom of the oceanic lithospheric plates.
The magnitude of individual forces depends on the tectonics of each plate and the local viscosity structure. Globally, the ratio of forces due to the excess buoyancy of slabs
including those in the lower mantle (slab pull) and those
due to lithospheric thickening (ridge push) is 70–80 %
to 30–20 % (Lithgow-Bertelloni and Richards, 1995;
Becker and O’Connell, 2001).
The effect of lateral variations in viscosity, deep continental roots, and parameterized slab strength (van
Summeren et al., 2012) shows that the effect of roots is
minimal and does not affect plate driving forces substantially, but the presence of a laterally varying asthenosphere
is critical and slab buoyancy and strength (slab pull)
remain the most important contribution to the force budget. Locally, however, the tractions exerted by large-scale
mantle upwellings can contribute significantly to the force
balance of individual plates (Lithgow-Bertelloni and
Silver, 1998).
Summary and conclusions
The plate-mantle system is one and indivisible. Plates are
part of mantle convection and interact with all parts of the
mantle through their buoyancy and rheology. The classic
view of plates decoupled from the mantle by a very
low-viscosity asthenospheric layer does not conform to
the present understanding. While the presence of the
asthenosphere is critical to making plates and matching
plate velocities (Tackley, 2000; Richards et al., 2001), it
does not preclude interaction with the deeper mantle as
far as the core-mantle boundary. The terms slab pull and
ridge push are parameterizations of the buoyancy and
strength of plates and slabs. Matching plate motions
and the state of stress of plates and slabs is critically
dependent on the rheology of the plates, slab, and mantle
as well as the plate boundary, which remains the most
uncertain physical property affecting the plate-mantle
system.
Bibliography
Alisic, L., Gurnis, M., Stadler, G., Burstedde, C., and Ghattas, O.,
2012. Multi-scale dynamics and rheology of mantle flow with
plates. Journal of Geophysical Research: Solid Earth
(1978–2012), 117, B10.
Anderson, D. L., 2007. The New Theory of the Earth. Cambridge:
Cambridge University Press.
Becker, T. W., 2006. On the effect of temperature and strain-rate
dependent viscosity on global mantle flow, net rotation, and driving forces. Geophysical Journal International, 167, 943–957.
Becker, T. W., and O’Connell, R. J., 2001. Predicting plate velocities with mantle circulation models. Geochemistry, Geophysics,
Geosystems, 2, 1060.
Bercovici, D., 2003. The generation of plate tectonics from mantle
convection. Earth and Planetary Science Letters, 205, 107–121.
Bird, P., 1998. Testing hypotheses on plate-driving mechanisms
with global lithosphere models including topography, thermal
structure, and faults. Journal of Geophysical Research: Solid
Earth (1978–2012), 103, 10115–10129.
Chapple, W. M., and Tullis, T. E., 1977. Evaluation of the forces that
drive the plates. Journal of Geophysical Research, 82,
1967–1984.
Chase, C. G., and Sprowl, D. R., 1983. The modern geoid and
ancient plate boundaries. Earth and Planetary Science Letters,
62, 314–320.
Coblentz, D. D., Zhou, S., Hillis, R. R., Richardson, R. M., and
Sandiford, M., 1998. Topography, boundary forces, and the
Indo-Australian intraplate stress field. Journal of Geophysical
Research: Solid Earth (1978–2012), 103, 919–931.
Conrad, C. P., and Lithgow-Bertelloni, C., 2002. How mantle slabs
drive plate tectonics. Science, 298, 207–209.
Dziewonski, A. M., Hager, B. H., and O’Connell, R. J., 1977.
Large-scale heterogeneities in the lower mantle. Journal of Geophysical Research, 82, 239–255.
Elsasser, W. M., 1969. Convection and stress propagation in the
upper mantle. In Runcorn, S. K. (ed.), The Application of Modern Physics to the Earth and Planetary Interiors. Hoboken:
Wiley-Interscience, pp. 1–41.
Elsasser, W. M., 1971. Sea-floor spreading as thermal convection.
Journal of Geophysical Research, 76, 1101–1112.
Forsyth, D. W., and Uyeda, S., 1975. On the relative importance of
driving forces of plate motions. Geophysical Journal of the
Royal Astronomical Society, 43, 163–200.
DRIVING FORCES: SLAB PULL, RIDGE PUSH
195