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Convective motion in the earth's mantle is considered
to be induced by cooling at its upper surface,
radioactive heating within, and heat flow coming from the core.
However, the characteristics of mantle flow are likely
to differ from those of simple Benard-Rayleigh-type thermal convection
due to material and geometric heterogeneities,
particularly those relating to viscosity variations,
the phase boundary near 670 km depth and the distribution of internal heating.
One geological factor likely to exert particular influence
on the mantle convection patterns is
the existence of oceanic and continental plates.
Such plates impose horizontally heterogeneous conditions
at the top boundary of the mantle
and result in different patterns of convection
than occur for a homogeneous boundary condition.
Continental plates consist of lower-density materials
than oceanic plates, and in contrast rest stably
on the top of the mantle without sinking.
It is also the case that continental plates are thicker
than oceanic plates.
Analysis of seismic data reveals that a positive seismic velocity anomaly region
commonly extends to 400 km depth beneath continents (Jordan 1975).
Thus, the continental plates also behave as thermal conduction layers
and affect the convective motion beneath.
The Wilson cycle of periodic continental collision and breakup
is believed to be one tectonic phenomenon caused
by the existence of the continental plates themselves (Wilson, 1966).
Since the continental plate behaves as a thermal conduction layer,
it impedes the transport of heat to the surface,
and is therefore presumed to result in the fluid beneath it becoming warmer.
This suggests that upwellings are easily produced beneath continental plates.
These features are considered to cause continental breakup
and subsequent re-collision (Anderson, 1982).
Observations of seismic data and gravity anomalies reveal
a negative density anomaly beneath the Atlantic Ocean and Africa,
which is inferred to correspond to the broken remnants
of the latest super continent, "Pangea" (Harger et al., 1985).
Many studies have been performed to investigate the effects
of continental plates on mantle convection.
Laboratory experiments of thermal convection by Guillou and Jaupart (1995)
employed a thermal conduction layer partly covering the top
of a fluid layer to represent the effects of a continental plate.
Those authors showed that an upwelling emerges beneath the thermal layer
and that the horizontal dimension of the convection cells
is equal to the horizontal extent of the thermal conduction layer above.
Laboratory experiments by Yanagisawa (1995) utilized a floating sponge plate
to investigate the temporal development of upwellings
beneath a continental plate and their response to changes
in the position of the plate.
Numerical calculations (e.g. Gurnis 1988, Lowman and Jarvis 1993)
have also succeeded in treating the collisions and breakups of plates;
the upwellings induced under the plates break up and distribute the
plates, and then reassemble them again.
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