Abstract
Numerical experiments are carried out to study the effects of continents on the structure of thermal convection in the mantle. The mantle is modelled by a viscous fluid occupying a horizontally extended rectangular 2-D region of aspect ratio 10:1. Continents are treated as thick rigid heat-conducting plates placed in the mantle, with free-slip and with no-slip conditions. Continents restrict the heat release from the underlying mantle; the mantle material heats up and becomes lighter; as a result, a hot upwelling flow replaces downwelling. We calculate the characteristic time τ of this restructuring for various values of model parameters and obtain analytical approximations for τ as function of Rayleigh number Ra , plate length L , and plate thickness d . For commonly accepted mean parameters of the Earths mantle, giving Ra =10 7 , with continental plate thickness about 300 km and length about 6000 km, the restructuring time τ is estimated as 2·10 8 a. As the continents are not fixed, but drift, the possibility of forming an ascending mantle flow exists if the continent does not significantly shift during an interval of about τ relative to the mantle by more than half of its length. Hence, under a continent drifting relative to the mantle with speed less than 1 cm⧹a an ascending mantle flow is expected, whereas under rapidly moving continents such an ascending flow has not enough time to develop.
Published Version
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