Other
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusm.u42a..03m&link_type=abstract
American Geophysical Union, Spring Meeting 2007, abstract #U42A-03
Other
1212 Earth'S Interior: Composition And State (7207, 7208, 8105, 8124), 1213 Earth'S Interior: Dynamics (1507, 7207, 7208, 8115, 8120), 5430 Interiors (8147)
Scientific paper
A broad range of phenomena are influenced by the behavior of thermal boundary layers in planetary mantles including plume temperatures, lithospheric stresses, resistance to plate motions, and the temperature structure of the mantle as a whole. The textbook picture of the temperature profile in a convecting layer consists of two boundary layers separated by a well-mixed, adiabatic interior. The sum of the temperature drops across the upper and lower boundary layers is equal to super-adiabatic temperature drop across the entire layer. This picture does not accurately describe, however, the horizontally averaged temperature structure derived from numerical solutions of the equations of infinite Prandtl number, Boussinesq convection. The sum of the average temperature drops across the boundary layers in such models is always greater than the super-adiabatic drop across the whole layer, with the result that some portions of the interior are sub-adiabatic. The excess average temperature drop across each boundary layer is due to the arrival of material from the other boundary layer which has not equilibrated with the well-mixed interior. It is this material which transfers heat conductively across the boundary and thus controls the heat transport of the layer. Internal heating breaks the symmetry of the boundary layers (as does temperature dependence of viscosity), and it is the interaction between the two boundary layers that sets the equilibrium temperature drops. The scaling of the temperature drop across each boundary layer is controlled by two competing factors which depend on the Rayleigh number in different ways: the scale of boundary layer instabilities and the velocity of plumes (hot and cold). Furthermore, these scalings change as the system becomes time-dependent at moderate Rayleigh number. At very high Rayleigh number, beyond that of most planetary mantles, the plumes do equilibrate with the interior and the textbook picture applies. A scaling theory for the average temperature drop across the boundary layers will be presented and compared to numerical solutions for isoviscous and non-Newtonian rheologies, with and without internal heating.
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