TRANSITIONS IN THERMAL-CONVECTION WITH STRONGLY VARIABLE VISCOSITY

One of the most important material properties influencing the style of convection in the mantles of terrestrial planets is the extreme tempe rature-dependence of viscosity. Three-dimensional numerical convection calculations in a wide (8 x 8 x 1) cartesian box and in a spherical s hell (ratio of inner to outer radius of 0.55, characteristic of terres trial planets) both display two fundamental transitions as the viscosi ty contrast is progressively increased from unity to a factor of 10(5) . These transitions not only mark changes in the style of deformation in the upper boundary layer from mobile-lid to sluggish-lid to stagnan t-lid but also have dramatic effects on the style, planform, and horiz ontal length scales of convection in the entire domain. Vertical varia tions of viscosity are the most important for determining the horizont al length scales of the convective patterns while lateral viscosity va riations play a role in shaping the relative structures of the upwelli ng and downwelling flows, Convection in Venus appears to be represente d most closely by the sluggish-lid regime of convection, whereas the E arth, with plate tectonics, more closely resembles the mobile-lid styl e of convection. Forcing plate-like characteristics onto the convectiv e flows in the form of imposed weak zones and prescribed surface veloc ities results in flow patterns dominated entirely by the form used to enforce the plate-like behavior and tells us little about why the mant le exhibits long-wavelength heterogeneity. (C) 1997 Elsevier Science B .V.