Jt. Ratcliff et al., 3-DIMENSIONAL VARIABLE VISCOSITY CONVECTION OF AN INFINITE PRANDTL NUMBER BOUSSINESQ FLUID IN A SPHERICAL-SHELL, Geophysical research letters, 22(16), 1995, pp. 2227-2230
We investigate a three-dimensional, spherical-shell model of mantle co
nvection with strongly temperature-dependent viscosity. Numerical calc
ulations of convection in an infinite Prandtl number, Boussinesq fluid
heated from below at a Rayleigh number of Ra = 10(5) are carried out
for the isoviscous case and for a viscosity contrast across the shell
of 1,000. In the isoviscous case, convection is time dependent with qu
asi-cylindrical upflow plumes and sheet-like downflows. When viscosity
varies strongly across the shell, convection is also time dependent,
but major quasi-cylindrical downflows with spider-like extensions occu
r at both poles and interconnected upflow plumes occur all around the
equator. The surface expression of mantle convection in the Earth (dow
nwelling sheets at trenches, upwelling plumes at hot spots, and upwell
ing sheets at midocean ridges) resembles structures seen in both the i
soviscous and variable viscosity models. The dominance of spherical ha
rmonic degree l = 2 in the variable viscosity model agrees with the l
= 2 dominance in the Earth's geoid, topography, and seismic tomography
. The overall pattern of convection in the variable viscosity case is
similar to the distribution of major highlands and volcanic rises on V
enus.