Laboratory and numerical studies of dynamical processes related to thermal convection in the Earth's mantle
Abstract
The results are presented of both numerical simulations and laboratory experiments designed to study various problems related to the structure of mantle convection. The effects were studied of internal heat generation, variable viscosity and plates on the structure of thermal convection. Numerical and laboratory studies of timedependent thermal convection in a fluid with constant properties were performed in large aspectratio configurations in which the fluid is heated from below as well as internally. These studies show that the structure of convection changes dramatically as the amount of internal heating is increased from zero. In situations where most of the heat is supplied internally, instabilities of the cold, upper boundary layer become the prominent features of the flow. In the absence of internal heating the convection planforms are the symmetric spoke pattern in which connected spokes of ascending and descending flow have, on average, equal strength. However, with large amounts of internal heat production, the planform becomes asymmetric with buoyancy concentrated in isolated descending acurate sheets and cylindrical plumes. With the exception of the plumes, the planform resemble the manner in which the midocean ridges and subduction zones are distributed. The planform of convection in a Boussinesq fluid with a temperaturedependent viscosity is also investigated with laboratory and numerical experiments. When the fluid is bounded above and below by rigid boundaries the planforms obtained are spoke patterns. However, when a stressfree condition is applied to the upper boundary the descending flows form a dendritic network of descending sheets and upwellings occur in the form of plumes as well as sheets. The coupling between plate motions and mantle convection is studied using a numerical model consisting of a thin nonNewtonian layer atop a thick Newtonian viscous fluid layer. The nonNewtonian layer has a simple power law rheology characterized by a power law index n and stiffness constant mu sub p. A systematic study of steady, single cell configurations shows that under certain conditions the nonNewtonian layer behaves like a mobile tectonic plate. Time dependent calculations indicate that the geometry and number of plates does not necessarily resemble the planform of convection below.
 Publication:

Ph.D. Thesis
 Pub Date:
 1992
 Bibcode:
 1992PhDT........19W
 Keywords:

 Boundary Layers;
 Earth Mantle;
 Free Convection;
 Mathematical Models;
 Planforms;
 Subduction (Geology);
 Temperature Effects;
 Aspect Ratio;
 Heat Generation;
 MidOcean Ridges;
 Newtonian Fluids;
 Plumes;
 Temperature Dependence;
 Time Dependence;
 Viscosity;
 Viscous Fluids;
 Geophysics