Thermal convection planforms in an infinite Prandtl number, isoviscous fluid layer with basal and internal heating are determined using three-dimensional finite difference calculations. With basal heating only, at Rayleigh number RaT=1.5¿105 the planform consists of a weakly time-dependent spoke pattern, with connected networks of equally buoyant up- and down-wellings. With volumetric heat sources the planform is strongly time variable. It consists of negatively buoyant flow concentrated in thermal trenches [discrete, arcuate sheets> and spots [cylindrical columns>, separated by diffuse, nonbuoyant upwellings. Rapid planform changes occur primarily by trench propagation, as observed experimentally by Weinstein and Olson [this issue>. The analogy between thermal trenches in internally heated convection and convergent plate boundaries on Earth suggests that the global pattern of subduction may evolve by propagation of Wadati-Benioff zones along the strike of the slab. ¿ American Geophysical Union 1990