The terrestrial planetary bodies display a wide variety of surface expressions and histories of volcanic and tectonic, and magnetic activity, even those planets with apparently similar dominant modes of heat transport (e.g., conductive on Mercury, the Moon, and Mars). Each body also experienced differentiation in its earliest evolution, which may have led to density-stabilized layering in its mantle and a heterogenous distribution of heat-producing elements. We explore the hypothesis that mantle structure exerts an important control on the occurrence and timing of geological processes such as volcanism and tectonism. We investigate numerically the behavior of an idealized model of a planetary body where heat-producing elements are assumed to be sequestered in a stabilized layer at the top or bottom of the mantle. We find that the mantle structure alters patterns of heat flow at the boundaries of major heat reservoirs: the mantle and core. This modulates the way in which heat production influences geological processes. In the model, mantle structure is a dominant control on the relative timing of fundamental processes such as volcanism, magnetic field generation, and expansion/contraction, the record of which may be observable on planetary body surfaces. We suggest that Mercury exhibits characteristics of shallow sequestration of heat producing elements and that Mars exhibits characteristics of deep sequestration.