While the underlying mechanisms governing thermal spallation in rock have been known since the 1930s, our ability to model this behavior remains largely empirical. Leading models of thermal spallation either rely on experimentally derived relationships linking applied thermal stresses to spall production, or employ idealized representations that ignore the effects of rock microstructure. Although such models are useful for describing systems within a given context, they are less suited to extrapolate outside the range to which they are fitted or to derive new insight into how mechanisms at smaller-scales influence spall production. This paper describes a numerical modeling tool designed to conduct explicit simulations of thermal spallation at the grain-scale. The model uses an Eulerian-Godunov scheme to simulate solid and fluid mechanical behavior, permitting both inter- and intra-granular fracture. Simulations conducted with the model illustrate how differences in rock properties, microstructural geometry and mineral volume fractions, combined with variations in thermal and mechanical loading conditions, influence spallation at the grain scale. We discuss the implications of these results on the processes controlling thermal spallation of rock, in particular, the role of micropores in the onset and extent of spallation. (C) 2013 Elsevier Ltd. All rights reserved.