Computational models have played a key role for estimating ages related to large scale geologic events such as the Ice Age and Genesis flood. Embedded within the computational model is a critical component called the material model, which in many cases can be the most important feature of the calculation. Simple material models have been applied to assess the timing of the Ice Age and Genesis flood. To achieve higher accuracy for time predictions, more physically based material models are necessary to simulate the complex theological behavior. In this paper, an internal state variable (ISV) unified-creep-plasticity model that captures history effects of microstructural features within a material is presented to describe high confining pressure, deviatoric behavior of mantle rheology. Stress is not just a function of strain, strain rate, and temperature but is a function of the history of the microstructure as well. The thermodynamics and internal state variable hardening rate equations are presented. The ISV model is compared to power law creep for polycrystalline ice and polycrystalline olivine to illustrate that simple material models cannot predict behavior that is often experienced in complex boundary value problems, such as, trying to simulate creeping/surging Glacial ice or thermal runaway of the subducting lithosphere that may have occurred in the Genesis flood.
