The Proceedings of the Eighth International Conference on Creationism (2018)

B. Recrystallization, dislocation-grain boundary interaction, and grain boundary sliding As the temperature increases, new crystal grains are nucleated, and their sizes grow (recrystallization) consuming dislocations (i.e., plastically stored energy) and smaller grains. Because this recrystallization process decreases the dislocation density, the mantle’s strength also decreases when recrystallization takes place. The process of recrystallization is affected by pressure, temperature, strain rate, and dislocation density. As the pressure increases, recrystallization is inhibited due to the increase in activation energy. By contrast, as the temperature and the dislocation density increase, the recrystallization rate increases. The strain rate effect is more complicated, but, in general, the static component of recrystallization increases as the strain rate decreases, while its dynamic component increases as the strain rate increases (Derby and Ashby 1987; Doherty et al. 1997). As mentioned earlier, the interactions between dislocations and grain boundaries can harden the rock. As the grain size decreases, the total surface area of grain boundaries increases, so that the likelihood of dislocation-grain boundary interactions increases. This effect causes the dislocation density to increase and the rock strength also to increase. Since the grain size depends on temperature, pressure, strain rate, and dislocation density, the strength change associated with dislocation-grain boundary interactions kinetics is also a function of these factors. Conversely, because the diffusion rate of point defects increases when grain size is small, the strength of rock can also decrease when the grain size is small. Because the mineral crystal is covalently bonded, dislocation movement tends to be slow such that the diffusion of point defects can become more important than the interactions between grain boundaries and dislocations. Therefore, both these competing mechanisms (weakening and hardening at a certain size of grain) must to be taken into account. During the Genesis Flood, the high strain rates most likely favored grain size reduction. Under conditions of high strain rate and moderate temperature that likely prevailed in the upper mantle, the dynamic recrystallization process would have resulted in grain size reduction. Once the grain size is small, other grain size sensitive mechanisms like “grain boundary sliding” can become dominant. Indeed, recent studies (Hansen et al. 2012; Ohuchi et al. 2015) on the deformation of mantle rocks suggest that grain boundary sliding may be one of the primary deformation mechanisms in the upper mantle. C. Effects of texture (deformation-induced CPO) Another mechanism that can greatly reduce rock strength in the mantle is the deformation-induced CPO. During plastic deformation, the grains rotate at different rates, so the rock ultimately has a distinctive average grain orientation within a distribution of orientations. As explained earlier, a rock’s viscous strength is related to its resistance to further deformation. When the deformation-induced CPO is developed (i.e., when the grains are spinning plastically), resistance to deformation and hence the material strength is reduced. In the mechanics and material science community, these texture effects are also referred to as the “plastic spin” at the microscale (Dafalias 2000) and as “torsional softening” at the macroscale. The latter term has come into use because the stress-weakening effects are more prominent under torsional (shear) deformation than under uniaxial compressive or tensional deformation (Horstemeyer and McDowell 1998). This texture effect may have played an important role during the Flood, especially in regions of the mantle that experienced exceptional amounts of mechanical shear. D. Phase transformation and multicompositional effects When a phase transformation takes place, the newly nucleated minerals initially have a tiny average grain size with a very low dislocation density. The dominant deformation mechanisms such as grain boundary sliding and diffusion creep in these small grains significantly reduce the material’s strength. This argues that notable strength weakening ought to occur at phase change boundaries. Moreover, experimental observations pertaining to grain growth kinetics (Hiraga et al. 2010; Ohuchi and Nakamura 2007; Tasaka and Hiraga 2013; Yamazaki et al. 2010) indicate that the grain growth rate is greatly reduced when a mineral is aggregated with other minerals. This suggests that, because most mantle rocks are composed of several different minerals, the actual grain growth rates in the mantle ought to be significantly smaller than what has been determined in the laboratory for single mineral compositions. Because such a large fraction of the mantle’s volume passed through the transition zone during the Flood, one might expect that weakening from grain size reduction may have played a significant role not only during the Flood but also for several centuries afterward. 3. A constitutive model for rheology of the earth’s mantle As just discussed, multiple rheological mechanisms must have operated simultaneously in the mantle during the Flood. Because the interactions amongmechanisms are so numerous and complex, a pencil and paper analysis is utterly incapable of yielding a confident level of understanding of the dynamics of the Flood cataclysm. Numerical simulations grounded in reliable observational data are necessary to explore such a complex problem. With this motivation, several authors (Baumgardner 2003; Sherburn et al. 2013) have developed numerical models in initial attempts to include some of the basic rheological mechanisms and explore the dynamics of earth’s mantle and lithosphere during the Flood. In particular, Sherburn et al. (2013) implemented a plasticity Internal State Variable (ISV) constitutive model originally developed at Sandia National Laboratories (Bammann 1990; Bammann et al. 1993; Horstemeyer 2000). This work demonstrated that the ISV model has an impressive capacity for capturing essential mechanical behavior of metals, polymers, and mantle rocks. The ISV model utilizes material history variables to track the elastic and the plastic attributes of a material, including its hardening and recovery mechanisms governed by the material’s internal structure. From Sherburn et al. (2013), a crucial microstructural mechanism was identified that likely played a key role in the Genesis Flood event, namely, the dynamic recovery. The ISV model used in the study had been calibrated against experimentally measured stress- strain data for the upper mantle rock type known as lherzolite. With dynamic recovery turned on in the ISVmodel, the TERRA2D simulations showed dramatic strength weakening in the mantle and catastrophic overturn of the mantle within a time span of only a few weeks. Cho et al. ◀ Strength-reducing mechanisms in mantle rock during the Flood ▶ 2018 ICC 710