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

Cho et al. ◀ Strength-reducing mechanisms in mantle rock during the Flood ▶ 2018 ICC 717 Figure 6. Representation of the Earth’s mantle initial temperature structure in TERRA2D. Boundary temperatures are fixed at 300 K at the top boundary and 3700 K at the bottom boundary. Laterally, temperatures vary in accord with a single period of the cosine function except at the boundaries. UM, MTZ, and LM represent the upper mantle, the mantle transition zone, and the lower mantle, respectively. RESULTS AND DISCUSSIONS 1. Mechanical effects of weakening mechanisms Once the cold blob of rock begins to sink, an increasing amount of mechanical energy is dissipated as heat in the volume surrounding the blob; consequently, the temperature within the envelope surrounding the blob increases (Fig. 7) as first explained by Baumgardner (2003). This increased temperature near the blob also influences the microstructural features. First of all, the increased annihilation of dislocations weakens the rock near the blob. Second, the grain size of surrounding rock of the blob increases because of the increased temperature from the shear heating (Fig. 8). As the grain size increased, the interaction between dislocation and grain boundary decreases, which also leads to weakening. Therefore, the reduced strength of the rock surrounding the blob contributes strongly to the runaway process. Noteworthy is the fact that the downwelling blob’s grain size decreases to approximately 5 µm as a result of strong deformation combined with dynamic recrystallization under cold temperature. However, average grain size of the leading part of downwelling blob remains at approximately the initial value of 100 µm. This occurs because the leading part of the blob does not undergo large deformations due to its high strength (and high viscosity) because of its relatively colder temperature and the extremely high pressure. In essence, it acts like a penetrator moving hydrodynamically through a thick viscous target. The most prominent mechanism that operates in the upper mantle is that of recrystallization. There the recrystallized volume fraction (i.e., the dislocation free volume fraction) increases to about 87%; this dramatic reduction of the dislocation density (see Fig. 9) leads to strong weakening. In particular, the rock surrounding the descending blob and the blob itself undergoes a large amount of the dynamic recrystallization. The reason for the absence of recrystallization in the transition zone and lower mantle, possibly may be attributed to the extrapolation that we were required to make beyond the limited pressure and temperature ranges provided in the available experimental data. Several experimental studies at high pressures and temperatures report the occurrence of recrystallization during deformation, although these results are largely qualitative at this point (e.g., Farla et al. 2015). Amodel for the recrystallization in the deep mantle is therefore an important future objective. One more important microstructural process that caused a high level of strength weakening was development of texture, or deformation-induced CPO, which is related to plastic grain rotations (plastic spin). Under a shear-dominated stress state, the texture can be highly developed, causing strong weakening of solid rock. The shear-dominated stress occurs around the downwelling blob as shown in the plots of the spin tensor and shear strain (Fig. 10). Therefore, the rock surrounding the blob was weakened additionally by the texture effect as we calibrated the shear stress- strain behavior against the compression stress-strain behavior (see Fig. 4). Fig. 11 shows the snapshots of plastic strains developed during the simulation. The heterogeneous degree of deformation (thus, heterogeneous microstructures) in the sinking blob can also be readily observed from the plastic strain. This result implies that the mantle’s physical properties and its microstructures would be very heterogeneous near the core-mantle boundary where the descending blob arrived. In fact, the seismic data from the earth’s interior have shown that strong heterogeneity currently exists near the mantle’s base, a region known as D˝. In addition, in the simulation most of the lower mantle was deformed only up to 0.5 plastic strain, which is a very small strain compared to that of the upper mantle. However, from an observational standpoint, a remarkably small amount of anisotropy throughout the lower mantle has been found in seismic investigations (Montagner and Kennett 1996), which implies a small degree of plastic deformation (and texture development). Even though more thorough studies are needed, these lines of observational evidence appear to match the simulated results of the present study reasonably well. The cooperative effects on strength weakening are clearly observed in the kinematic hardening, isotropic hardening, stress, and viscosity (Fig. 12) in our simulations. As shown in these stress and viscosity snapshots, the rock surrounding the sinking blob displays very low stress and viscosity values. An interesting feature in the stress and viscosity plots is the presence of distinct low viscosity bands. These low viscosity bands imply that the entire mantle feels the effects of the runaway instability that produces a catastrophic