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

mantle overturn. For this work the ISV model was implemented with the associated microstructural features successfully into the TERRA3D code making 3D simulations possible. Figs. 14-18 show the temperature, stress, average grain size, viscosity, and plastic strain, respectively, of an illustrative simulation. Just as in the 2D simulation, extreme strength weakening due to effects of recrystallization, grain size, and texture were observed. Strong mechanical and microstructural heterogeneities produced by the cold material that descended catastrophically from the upper boundary was also observed at the core-mantle boundary. Most notably, the low viscosity bands present in the 2D calculation were also clearly evident in this 3D case, even though the grid resolution was not that high in the radial direction. These low viscosity bands revealed that the entire mantle participated in the catastrophic runaway event. 2. Comparison of weakening mechanisms Finally, we have undertaken a limited study to compare the relative importance of three of the strength weakening mechanisms: recrystallization (REX), grain size (GS), and texture (SS). Fig. 13 summarizes the results of four comparison cases: 1) REX, GS, and SS; 2) REX and GS; 3) REX and SS; and 4) GS and SS. Fig. 13 displays snapshots at a time of 30 days of the base 10 log of the norm of stress. To summarize the findings, without any of these three microstructural mechanisms, the mantle still shows a tendency to overturn in the catastrophic manner due to the shear heating and the recovery process as shown in the previous studies of Baumgardner (2003) and Sherburn (2013). However, with the three mechanisms, the strength of mantle is reduced even more. When the texture effect is turned off (Case #2), the overall mantle strength increases significantly such that the maximum flow velocity decreases by almost half. By comparison, the grain size and recrystallization mechanisms appear to have much smaller effects, but they nevertheless reduce rock strength in localized regions such as in the upper mantle and mantle near the cold descending blob. As discussed earlier, our recrystallization treatment in the transition zone and the lower mantle is limited in its realism by the lack of experimental data. In addition, our treatment of grain size dependence on rock strength, particularly for grain size sensitive mechanisms (i.e., weakening mechanisms when the grain size is small) such as diffusion creep and grain boundary sliding, needs improvement. The present model focuses primarily on the interactions between grain boundary and dislocations without deep consideration of the grain size sensitive mechanisms. When the grain size is significantly decreased by dynamic recrystallization, these grain size sensitive mechanisms might conceivably produce much more weakening than our present results indicate. In spite of these limitations, our preliminary study of weakening Cho et al. ◀ Strength-reducing mechanisms in mantle rock during the Flood ▶ 2018 ICC 721 Figure 10 . Snapshots of norm of spin tensor (or vorticity) and total shear strain ( i.e. , sum of elastic and plastic shear strain) at the first day. Both of the spin and shear strain were very high around the descending slab. From these snapshots, it is clearly shown that the surrounding rock of slab underwent high shear deformation by plastic spin.