Abstract Compaction bands, desiccation cracks, and melt segregation structures are geological patterns relying on the same fundamental manifestations of a universal Turing‐type instability mechanism, as predicted by the thermodynamically consistent reaction–cross‐diffusion framework developed in Part I (Regenauer‐Lieb et al., 2025, https://doi.org/10.1029/2025GC012710 ). We demonstrate that localization is driven by a state‐dependent “role‐swap” between coupled mechanical and hydraulic phases. While the solid acts as the activator during softening, the hardening regime triggers a critical transition: permeability collapse suppresses inhibitor diffusion, forcing the fluid phase into the activator role. Driven by anti‐symmetric cross‐diffusion (), this mechanism enables the fluid to “pump” the hardening solid into compaction bands (cnoidal waves) via thermodynamic uphill diffusion, maintaining positive global entropy production. We resolve the long‐standing discrepancy in Olsson's experiments by mapping discrete bands as stationary cnoidal solutions, physically distinct from the traveling dispersive waves characterizing the diffuse regime. These results identify field‐scale transport coefficients as signatures of rate‐dependent micromechanics rather than measurement discrepancies between laboratory and field, suggesting that pre‐fracture microstructural evolution dictates the ultimate complexity of geological networks.
Regenauer‐Lieb et al. (2026) studied this question.