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To conserve or not to conserve (mass in numerical models)

Mass conservation is a fundamental physical law. Yet, computer models aiming to simulate Earth evolution commonly fail to respect it. Indeed, most geodynamic models implement phase transitions of rocks – such as metamorphism (solid–solid phase change) or melting (solid–liquid phase change) – following a simplifying assumption dating back to 1897 which is conserving volume rather than mass. The underlying problem is present at different scales, illustrated here by three examples: metamorphism in the continental crust, phase changes in the mantle transition zone and melt crystallization during columnar jointing. These illustrate that phase changes may become a driving force of a system's deformation and point to important differences with respect to simplified models. Developing and applying mass‐conserving approaches in future modelling tools are therefore not an option, but a necessity.