Geodynamic models incorporating metamorphic phase transformations almost invariably assume the validity of the Boussinesq approximation that violates conservation of mass. In such models metamorphic density changes take place without volumetric effects. We assess the impact of the Boussinesq approximation by comparing models of orogeny accompanied by lower crustal eclogitization both with and without the approximation. Our results demonstrate that the approximation may cause errors approaching 100 per cent in characteristic measures of orogenic shape. Mass conservation errors in Boussinesq models amplify with model time. Mass conservative models of metamorphism are therefore essential to understand long-term tectonic evolution and to assess the importance of the different geodynamic processes.