We have measured the evolving three-dimensional (3D) morphology of patterned SiO 2 stripes on Si substrates induced by 3 MeV O ++ ion irradiation. We develop a 3D constitutive relation to describe anisotropic deformation, densification, and flow. We use this constitutive relation in a finite element model that simulates the experimental morphology evolution, and we find excellent agreement between simulated and measured profiles. The model should be useful in predicting morphology evolution in complex three-dimensional structures under MeV ion irradiation. © 2006 American Institute of Physics.