Nanoporous anodic aluminum oxide (MO) can be created with pores that self-assemble into ordered configurations. For more than 60 years it has been assumed that field-assisted dissolution of the oxide leads to pore formation, despite a lack of direct experimental evidence that confirms this expectation. In this work, we have developed a method for separately studying the onset of field induced growth instabilities and the instability that leads to pore formation. We find that field-assisted dissolution models are consistent with the observed dependence of the Al2O3 dissolution rate on the electric field, as well as the existence of a critical field for pore initiation. However, we further show that the well-known porous structure, which has a significantly different length scale, does not result from a field-induced instability, but is instead the result of a mechanical instability with forced plastic deformation and flow of the oxide during further anodization. Through interpretation of these results we develop a generalized mechanism for pore formation in MO, and by analogy, for pore formation in other anodization processes. (C) 2011 Elsevier Ltd. All rights reserved.