This paper presents a qualitative description and quantitative model of the technologically important problem of the transition from ineffective to effective internal gettering (IG) states in CZ silicon wafers as the precipitation of oxygen proceeds at high temperatures. Of central importance to the problem is a morphological transformation of growing oxide preciptitates from an initial strain free to a strained platelet state. The transition to effective IG occurs when approximately 107 cm-3 precipitate sites are transformed into the strained state. The transformation rates as a function of oxygen concentration and precipitate site density, deduced from etching and TEM, are presented. A model for the morphological transformation to the strained state is developed and with it a model for the annealing criteria for the threshold for effective gettering.