Using thermodynamic and kinetic considerations, we explain the quasiequilibrium, morphological, and structural characteristics of thermally induced oxide precipitates in Czochralski silicon. A model based upon the formation of Frenkel defects at the silicon/silica interface is used to explain the experimentally observed precipitate shapes: 〈110〉 coesite rods, (100) vitreous silica discs, and silica polyhedra at low, intermediate, and high temperatures, respectively.