A quantitative model of environmentally sensitive crack growth of amorphous silica (a-silica) is based upon semiempirical molecular orbital (MO) calculations (AM-1 method) of a water molecule interacting with strained three- and four-fold silica rings and a five-fold ring-chain structure. The energy barrier for hydrolysis of strained 3-fold rings is only 7 kcal mol(-1), the energy barrier for hydrolysis of strained four-fold rings is 29 kcal mol(-1); for a five-fold ring-chain it is 39 kcal mol(-1). Thus, the MO model predicts that the energetics of Region 1 slow crack growth is controlled primarily by the distribution and hydrolysis of three-membered silica rings in the a-silica structure, and Region III is controlled by the distribution and energy of contraction of four and four + membered rings.