A quantitative ring contraction model for the fracture of amorphous silica is described based upon AM-1 semiempirical molecular orbital calculations of strained three- and four-fold silica rings and a five-fold ring-chain structure. The fracture barrier for five-fold ring-chain structures is 1 03 kcal mol-1. The barrier for fracture of a three-fold ring is 96 kcal mol-1. Fracture by contraction of four-fold rings has a lower energy barrier of 77 kcal mol-1 due to formation of pentacoordinate silicon transition states which produce trisiloxane rings and a broken siloxane bond. Thus, the ring contraction model predicts that a crack will follow a path which depends on the distribution of four-fold (or larger) rings in vacuum or fast fracture.