Abstract Processes leading to the creation of both transient and permanent broken SiO bond defects in fused silica are discussed. An exciton self-trapping causes a transient rupture of the SiO bond characterized by short-lived E'centers and 2.6 eV luminescence band. A stabilization of the broken SiO bond components occurs, if the rotation of the tetrahedron, which contains a non-bridging oxygen, separates them spatially. The defect separation increases further, if a diffusion of the neighbouring tetrahedron's Si atom towards the non-bridging oxygen atom takes place. A model of the non-bridging oxygen defect and its spectroscopic parameters is presented. The induced absorption band at 2.0 eV and the luminescence band at I.85 eV are due to the transitions between the atomic type 2p orbitals in the non-bridging 0 atom, split by a strong dynamic Jahn-Teller effect.