Abstract Using molecular dynamics computer simulations, we have studied the nature of structural transitions in crystalline and amorphous silica. Atomic interactions were modeled using a charge-transfer three-body potential that was developed to describe the charge redistribution upon breaking and forming of bonds, especially in systems that exhibit mixed covalent-ionic bonding. Two types of transitions, namely reversible and irreversible, occur in silica glass. The reversible transition is based on the same mechanism as in the α- to β-cristobalite phase transformations in crystalline silica, i.e., the abrupt rotation of Si–O–Si bridges around the Si–Si axis. Unlike in the crystal, in the glass, these rotations occur sporadically and remain localized, spreading over a range of temperature or pressure. Thus the transition is gradual. We show that this mechanism is responsible for the anomalous thermo-mechanical behaviors of silica glass, e.g., stiffening upon heating and softening upon compression. The irreversible transition is based on bond exchanges. In particular, smaller rings in the structure dissolve in favor of larger ones as the density of the structure increases. This mechanism causes permanent densification of silica glass and constitutes the basis for polyamorphism in this system.