Water molecules have a great influence on the mechanical performance of silicate materials. In this study, the hydrolytic weakening mechanisms of crystal α-quartz and amorphous silica glass were investigated in light of molecular simulation. A reactive force field was used to simulate the structure, dynamics and mechanical properties of the silica with water molecules during a uniaxial tensile process with loading rates from 0.02 to 0.16 ps. Dry silica samples, obtained by the temperature quenching method, and wet silica samples, formed by Grand Canonical Monte Carlo water adsorption, were prepared for mechanical tests. Structurally, water penetrating the silica cavity can form H-bonds with the neighboring bridging oxygen in the silicate network, leading to the elongation of the Si-O bonds and the enlargement of the Si-O-Si angles. Dynamically, the confined water molecules, with a high diffusion rate, collide with the silicate network, reducing the stability of the siloxane bonds. The reactive force field, coupled with the chemical and mechanical responses, provides new insights into the molecular structural evolution. During the tensile process, the siloxane bonds and Si-O-Si angles are initially stretched in the elastic region. Subsequently, the silica network depolymerizes into branch structures and, finally, the structure undergoes a tensioned fracture in a catastrophic manner. When the stress exceeds 30% of the tensile strength, water molecules in the silica system begin to dissociate into two reaction pathways, i.e. one governed by the chemical adsorption of water at a low stress level and the other dominated by the breakage of siloxane bonds at a high stress level. The stress-dependent water dissociation accelerates the depolymerization of the silica network, and brings about further irreversible deformation, which weakens the cohesive force of the silica. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.